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CONTRIBUTIONS TO PRE-CAMBRIAN 
GEOLOGY 



GEOLOGY OF LIMESTONE MOUNTAIN 



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WAIN LIBHARY 



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MICHIGAN GEOLOGICAL AND BIOLOGICAL SURVEY. 

Publication 18. 
Geological Series 15. 



CONTRIBUTIONS TO THE PRE-CAMBRIAN GEOLOGY OF 
NORTHERN MICHIGAN AND WISCONSIN. 

R. C. ALLEN AND L. P. BARRETT. 



THE GEOLOGY OF LIMESTONE MOUNTAIN AND SHER- 
MAN HILL IN HOUGHTON COUNTY, MICHIGAN. 

E. C. CASE AND W. L ROBINSON. 




PUBLISHED AS A PART OF THE ANNUAL REPORT OF THE BOARD OF 
GEOLOGICAL AND BIOLOGICAL SURVEY FOR 1014. 



LANSING. MICHIGAN 
WYNK(;()I' IIALLKNBECK CRAWFORD CO.. .STATE PRINTERS 

1015 



Digitized by the Internet Archive 

in 2010 with funding from 

University of British Columbia Library 



http://www.archive.org/details/contributionstopOOalle 



LETTER OF TRANSMITTAL. 

To the Honorable, The Board of Geological and Biological Survey of 
the State of Michigan: 

Gov. Woodbridge N. Ferris. 
Hon. Fred L. Keeler. 
Hon. Wm. J. McKone. 

Gentlemen: — I have the honor to transmit herewith two manu- 
scripts, viz., Contributions to the Pre-Cambrian Geology of Northern 
Michigan and Wisconsin, by R. C. Allen and L. P. Barrett, and The 
Geology of Limestone Mountain and Sherman Hill in Houghton 
County, Michigan, by E. C. Case and W. L Robinson, with the rec- 
ommendation that they be printed and bound as Pul)lication 18, 
Geological Series 15. 

Very respectfully, 

R. C. ALLEN, 

Director. 
Lansing, Michigan, 
June 16, 1915. 



CONTENTS. 

Letter of Transmittal 



Page 
5 



region, R. C. Allen 




15 



CHAPTER I. 
Introduction and acknowledgements, R. C. Allen 

CHAPTER II. 
A revision of the correlations of the Huronian group of Michigan and the Lake Superior 



23 



Cc^^l^icSof the Anin^kie series as Mid^^^ 23 

g^r^rlllllon ^i Z ^uTa'^rroTb^elK) se^r^e's with the Negaunee (.ron bear- 

ing) series „ ' ^- ♦ ■ f • •■ ^^ 

Marf4uette and Crystal Falls districts^ series with the Vulcan (iron bear- 

trough and Menominee range 27 

Sturgeon River synchne 27 

Felch Mountain district 28- 

Calumet trough ''..........■■■ ^* 

29 

Gen^'^a^S'^n'thVcorrdatTSn of the Animikie series witn tne .uacue ^^ 

Huronian 

CHAPTER III. 

Tntrn(hictorv statement ,: ; .S3 

Svnmnarv of the geology based on earlier work -^I 

Summary of the writers' conclusions 37 

Archeau system 37 

Algonkian system 37 

Huronian group 37 

Lower Huronian 37 

Sunday quartzite 37 

Lithology , 37 

Relation to adjacent rocks ^g 

Bad Hiver Umestone 3g 

Lithology . ■ ■ 3g 

Middle Huronian (Animikie) ^g 

Introductory statement 39 

Relations to adjacent formations ^^ 

I'ahns formation 40 

Distrit)Ution and exposure 4q 

Lithology 41 

M etamorphic phases 43 

Relations to adjacent formations ^^ 

Ironwood formation 43 

Distribution and exposures ^^ 

Lithology 44 

I„l.TlH.(td.Ml slat.- and graywacke. . ..... ■ - • • v.™ 45 

I.itlH.logy of Ironwood forma ion m I 47 N^ R 44 W 4ft 

riif Ironwood formation m 1 47 N , R 4,3 W . ... . .^ • ■ • • • ^^ 

iVclaliot. of the Ironwood formation to a<ijac-ent formations.. . . 47 

Middle Huronian t-xtrusives • . 1 , ^l-o 47 

Creenstoiif agglomerates and associated rocks |^ 

Basic intrusives. .......■■■ _ ■ ■■■ 4<) 

Diabase in sections 22, 2(> and 27 .^.^_ . . ^ - .„ 

C.reenstone in sections 20, 29 an<l 30, T 47 N, R 4.1 W 4J 

Presnue Isle granite _^q 

Location and extent g^ 

UelaVions of ilx- I'res.,ue Isje graiiite to" tlie Middle Huronian 

( Ammikic) formations .,;, .•■.■.' ' \± 

Relation of the I'res(iue Isle granite to the C opps formation ... o4 



8 CONTENTS. 

Page 

Upper Huronian 54 

Copps formation 54 

Location 54 

Lithology 54 

Thicicness of tiie Copps formation 57 

Relations to adjacent formations 57 

Keweenawan series 58 

Relation of the Keweenawan to adjacent formations 58 

Keweenawan (?) Rabbro in T 47 N, R 44 W 58 

Structure of the Gogebic range east of Wakefield 59 

Structure of the Keweenaw series 60 

Structure of the Copps formation 60 

Structure of the Middle Huronian series 61 

CHAPTER IV. 

Geology of the Marenisco range, R. C. Allen and L. P. Barrett 65 

Introductory statement •. 65 

Archean system 66 

Northern area 66 

Southern area 66 

Location and general statement 66 

M ica schist 67 

Origin of the mica schist -. 68 

Intrusives 69 

Relations to adjacent rocks 69 

Relations to Keweenawan 69 

Relations to Huronian 69 

Middle Huronian series (Animikie) 69 

Graywacke-quartzite 70 

Iron formation 71 

Lithology 71 

Ore bearing possibilities of the iron formation 74 

Igneous intrusives in the iron formation 74 

The slate formation 75 

Lithology 76 

Relations of the slate to adjacent formations 77 

Igneous rocks 77 

Extrusives , 77 

Altered porphyrites 77 

Hornblende schists 79 

Relation of the effusives to the Huronian rocks 79 

Intrusives 80 

Granite 82 

Diabase 83 

Relations of the intrusives to the Algonkian and Archean 83 

CHAPTER V. 

Geology of the Turtle range, R. C. Allen and L. P. Barrett 87 

Inroduction 87 

Succession on the Turtle range 89 

Archean 89 

Huronian group 90 

Lower Huronian series 90 

Dolomite and quartzite 90 

Middle Huronian (Animikie) series 90 

Ford-Lucas section 91 

Whiteside exploration 93 

M ichigan mine "". 95 

Broomhandle exploration 96 

Mercer section • 97 

Winegar section 99 

Banner location 103 

Igneous rocks 104 

Intrusives 104 

Extrusives 105 

CHAPTER VI. 

Geology of the Manitowish range, R. C. Allen and L. P. Barrett Ill 

Results of diamond drilling 114 

General summary , 117 

CHAPTER VII. 

Geology of the Vieux Desert district, R. C. Allen and L. P. Barrett 119 

Conclusions 121 



CONTENTS. 9 

CHAPTER vni. 

Page 

'Geology of the Conover district, R. C. Alien and L. P. Barrett 123 

Petrographic description of Conover slates 124 

CHAPTER IX. 

'The Paint slate and the Wolf Lake granite, gneiss and schist, R. C. Allen and L. P. 

Barrett 131 

The Paint .slate fromation 131 

Petrographic description of the Paint slate 132 

Graj-wacke and gravwacke schist 133 

Slate 133 

Mica schist, hornblende schist and altered grajrwacke 134 

Summary statement 134 

Granites, gneisses and schists of the Wolf Lake area 134 

The Wolf Lake granite 135 

The Wolf Lake schists 136 

Green schist 136 

Mica schists 136 

Relations of the Wolf Lake granite and mica schist to the Paint slate formation.. 138 

CHAPTER X. 

"Correlation and structure of the pre-Cambrian formations of the Gwinn iron bearing 

district of Michigan, R. C. Allen 141 

Location and topography, etc 145 

Notes on the structure of the Gwinn synclinorium 145 

Archean system 147 

Algonkian system 147 

Middle Huronian 147 

Gwinn series 147 

Conglomerate-arkose 147 

The lower slate 149 

The iron bearing member 149 

The upper slate 150 

L'pper Huronian 150 

Princeton series 1 50 

Keweenawan series (?) 151 

Paleozoic 151 

Correlation of the Gwinn and Princeton series 151 

CHAPTER XL 

;Evidence of the Middle-Upper Huronian uncomformity in the quartzite hills of Little 

Lake, Michigan, R. C. Allen and L. P. Barrett 153 

Structure of Little Lake hills 155 

The Lower (Gwinn) series 157 

Arkose and conglomerate 157 

Upper ( Princeton) series 157 

Conglomerate 157 

Quartzite and quartz slate 158 

Note.s on the correlation 159 

CHAPTER Xn. 

Relative to an extension of the Menominee iron range eastward from Wauce<lah to 

Escanaba, Michigan, R. C. .\llen 161 

Conclusions 163 

The geology of Limestone Mountain and Sherman Hill in Houghton county, Michigan, 

E. C. Case and W. I. Robinson 165 

Stratigraphy and correlation of beds 171 

Mid-Devonian 173 

N iagaran 1 73 

Middle or Upper Richmond 173 

Uiiper part of Lower Riclimond 173 

Lower Richinotid 174 

fJalena (Stcwartville or Upper Galena) 174 

Decorah (I'pper Mhie) 175 

Upper Black (Ipper- bluff) 175 

Pot.sdani (Jacobsville) 176 

Structure 176 

History 178 

Tlie fault between Big and Little Limestone. . . 170 



LIST OF ILLUSTRATIONS. 

CONTRIBUTIONS TO THE PRE-CAMBRIAN GEOLOGY OF NORTHERN MICH- 

IGAN AND WISCONSIN. 



Plate I. 
Plate II. 
Plate III. 



Plate IV. 
Plate V. 



Plate VI. 
Plate VII. 
Plate VIII 
Plate IX. 
Plate X. 
Plate XI. 
Plate XII 



PLATES. 

\ ^mgatofpont Looking north from the south end of Lake Gogebic 
n Fall! on the Middle Branch of the Ontonagon river, section 32, T 46 
B. Falls^n me^Aimai^^^^.^^ jointing in amygdaloidal greenstone 

Band'ed mica schist. Section 30, T 46 N, R 42 " ■ - ■ ■ ■ ■ - 

^ --«rc^i^^go"!^^^tSi^n"i^^i^^i^r^ 

tou"h of theh^tatifn of Marenisco, Michigan < Mjerophot^graph 
K Greenstone porphyrite from SWJ of N\\ i, section , , T 46 N, R 41 

W Michigan. (Microphotograph) ■ ■ • ■ • ■ ■ ■ 

B Feldspithic-biotite-schist from Archean (?\ area lying south of the 
B. ^eiaspa mc^j o ^i^.enisco range. (Microphotograph) 

A Black sate from test pits of Ford-Lucas exploration, bEJ of SEj. 

sec fon 5 T 41 NR 1 E, Wisconsin. (Microphotograph) , . . 
B Grunldte-magnetite-schist from drill hole in SEi of S>A i of section 

H. v^ruiieiur s Wisconsin. (Microphotograph) 

A Altefed porphyrite from section 22, T 42 N, R 1 E, U.sconsm, near 

Michigan Mine. (Microphotograph) • • _ 

B \ltered porphvr te from east end of Turtle range near south quarter 
^ -^^ pos se'.^^tiVil 27. T 46 N, R 41 W. Mici^igan. UM.crophotograph) 
.\ Fine graine.i .^chist from SEJ of NEi of section 33, T 46 N , R 41 \^ , 

Miohiean (Microphotograph) i ' .■' ' / 

B ^Iterecl bate amvg.ialoidal lava from near the center of section 4, 

T 4'> N R 3 E, Wisconsin. (Microphotograph) .^^.. . . . ■ -^ 

A. DiabLe f rom dike cut ting basic lavas in t h. N \\ ; of SW i of section 

34 T 43 N R 3 E, Wisconsin. {Microphotograph). 

B. Altered <liaba.-^V in the SW J of section 1.5, T 42 N , R 2 E, Wisconsin. 

(Micropliotograph) ■ ■ ■ ■ ■ \^ ■■ v,- 'ri ■; t^ ' wiV 

\ Kyanitic. garnet if erous gneiss from section 30, T 42 N, R 4 E. W is- 

consin. (Microphotograph) ;• " ' V 'f'io m ' vi '< v 

B Garnetiferous, pyritic, graphitic rock from section 2, T 42 N. K 5 i;.. 

Wisconsin. (Micropliotograph) . _, . ... ^^ ■ • ■ ■ • -.■- 

\ Sernentine-magnetite-carbonate rock from NEi of NEJ of section 
A. herpenune niag ^ ^^ ^, .^.j^^.^^^j,^ (Microphotograph) . 

B Same is A showing obliteration of the granular structure under 
crossed nicols. (Microphotograph) 



Page 

39 
65 
6.5 
65 

65 
69 

SI 

81 
81 
81 
97 
97 
97 
97 
97 
97 
97 
97 
129 
129 
129 
129 



Figure 1 . 
Figure 2. 



Figure 3. 



Figure 4. 

Figure 5. 

Figure 6. 

Figure 7. 

Figure 8. 

Figure 9. 

Figure 10. 

Figure 11. 



FIGURES. 

GeoloL'ical map of parts of Michigan an<l Wisconsin. ... , 

A (;^- .1 nd • map of the east en.l of the Gogebic iron range, Michigan 

B Map (.f tlie east cikI of the Gogebic iron range, Michigan, showing 

outcrops, drill holes, explorations, etc . ■ 

Diagram showing relations between Keweenawan «?"'^«l«"V'''?\fter 
intrusive diabase in .sections 11 and 14. T 46 N, R 41 W . (Alter 

Plat 'a'nd 'section of Lucas-Ford explpration, section 5, T 41 N, R 1 E, 

Wisconsin ^- ■ ■ ;, v^r. 

M.al s.ction. Section 3, T 41 N, R 1 K. Wisconsin, .. ■■ ^ ^ . 

Plat of test pits and section at Michigan Muie, section 22. 1 42 N. K l 

I-'.. Wisconsin ,,.. kx^- ' .,o;„ 

Drill liole section across the Turtle range at W inegar. >\ isconsin _ 
Geological i>lat an.! cross section. Banner location, SKJ of section 1, T 

f;eologicai iiiai'i of the Pr'-ranibrian formations of the (Jwinn iron bear- 
ing district of .Michigan ..,■,, Vi. •: 

Plat of oubrops in l' ir, N, K 24 W. near Little Lake, Michigan 

Map showing the hypothetical extension of the Menominee iron range 
from Waucedah to Kscanaba, Michigan. 



17 
43 



83 



91 
94 



95 
100 



103 



146 
1.56 



162 



12 ILLUSTRATIONS. 



THE GEOLOGY OF LIMESTONE MOUNTAIN AND .SHERMAN HILL IN HOUGH- 
TON COUNTY. MICHIGAN. 

FIGURES. 

Page 
Figure 1. Limestone Mountain and vicinity modified from a map l)y W. L, Honnold 

and A. C. Lane. Contour interval .50 feet 168 

Figure 2. The succession of the beds 172 

Figure 3. Cross section of soutliern slope of Big Limestone along B.B. (See map). 176 

Figure 4, A generalized section of Big and Little Liine.stoiie 177 

Figure .5. The upper figure show.s a hypotlieticul i)r()[,'rcssive overlap. The lower 

figure shows the present condition due to faulting and erosion 180 



CONTRIBUTIONS TO THE PRE-CAMBRIAN GEOLOGY OF 
NORTHERN MICHIGAN AND WISCONSIN. 



R. C. ALLEN AND L. P. BARRETT. 



CHAPTER I. 
INTRODUCTION AND ACKNOWLEDGMENTS. 



R. C. ALLEN. 



In 1909 the writer became interested through, studies in the Iron 
River district, in the problems of the pre-Cambrian terranes which 
occupy the great area west of the Crystal Falls and Iron River 
districts and south of the Marquette and Gogebic iron ranges to the 
state boundary line and beyond into Wisconsin. This region is a great 
Huronian interior into which the better kno\NTi and structurally dis- 
tinct "ranges" on its north and east borders coalesce and lose their 
identity. It is unattractive to the geologist because of the general 
scarcity of rock exposures, its apparent lack of structural individuality, 
its unimportance from the mining standpoint and the difficulty of 
access to many of its parts. Notwithstanding all of these it has seemed 
to the writer that a serious attempt should be made to acquire what 
information there is to be had of the geology of this region in order to 
reduce this geological "no man's land" to terms of general description 
even if nothing further could be expected to be accomplished. Although 
the conditions to be faced were discouraging, the labor great, and the 
results certain to be disproportionate to the cost in time and money in 
comparison with equal expenditures in more favored regions, it was 
finally determined to embrace the task of making a field examination 
of the entire area so far as it lies within the state of Michigan. Field 
work was prosecuted during the summers of 1910, 1911, 1912, and 
1914. Fortunately for the progress of geologic knowledge of this dis- 
trict, a number of individuals became interested in 1911 in exploration 
for iron ore in northern Wisconsin adjacent to the Michigan boundary 
and a syndicate was formeil under the direction of Dr. F. 1. Carpenter 
to finance the geologic examination and reconnaisance exploration by 
drilling of a large territory extending westward from the vicinity of 
Lake Vieux Desert to near Butternut, Wisconsin, a distance of about 
65 miles. The supervision of the geologic work, which fell to the writer, 
has enal)le(l him to combine in this volume th(> n^suits of private enter- 
prise in Wisconsin with the work of the Michigan (Geological Survey 
in Michigan. 

The area which has been examined in the field contains not less than 



16 INTRODUCTION AND ACKNOWLEDGMENTS. 

2,000 square miles including 42 townships in Michigan and 32 town- 
ships in Wisconsin. (See Fig. 1.) Nearly all of the townships on the 
Michigan side were examined in the 80's by the Michigan Geological 
Survey but the information thus aciuired was never used to any practical 
extent, if indeed it was understood. With the exception of the admir- 
able notes of Prof. A. E. Seaman on certain townships none of this early 
work has been of much value in the present investigations beyond a 
saving in time in search of some of the exposures and in mapping parts 
of a few of the magnetic belts. These magnetic belts heretofore have 
not appeared on geologic maps but parts of some of them, more or less 
correctly located, appeared 20 to 30 years ago on commercial maps 
issued by land holding companies 

A large area of Huronian rocks extending south of the Marquette 
iron range to about the line between townships 46 and 47 North, west 
to the Eastern sandstone and northeast in a great tongue fringing: 
the Archean mass north of the Marquette district remains to be ex- 
amined. It is planned to continue the work in this region in this and 
subsequent years. 

In many townships in Wisconsin and in a number of those in Michigan 
rock outcrops are entirely wanting. The drift covered townships, and in 
fact the entire area, has been magnetically surveyed by the use of the dip 
compass and most of the magnetic belts thus located have been explored 
to some extent by diamond drilling, but the geology of the non-magnetic 
drift covered territory still remains and doubtless will remain for many 
years the subject of speculation and inference. 

The geology of the area between the Gogebic and Iron River dis- 
tricts in Michigan and southward into Wisconsin is treated in the 
following chapters as a composite of several areas. Each of these 
has a well defined geologic individualism but is separated from adjacent 
ones by tracts in which this individualism is lost, through partial or 
complete lack of geologic data and disappearance of identifiable struc- 
tures and stratigraphic horizons. Stated in clearer manner perhaps,, 
there are (1) some areas in this composite in which the geologic data^ 
while not so complete as to afford an entirely satisfactory view, are 
relatively ample, (2) some areas respecting which freedom of inference 
from meager facts alone renders possible any connected ideas of the 
geology, however hypothetical, and (3) some areas of which such ideas- 
as may be advanced must rest solely upon inference. 

For purposes of description it is necessary to consider these several 
areas separately. The discussion of each unit or separate area should,, 
however, be approached with an understanding of its relations to 
adjacent areas and to those factors in the geology which have a general 
application to the entire region. 



INTRODUCTION AND ACKNOWLEDGMENTS. 17 

Let US refer for a moment to some earlier conceptions of the geology 
of this region. On early general geologic maps a line was drawn from 
the south end of Lake Gogebic southeasterly to the headwaters of the 
Brule river, separating the Algonkian rocks northeast of it from the 
supposedly Archean rocks to the southwest, forming what was then 
thought to be a part of the great Archean "Isle of Wisconsin." To 
this conception may be traced the idea, which was prevalent among 
explorers for iron ore as recently as 1909, that the Huronian series 
swing from the Iron River district northwesterly parallel to this hypo- 
thetical line and connect more or less directly with the Huronian of 
the Gogebic range. In 1911 Xan Hise and Leith shifted this hypo- 
thetical line in general westward** and introduced an area of Archean 
"green schists and iron bearing formations" several miles broad extend- 
ing from the south end of Gogebic lake eastward some 20 miles, bounded 
on the north by the South (Keweenawan) Trap Range and on the 
south by a narrow belt of Laurentian granite. The territory to the 
southwest was mapped as "undifferentiated pre-Cambrian." This 
classification of the rocks south and east of Gogebic Lake has not 
survived recent studies but the general mapping makes an advance 
over earlier attempts because it expresses doubt concerning the Archean 
age of all of the rocks included in the "undifferentiated pre-Cambrian," 
formerly mapped as Archean. 

The assignment of an igneous complex or metamorphic schist series 
to the Archean in the absence of definite evidence of later age may be 
permissible for purposes of general mapping if qualified by an appropri- 
ate expression of doubt concerning its correlation. It sometimes hap- 
pens that a correlation which was sufficiently qualified by the author 
as regards the points of doubt and uncertainty, through long stand- 
ing and repetition in text books and other geological literature, be- 
comes fixed in the minds of later workers as fact in distinction 
from inference, and to this extent acts as a bar to inquiry which other- 
wise would be directed toward a final solution of the uncertainties. 
So far as concerns the region under examination we are unable to 
say that rocks older than Huronian are present. On the other hand 
there is indisputable evidence that )7iost of the area heretofore mai^ix'd 
as Archean and undifferentiated pre-Cambrian is in reaUty Huronian. 
The granite wliicli has l)een considered as underlying most of the sup- 
posedly Archean area is intrusive into the Lower antl Middle Huronian 
(Animikie) sediments. It constitutes a great batholith which seems 
to ()C(-upy many thousands of scjuare miles in northern Wisconsin 
and is rei)resented by outlying bosses in the east end of the (u)gebic 

♦*See "Geologic Map of Ihf Lake Superior He^iio". wi"> Sections," accoiiipanyiiiK Mono- 
graph No. .52, V. S. Geological Survey, I'.Ul. 

3 



18 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

range and southward and eastward in Michigan. There is clear evi- 
dence that this granite has (Usplaced large masses of pre-Cambrian 
sediments. Nearly all of the sediments which remain have been 
rendered schistose, gneissose and thoroughly crystalline and in 
many areas they have lost all evidence of sedimentary origin. The 
profound exomorphic effect of granitic intrusion is apparent, par- 
ticularly in the Manitowish range, in the Vieux Desert-Conover dis- 
trict and in the vicinity of the Wolf Lake granite north of Watersmeet. 
While there are other intrusives it is apparent that this enormous intru- 
sive granite mass ivhen considered with its profound metamorphic effects 
on the pre-Camhrian sedimentaries is the great, significant, outstanding 
feature of the geology of this region. 

The region may be described in the most general terms as a series 
of highly folded belts of Huronian sedimentary rocks and, in some dis- 
tricts, associated contemporaneous igneous efTusives, showing marked 
parallelism to the axis of the great Lake Superior geosyncline, and 
which are invaded, here and there terminated or interrupted, and in 
great part separated, one from another, by intrusive granite and schists 
of doubtful origin. The character of the folding is such as to support 
the inference that the Archean rocks were exposed by erosion on the an- 
ticlinal areas prior to the granitic intrusion of late Middle Huronian (An- 
imikie) time and inasmuch as the sedimentary rocks were not entirely 
replaced or absorbed by the invading granite it must be presumed that the 
Archean also still remains in some of the territory not occupied by the 
sediments. That it has not been identified wdth certaintj^ must be laid 
to total lack of exposures of contacts between the sediments of the 
Huronian group and the older terranes on which they -were deposited, 
as well as a general scarcity of outcrops, and the fact that drilling 
and other exploratory operations have been confined to the vicinities 
of exposures of the sedimentaries and, in drift covered areas, the mag- 
netic belts by which the positions of the sedimentary rocks are to con- 
siderable extent marked. 

The geologic field investigations which are discussed in following chap- 
ters together with the data which has become available through explora- 
tory drilling and underground mining have thrown much added light on 
the problems of correlation of the Huronian group and have furnished a 
body of evidence which, in our opinion, may be interpreted satisfactorily 
only through a revision of the present correlation of this group through- 
out the Lake Superior region in the United States. The following chapter 
is a discussion of this evidence and a revision of the correlation in the 
direction of a clearer and more adequate interpretation of present 
information. This chapter precedes the discussion of the various 
districts because it serves as an introduction to the general theme 



INTRODUCTION AND ACKNOWLEDGMENTS. 19 

and explains the correlations which have been applied throughout to 
the Huronian group in Michigan. 

This volume is presented to the public under a full appreciation 
of its inadequacies. These, however, do not express lack of diligence 
on the part of the writer but rather the measure to which he has been 
able to surmount the difficulties imposed on him by the meagerness 
of the geologic data afforded by an exhaustion of available sources 
and present means. While he assumes full responsibility for the short- 
comings of this presentation he desires to express his indebtedness 
to his assistants and to share with them any merit which it may possess. 
Messrs. R. E. Hore, R. W. Clark, R. E. Ascham, R. A. Smith, Geo. B. 
Corless, L. J. Youngs, and L. P. Barrett, geologists, have rendered 
valuable service in the Michigan field, Messrs. P. G. McKenna, H. J. 
Allen, L. P. Barrett, W. I. Robinson, J. P. Goldsberry, and Geo. B. 
Corliss, in Wisconsin. I am especially indebted to Messrs. McKenna 
and Allen who have had joint charge of both field work and drilling 
operations in Wisconsin, to Mr. R. W. Clark for petrographic descrip- 
tions of thin sections of the Paint slate series, and above all to Mr. 
L. P. Barrett who executed the field mapping of the eastern Gogebic 
range, made the greater part of the microscopic examinations of thin 
sections of the rocks in both Michigan and Wisconsin, and collaborated 
in the preparation of manuscript and illustrations. My indebtedness 
to Mr. Barrett is expressed in joint authorship of the chapters on the 
eastern Gogebic, Marenisco, Turtle, and Manito\vish ranges, the Paint 
slate series and the Conover-Vieux Desert district. Thanks are due 
to the F. I. Carpenter syndicate for placing at our disposal all of its 
information relative to the geology of the area adjacent to Michigan 
in Northern Wisconsin without which it would be impossible to discuss 
the geology of that region and which has thrown much additional 
light on the geology of the contiguous area in Michigan. 

For their bearing on the general subject of Huronian correlations I 
have introduced a description of the Gwinn and Little Lake areas in 
Mar(i[uette county and the east end of the Menominee range. The 
article by Dr. E. C. Case and W. L Robinson on the geology of Lime- 
stone Mountain and vicinity has no bearing on the main theme of this 
volume but is append(>d, with apologies to the authors, for want of a 
better present means of publication. 

Lansing, Michigan. 

.luiK- 17, lUl."). 



CHAPTER II. 



A REVISION OF THE CORRELATIONS OF THE HURONIAN 

GROUP OF MICHIGAN AND THE LAKE 

SUPERIOR REGION. 

R. C. ALLEN. 

The Huronian group comprises at least three unconformable series 
of pre-Cambrian sedimentary and associated igneous rocks separated 
from the younger Keweenawan series by a great unconformity^ and 
from the older Archean system by another more profound unconformity. 
The position of the group has been defined by the Uniied States Geo- 
logical Survey and by a committee of Canadian and United States 
geologists* (1904) as follows: 



Paleozoic 



Algonkian 



Archean 



Keweenawan 



Huronian 



Keewatin 



Laurentian 



Unconformity. 

L^nconformity 
Upper 

Unconformity 
Middle 

Unconformity 
Lower 

Unconformity 

Eruptive contact 



Former correlations of the Huronian group as well as the entire pre- 
Cambrian of the various districts of the Lake Superior region in the 
United States are mainly the results of detailed field study and majiping 
by the United States (ieologicai Survey. These studies have been pub- 
lished in a series of monographs on the most iinjxjrtant districts and 
a final summary covering the entire^ region.** 

♦Joiir. (leol. Vol. IH, 1<)0.'., ]i]i 8<)-101. 

**Thf I'fiioktM' iron Ix-ariiiK serie.s of Mkhipiiii and Wisconsin, by R. D Irvine and C R 
Van llisc, Mono Mt, {' . S. C. S.. 1892. f. ■ 

IIk' .Mannii'ttc iron tx-iirinj; series of Mictiinan, by C. R. Van Hise and W. S. Baylev with 
a rhaptcr on tlif Kc|)ul)lic troiKjli, l)y II I,. Sniytli. Mono. 28, V. S. G. S.. 1896 

Till- Crystal Kails iron bcaritiK (list rut of Nli(lil>,Mii, by C. R. Van Hise, J. M. Clements. 
W. S Uayley, and II. I,. Smyth. Mono. .<(>, T. S. C. S., 1899. 

The Me.sat)a iron bearing district of Minne.sota, by C. K. Leith Mono. 4.3, V. S. G. S.. 

The Vermilion iron beuriiiK district of Minnesota, by .1. M. Clements, Mono. AH, V . S. G. 

I'he Menominee iron beariiiR district of Miclii),'an, by W. S. Raylev. Mono 4(5 I' S G S 
1904. ■ ....v.,^. 

GeoloKy of the Lake Superior rej^ioii, by C |{ \ an lli.se and C. K I.eith, Mono .V2 V S 
G. S.. 1911. •<.--. 



22 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

In 1892 Irving and Van Hise published their completed work on the 
Gogebic iron range. They beheved that the Huronian group in the 
Gogebic district comprises two unconformable series, the Upper and the 
Lower Huronian, and that the Upper Huronian or iron bearing series 
is probably eciuivalcnt to the Animikie of the north shore of Lake 
Superior. In 1896 Van Hise, Bayle}^ and Smyth issued a monograph 
on the Marquette iron range. There were found here two unconform- 
able Huronian series, the Upper and the Lower Huronian, which were 
correlated with the Upper and the Lower Huronian of the Gogebic range, 
the Negaunee iron-bearing series of the Marquette range falling in the 
Lower Huronian. This is the dual classification which was gradually 
extended to cover all of the other districts of the Lake Superior region 
until A. E. Seaman discovered about 1902 that the "lower" Huronian 
of the Marquette range is di\'isible by a great unconformity at the 
base of the Ajibik quartzite. In 1904 Seaman's discover}^ was formally 
recognized by an international committee of C'anadian and United 
States geologists which adopted the tripartite classification of the Hu- 
ronian group. The Negaunee iron-bearing series was separated from 
the Lower Huronian to form the new Middle Huronian but the old 
correlations outside the Marquette range w^ere preserved. Nine j^ears 
later (1913) Allen accounted for a tripartite division of the Huronian 
in the Gwdnn synclinorium south of the Marquette range and cor- 
related the Gwdnn iron-bearing series -wath the Negaunee iron-bearing 
series of the Marquette district*. In 1914 Allen and Barrett found 
that the Upper Huronian as described by Van Hise and Irving on the 
east end of the Gogebic range includes two unconformable series. This 
discovery, in their opinion, has finally opened the way for a revision of 
the correlation of the Huronian group of the Lake Superior region on 
the tripartite basis in which most of the difficulties and inconsis- 
tencies in the old dual classification largely disappear. 

The new correlations place the Animikie series in the Middle Huronian, 
rather than in the Upper Huronian as in former correlations. Briefly 
stated, the steps in the argument are these : 

(1). The Animikie (iron-bearing) series of the Gogebic range is 
the equivalent of the Negaunee (iron-bearing) series W'hich constitutes 
the Middle Huronian of the Marquette range; therefore, the Animikie 
series of the other Michigan districts, Minnesota and the north shore 
of Lake Superior is also the equivalent of the Negaunee series, i. e., 
Middle Huronian. 

(2). The Negaunee series of the Marquette range is unconformably 
overlain by the Upper Huronian. The Ironw^ood (Animikie) series of 
the Gogebic range is also overlain unconformably by a series which is 

*Jour. Geol., Vol. XXII, No. 6, 1914. 



REVISION OF HURONIAN CORRELATIONS. 



23 



unconformably beneath the Keweenawan. This series is equivalent to 
the Upper Huronian of the Marquette range. Therefore the Animikie 
series is Middle Huronian. 

CORRELATION OF THE ANIMIKIE SERIES AS MIDDLE HURONIAN. 

Correlation of the Ironwood series with the Negaunee series: The 
Ironwood (Animikie) iron-bearing series of the Gogebic range is corre- 
lated with the Negaunee (Middle Huronian) iron-bearing series of the 
Marquette range because (1) these series occupy identical positions 
in the Huronian succession of these districts, (2) are essentially .similar, 
(3) are underlain and overlain by essentially similar series, (4) are in 
practically adjacent territory, (5) there is substantial evidence of their 
equivalence through direct connection of the Negaunee series with 
the Vulcan (iron-bearing) series of the Crystal Falls-Iron River district 
which bears the same relation to a great granite batholith and its 
outliers as does the Ironwood series of the Gogebic range and its cor- 
relatives in the Marenisco and Turtle ranges, and (6) there is no direct 
evidence in favor of any other correlation. 

The similarity of the successions in the Gogebic and Marquette 
districts is striking and coupled with proximity would ordinarily de- 
termine a direct correlation of the similar series occupying identical po- 
sitions in the group even were there no further evidence of identity-. 

CORRELATION OF THE HURONIAN GROUP IN THE GOGEBIC AND MAR- 
QUETTE DISTRICTS. 

Marquette District. Gogebic District. 



Greenstone intrusives and 

extrusives. 
Clarksburg volcanics i)art- 
ly replacing Michi- 
gamme slate. 
Upper Michigamme slate carry- 
Huronian ing iron-bearing lenses 
(Bijiki schist). 
Goodrich conglomerate- 
quartzite. 
uncoil fonnitv 



Graywacke and slate. 



(Deep erosion) 
Negaunee (iron-lx^aring) 
Mi(klle formation and extru- 

Huronian. sivc greenstoiu*. 

(Animikie) Siamo slate. 

Ajibik (juartzite. 



Ferruginous and cherty 
slates and jasper. 

Conglomerate. 

unconformity 

Presque Isle granite. 

Tyler slate. 

Ironwood (iron-bearing) 
formation and extru- 
sive greenstone. 

Palms (luartzite and 
(juartz slate. 



24 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 





unconformity 


unconformity 


Lower 


Wewe slate. 


(Deep erosion) 


Huronian . 


Kona dolomite. 


Bad River dolomite. 




Mesnard quartzite. 


Sunday quartzite. 




unconformity 


unconformity 



Archean 

The absence of slate above the Bad River dolomite should be con- 
sidered with the evidence of deep erosion in middle-lower Huronian 
time in the Gogebic district which not only removed the slate if it 
was ever present there but also the entire Lower Huronian over the 
greater part of the range. In respect to the Middle Huronian (Animikie) 
of these districts the situation is reversed, i. e. the thick Tyler slate 
formation above the Ironwood series is to be considered with the 
evidence of deep erosion of the Negaunee series and the development 
of iron ores on the exposed surface of the iron formation (now repre- 
sented by the hard ores of the upper part of the Negaunne formation) 
prior to the deposition of the Upper Huronian. If the correlative of 
the Tyler slate was ever deposited on the Negaunee formation it had 
been removed prior to the deposition of the Goodrich quartzite. The 
Middle Huronian (Animikie) of both districts is characterized by 
volcanic activity which continued on into and culminated in the Upper 
Huronian in the Marquette range but apparently terminated prior to 
the deposition of the Copps formation in the Gogebic range. 

There is a strong resemblance of the Copps formation of the Gogebic 
to the Upper Huronian of the Marquette range in respect to lithology 
and order of succession. At the base of these series is a great con- 
glomerate which is overlain by slate and graywacke. Neither series 
carries a great productive iron bearing member but both contain jasper 
and ferruginous beds near the base, which in the Marquette district 
are locally iron ore bearing. 

It can not be easily doubted that if Van Hise and Irving had dis- 
overed in 1892 the great unconformity at the base of the Copps forma- 
tion on the Gogebic range and Van Hise, Bayley and Smyth in 1896 the 
great unconformity at the base of the Ajibik quartzite on the Marquette 
range, the correlation of the Huronian group of the Lake Superior 
region would have bgsn tripartite from the beginning and not dual for 
the correlations have been built up on these two type districts which 
were earliest studied by the U. S. Geological Survey. The Ironwood 
(middle) series would then have been correlated with the Negaunee 
(middle) series because, as above stated, they occupy identical posi- 
tions in the Huronian succession, are essentially similar, are overlain 
and underlain by essentially similar series, and are in practically adja- 



REVISION OF HURONIAN CORRELATIONS. 25 

cent territory. The character of the reasoning which was employed in 
separating the N.gaunee and Ironwood series in the correlations would 
have united them had present information been available. There 
could then have been httle, if any, reason for assigning these two iron 
bearing series to different positions in the Huroman group; certamly 
there is no reason for such assignment today particularly as there 
is additional substantial evidence for the correlation of the Ironwood 
and the Negaunee series which will be discussed below. 

CORRELATION OF THE VULCAN (iRON BEARING) SERIES WITH THE NEGAU- 
NEE (iron bearing) series. 

Marquette and Crystal Falls Districts: The basis of correlation of 
the formations of the Marquette and Crystal Falls districts is afforded 
by an indicated actual continuity of the Negaunee and Vulcan iron 
bearing formations. In 1903 H. L. Smyth traced the Negaunee iron 
formation from the Republic trough southwest around two major 
anticlines to the northeast side of the great oval anticline in the northern 
part of the Crystal Falls district*. The relation of the Negaunee 
formation in this area to a persistent magnetic line may be seen on 
Fig. 1. From the vicinity of Michigamme Mountain, T 44 N, R 31 
W, where the Negaunee formation is exposed, a magnetic line extends 
north through the Sholdice and Doan explorations, where the Negaunee 
is again exposed, and thence north, northwest, west, and southwest 
around the great oval anticline into section 27, T 46 N, R 33 W, where 
it still coincides with the position of the iron formation. A short dis- 
tance beyond the latter locality the line is l^roken but it reappears after 
an interval of about two miles and passes through the Red Rock and 
Hemlock mines at Amasa and beyond connecting with the iron forma- 
tion which has been mapped as Vulcan or Upper Huronian. 

The U. S. Geological Survey in 1911' accepts the conclusion 
that the magnetic line, exposures, explorations, and drift bould- 
ers prove the practical continuity of the Negaunee formation from 
Michigamme Mountain for a distance of 25 miles around the 
great anticline to a point about a mile south of section 27, T 46 N, 
R 33 W, but from a point al)out two mil(>s further on, through the 
Red Rock mine anil southwartl, the iron formation is correlated with 
the Vulcan or Upper Huronian despit(> the facts (1) of similar position 
with reference to the und(Tlying IbMulock volcanics, (2) that the strik(> 
of the magnetic line north of the Kcd Hock iniiu> indicates a continuity 
of the "Vulcan" iron formation there with the Xc^gaunec a little farther 
north, (3) and that there is no evidence whatever Ix-yon.l sonic^ difTcM- 

*8ee Smyth's diHciissioii in Mono. 30. V. S. G. S., pp. 4.V2-5. 
'Mono. !i'2. 



26 CONTRIBUTIONS TO PRE-CAMBRTAN GEOLOGY. 

once in degree of metamorphism to show that the iron formations in 
section 27, T 46 N, R 33 W, and at the Red Rock mine are not one 
and the same. 

The reason why the iron bearing series (Vulcan) of the Crystal 
Falls-Iron River-Florence district was not correlated with the Negaunee ' 
by the U. S. Geological Survey is a simple one. In the Menominee 
range Van Hise and Bayley found that the iron bearing series is 
unconformable^ above a quartzite-dolomite succession similar to the 
Lower Huronian of the ]\Iarquette range and, under the dual classifica- 
tion, therefore Upper Huronian. Since the Negaunee formation had 
been correlated prior to 1904 with the Lower Huronian the question 
arose as to the position of the iron bearing series in the intervening 
districts. It was reasoned that (1) because the iron bearing series of the 
Crystal Falls district was at that time inseparable from the great Upper 
Huronian slate area opening out south and west from the Marquette 
district, and (2) because the Hanbury slate of the iron bearing series 
of the Menominee range seemed to have areal connections with the 
slates of the Florence-Crystal Falls district, therefore the iron bearing 
series in the latter district must be considered Upper Huronian. This 
conclusion was preferred notwithstanding the evidence of continuity 
of the Vulcan and Negaunee formations in the northern part of the 
Crystal Falls district and as we shall see became a source of consider- 
able difficulty in applying the dual classification to the facts of succes- 
sion in the Felch Mountain and Florence districts. 

Recent developments in the Hemlock and ^Michigan mines at Amasa 
fortunately determines conclusively that the Vulcan iron forma- 
tion there which has been correlated as Upper Huronian is in reality 
Negaunee or Middle Huronian. On the thirteenth level of the Hem- 
lock and Michigan mines the folds in the iron formation are truncated 
by a heavy conglomerate and quart zite carrying fragments of the 
Negaunee formation of all sizes up to several feet in diameter, including 
small angular hard jasper fragments, rounded pebbles of chert and ore 
and great boulders of the iron formation. It is reported that this same 
conglomerate was found by drilling in similar relations to the iron 
formation about three miles south of Amasa and in section 36, T 44 N, 
R 33 W, about four miles further southward. There can be no reason- 
able doubt that this conglomerate-quartzite is the Goodrich formation 
of the Marquette range where exactly similar relations are observed. 

The productive iron formation at the Hemlock mine extends with 
only a few unexplored breaks in drift covered country southeastward 
around the great anticUne of Hemlock volcanics and other rocks and 
is believed to be almost if not quite continuous with the iron formation 
passing through the Hollister, Armenia, and other mines in the vicinity 



REVISION OF HURONIAN CORRELATIONS. 27 

of Crystal Falls. Such continuity is indicated, so far as definite informa- 
tion is available, by drilling, underground openings and magnetic 
surveys. Furthermore, the iron formation on the west and southwest 
sides of the great Crystal Falls oval anticline maintains the same posi- 
tion with reference to the underlying Hemlock volcanics that it does 
on the north and east sides. Therefore if the iron formation at the 
Red Rock and Hemlock mines is Negaunee the burden of proof rests 
on those who would assert in the absence of any supporting facts that 
the Upper-Middle Huronian unconformity cuts out the Negaunee iron 
formation and occupies an inferior position with reference to the Vulcan 
iron formation at any or all points southward. As a matter of fact this 
practical continuity was accepted by Clements in 1899 and by Leith 
and Van Hise in 1911 as shown on the maps issued in Monographs 36 
and 52 of the United States Geological Survey. In fact Leith and Van 
Hise argue in this work that the iron formation at the Hemlock mine 
is not Negaunee 07i the assumption of practical continuity with the Vulcan 
formation of the Crystal Falls district to the south and lack of continuity 
with the Negaunee formation a few miles northward. 

CORRELATION OF THE NEGAUNEE (iRON BEARING) SERIES WITH THE 

VULCAN (iron bearing) SERIES OF THE STURGEON TROUGH, 

FELCH MOUNTAIN DISTRICT, CALUMET TROUGH, AND 

MENOMINEE RANGE. 

These ranges lie south of the Marquette district and east of the Iron 
River-Crystal Falls-Florence district and form eastward projecting 
tongues of the Huronian series of this great area. (See fig. 1.) 

1. Sturgeori River Syncline. The Negaunee formation has been 
traced by outcrops, exploration and magnetic survcj-s from the Mar- 
quette Range into the north limb of the Sturgeon River syncline. From 
near Witch Lake, about eight miles south of Republic, the Negaunee 
formation is shown by the mapping of the U. S. Geological Survey to 
rest directly on the Archean. In the Sturgeon syncline, however, the 
Archean is overlain ])y the Randville dolomite and Sturg(>()n quartzite 
equivalent to the Kona and Mesnard of the Lower Huronian in the 
Marquette range. ( )ii the south limb of this syncline the iron formation 
reap))ears above the Randvilh' dolomite and has naturally b(>en cor- 
related with the Negaunee of the north limb. 

2. Fclch Mountain District. The Felch Mountain district is a nar- 
row syncline of Huronian rocks downfolded in the Archean, from on(> 
to two miles Avide. trending east -west and, like the Sturgeon trough, 
opening out westward into the great slat(> area of the Crystal Falls 
district wlier<'in the structure is ol)seure<l. It is sejiarated from the 



28 CONTRIBUTIONS TO PRB-CAMBRIAN GEOLOGY. 

Sturgeon trough north of it by an anticUue on which the Archean 
appears as a belt of granite about 2| to 3 miles wide. In the Felch 
syncline there is an iron formation (Groveland) similar to that in the 
Sturgeon trough, separated from the quartzite-dolomite below by con- 
formably underlying sedimentary schist (Felch schist) which has not 
been observed in the Sturgeon trough although it may be present there 
also. Bay ley makes no mention of the Negaunee formation of the 
Sturgeon trough in 1899* although it was subsequently discovered 
through exploration and is shown on the maps of the U. S. Geological 
Survey published in 1911**. On these maps the iron formation in the 
Sturgeon trough is called Negaunee (Middle Huronian) and, in the 
Felch syncline, Vulcan (Upper Huronian). This seems to be a purely 
arbitrary classification but it fulfils the necessity under the old correl- 
ation of making the jump somewhere from the Negaunee of the Sturgeon 
trough to the Vulcan of the Menominee range. The Felch schists were 
regarded by the U. S. Geological Survey as Upper Huronian because 
they open out and seem to connect with sediments to the west which 
had been correlated as Upper Huronian although the area in which 
the connection is indicated is deeply drift covered and wholly devoid of 
rock exposures. But even if this connection were a fact, as it may well 
be, it appears noiv that it constitutes merely an added reason why the 
Groveland iron formation should be correlated with the Negaunee since 
the Crystal Falls slate-iron formation series has been shown to be more 
probably Middle rather than Upper Huronian. 

As a matter of fact Smyth did correlate the Groveland with the Ne- 
gaunee formation in 1899 but after Seaman's discovery of the uncon- 
formity at the base of the Ajibik quartzite necessitated the correlation 
of the N-gaunee series with the Middle Huronian (1904) Van Hise 
and Leith in 1911 took the Groveland out of the Lower Huronian 
and placed it in the Upper Huronian. In order to make this change it 
was necespary to assign a highly metamorphic quartzite-mica schist 
series which is unconformably above the Groveland to the Keweenawan 
or Paleozoic for the reason that no place was then left for it in the 
Huronian group. Smyth was obviously right in correlating the Grove- 
land with the Negaunee formation. The quartzite-mica schist series 
above the Groveland bears no resemblance to the Keweenawan or 
Paleozoic. It is Huronian and we believe, should be correlated with 
the Copps, part of the Michigamme and the Princeton series. The 
quartzite which is unconformably above the slate-iron formation series 
(Animikie) of the Florence district is similarly correlated. 

3. Calumet Trough. In the Calumet trough, about four miles south 

♦Monograph 36, U. S. Geological Survey. 
♦♦Monograph 52. 



REVISION OF HURONIAN CORRELATIONS. 29 

of the Felch syricline, the situation is practically identical with that in 
the Felch Mountain district, and the same arguments for revision of 
the correlation apply here as in the Felch district. In other words, 
the iron formation now assigned to the Upper Huronian is beheved to 
be really an equivalent to the Negaunee of the Middle Huronian. 
Rocks similar to the quart zite-mica schist series of the Felch sjaicline 
are exposed at the old Hancock exploration in T 41 N, R 27 W, and in 
at least one or two places near the southern edge of the Calumet trough. 
Each of the three Huronian series seems to be represented here exactly 
as in the Felch syncline a few miles north. 

4. Menominee Range. 'The general similarity of the iron bearing 
series of the Menominee district with that in the Sturgeon, Felch, and 
Calumet troughs and in turn with certain phases of the Negaunee 
formation of the Marquette district, its similarity in relation to the 
underlying Lower Huronian and its areal connections with the great 
slate-iron formation series of the Florence-Crystal Falls district de- 
termine that the Vulcan of the Menominee range is probably of Ne- 
gaunee age, that if the iron formation in the districts intermediate 
between the Menominee and the Marquette ranges is Negaunee, there 
is no basis on lithological, structural, or other grounds for assigning 
the iron formation of the Menominee range to any horizon other than 
Negaunee, i. e., Middle Huronian. 

Leith has described a remnant of cherty quartzite, of a maximum 
thickness of 70 feet, lying apparently unconformably between the 
Randville dolomite below and the Traders member of the Upper 
Menominee series above in the vicinity of Norway* but he places no 
emphasis on it so far as concerns its significance in the correlations. 
This formation according to the more recent opinion of Dr. Leith** is 
a remnant of regolith unremovcd by erosion in Lower-Middle Huronian 
time. 

CORRELATION OF THE IRONWOOD SERIES (aNIMIKIE) OF THE GOGEBIC 
RANGE WITH THE VULCAN (MIDDLE HURONIAN) SERIES OF THE 
CRYSTAL FALLS-IRON RIVER-FL0RENCE-MP:N0MINEE DIS- 
TRICT ON THE BASIS OF SIMILAR RELATIONS TO 
INTRUSIVE GRANITE. 

Area southeast of the Gogebic range including Marenisco, Turtle, Man- 
itoivish, Vieux Desert, ConoiHr, Iron Hirer, Cri/stal Falls, and Mcnoiuinee 
Districts. Probably the most striking feature of the Mid(ll(> Huronian 
(Animikie) of these districts is the general prevalence^ of intrusive 
granite. Heretofore these granites have been vMriously conu'lattMl, from 

♦Monograph .12, r, S. Geologirul Survey, pp. 2.34-.'>. 
♦♦As ex|)res8e(l in conversation with the writer. 



30 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

Laurentian in tho Gogebic, northern Wisconsin, and M(>nomineo dis- 
tricts, through the Upper Huronian in the Crystal Falls district to 
Keweenawan in the Florence-Menominee and northeastern Wisconsin 
areas. 

In the Menominee district Bayley found that the granite south of 
the Menominee River intrudes a series of basic volcanics called the 
Quinncsec schist which he correlated erroneouslj^ with the Keewatin. 
Although it was realized that the correlation of the Quinnesec schist 
as Keewatin introduced a conception of structure quite out of accord 
with natural inferences on the basis of the facts, it remained for Corey 
and Bowen, working under the direction of Van Hise and Leith, in 
1905* and Hotchkiss in 1910** to show conclusively that the Quin- 
nesec schist is partly intrusive into but in greater part interVjedded 
with the upper part of the Upper Huronianf, i.e. Animikief. 

Inasmuch as the granite was thus proven to be the youngest rock in 
these districts, and the youngest pre-Cambrian sediments had been 
correlated with the Upper Huronian, Leith and Van Hise in 191 Iff 
correlated the granite with the Keweenawan and extended the boun- 
daries south and east to include several thousand square miles of acid 
intrusives in north central Wisconsin which had been mapped and de- 
scribed by Weidman in 1905ttt- 

It is interesting to note here that Brooks and Wright had correctly 
interpreted these relations as early as 1876, although they did not, at 
that time at least, comprehend the great mass of the intrusive granite. 
To quote from Brooks :"In the summer of 1874 Chas. E. Wright and 
mj^self exploring the country west and south of the Menominee 
River about 90 miles from its mouth, under the auspices of the 
Wisconsin Geological Survey, observed a large granite area, the 
north edge of which was bounded by dark colored hornblendic and 
micaceous schists of Huronian age, which I have since concluded 
are equivalents of the youngest member of that series j^et observed in 

the Marquette iron region The lithologic character of 

this w:ide granite belt bore so much resemblance to the Laurentian 
rocks, which are extensively developed on the waters of the Sturgeon 
River in Michigan, 10 to 20 miles to the northeast, that we were dis- 
posed at the time to believe that some phenomena of folding or faulting 
had brought rocks belonging to that system to the surface in an unex- 
pected quarter. Professor Pumpelly and myself, several years pre- 
viously had observed, farther to the north and west, similar granite 
rocks crossing the Michigamme and Paint rivers (branches of the 

♦Unpublished notes of field work done in 1905 by G. W. Corey and C. F. Bowen. 
**Uripuhli.shed field notes of W. O. Hotchkiss. 
tMiiidlf Huronian of this article. 
ttMono. 52, U. S. G. S. 

tttThe Geology of North Central Wisconsin, by S. Weidman, Bull Wis. Geol. & Nat. Hist. 
Survey No. 16, 1907. 



CORRELATION TABLE. 
(Showing changes In Huronlan corrclution mi llic basis ihat the / 





Mnrquette. 


--■ 


Slurseiin, 


M OUT) lain. 


« j — 


--"- 


Crystal FallB. 


>™,„,ver. 


'''&l?,T' 


Manitowish. 


■r„.,.. 


.„.,.,„. 


Gogebic. 


Cuyuna, 


M..,.,. 


v„,„,„.„. 


Gunflint inimitiB 
Lake 1 Ammilcie. 


',=,„ 


Greenstone in- 

Micliigttmme 
slate pftrtlr 

(iron bearing) 
Goortrirh 
quarlxite 


Michigamme 

Goorlrinh pray- 
conglnnierate 


quarlzite and 
slate. 


Mica schist. 
Ferruginous and 






QuartKite and 
Conglomerale. 


M,c,,,.™™ 


MH,i.„„„e 










Copps 
(Conglomerale, 












"tBis.. 


Negaunee iron 
Ajibik t|iiartz- 


[ormallon. 


,™,o™a„o„ 


Feldi s.-hisl. 


,r .jibury slate. 


Quiiineaec 
Planbury slate. 


Granite. 
Qiiinnesec 

Hanljury slate. 


iiilrusivRs and 
Hanbury slate. 

Vulcan-Negau- 
nee iron tonna- 

Mans^eld'^ 
slate, Hein- 


intrusives and 

formation. 
Slate. 


— • 


Kyanitic bio- 
titlc garnetif- 

Iron forma linn 

(Amphibole- 
magnetite 


Granite- 
Quart/ite 


Presque Isle 

estrusives and 
^Intrusives. 

Grai-wacke and 


Tyler slate. 

Palms forma- 
tion (quartz! 19 


Quari .!.■ 


Embarraas 
granite? 
Acidic' ami basic 

Pokegama 
i|UarlKite. 


Rove slate. 


^~' 


r,::: 


\r:z... 


(|iiartzite. 




Ilun.lviUe <lolo- 
quarlElte. 


Sturgeon 
quart zite. 


■-""«'°- 


Quurtzite 
Sturgeon 






Dolomite and 






Dolomite 
qnartxite. 




Sim.lay quarts- 




Giants Range 
CO ngloni orate. 


below. 
Knife Lake 

Ogu'like con- 
glomerate. 


granite inlm- 1 Iniswe Imo 

Graywacke. 1 

conglomerate. 


^ 


ill 


Granite 


"rn-Si' "'"' 


°^"£""" 


' ,:;i"""' 


Gmnite and 
Green schist. 






greeiiscliisls 
and tufTe. 








Amphibolite 

Granite and 
gneiss. 


Granite and 
gneiss. 

Greenstone, 
greeiischist 
and slate. 




Granite and 
porphyries. 

liorphyries. 


Granite and 
Ely greenstone. 


S^nnT ■ SoTe'* 

Mashed por- Mashed por- 
phyries. 1 phyries. 



REVISION OF HURONIAN CORRELATIONS. 31 

Menominee) presenting similar puzzling relations with beds known to 
be Huronian, and younger as well as lithologieally different from any 
rock then known to be of that period. 

"A careful consideration of all of the facts to be observed in the 

Menominee region confirms me in this hypothesis, "*. The 

conclusion of Brooks is confirmed l)y the work of Hotchkiss in the 
Florence district of Wisconsin in lOlOf. 

In 1911-14 the writer and assistants, through field mapping and dia- 
mond drilling, traced what appears to be this same granite from the 
Iron River district westward through the Animikie series of the Vieux 
Desert-Conover district and Manitowish, Turtle, and Marenisco ranges, 
in all of which it is in intrusive relation with the sediments, and con- 
nected it with the "eastern Laurentian area" of Van Hise and Irving 
on the eastern Gogebic range which was considered by these geologists 
to form a part of the Archean basement complex whereas it actually 
intrudes the Lower and Middle Huronian (Animikie) series and is un- 
conformably overlain only by the Upper Huronian (Copps) series. Thus 
the great granite hatholilh of Northern Wisconsin has been fairly demon- 
strated to be not only in intrusive relations with the Animikie sediments 
over several thousands of square miles but also to be overlain unconformably 
by a pre-Cambrian series which is unconformably below the Keweenawan. 

Inasmuch as the Vulcan (Animikie) series of the Crystal Falls, Iron 
River, Menominee, Florence and other Michigan districts has been 
shown to be very probably equivalent to the Negaunee series of the 
Marquette range and the Ironwood series of the Gogebic range, the 
age of this great granite batholith and its outliers may be considered 
as late Middle Huronian. 

GENERAL REMARKS ON THE CORRELATION OF THE ANIMIKIE SERIES 
WITH THE MIDDLE HURONIAN. 

The correlation of the Animikie series with the Middle Huronian 
eliminates what would otherwise be the necessity of assuming a fourth 
Huronian series of which the Copps formation of the Gogebic range 
would be the sole representative. The practical identity of the Huronian 
succession of the Gogebic and Marquette ranges together with the 
marked similarity of the Copps formation and the Ui>i)er Huronian 
of the Marc|uette and Gwinn districts is an adec^uate basis for moving 
the Animikie of the Michigan and otlui- Huronian districts downward 
into the Middle Huronian, particular!}- as tliis correlation, as W(> have 
se«'n, cliiniiKitcs what would otherwise \)c a further necessity of includ- 
ing the uppermost series of metamorphic sediments in the l'\'lch, Gal- 
lon tht" yoiiiiKfsl Hiiroiiiiiii rocks south of Lake Superior iiiul tin- iiv'c of ilic copiuT bfuriiis 
serins, by 1 M Mrooks Am .lour. Sci. Vol. II. 187(1. pp. :iOl)-7 
triipuhlislifd itiiiiMiscript 



32 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

umet, and Florence districts in the Keweenawan or the Paleozoic where 
they obviously do not belong. 

The anomalous position in the correlations of the great Negaunee 
iron bearing series has been unsatisfactory to a great many students 
of the pre-Cambrian for many years.' We now have a firm basis for 
the correlation of the Negaunee series with the great productive iron 
bearing series of the Animikie of all of the other districts of the Lake 
Superior region and are able to recognize the consideration that the 
unique conditions which resulted in the deposition of the great Huronian 
iron formations were regional rather than local and should be corre- 
lated in time. While it is true that the names now used have come to 
have well understood significance because of long usage, it can not be 
held that this is a valid argument for the retention of a classification 
which no longer correcth^ interprets the facts of present knowledge. 

'See Geological section of Michigan, A. C. Lane and A. E. Seaman, Mich. Geol. Survey, 
Annual Report, 1908, pp. 2.S-30. Lane and Seaman correlate the Animikie iron-bearing series 
of Michigan throughout with the Negaunee iron-bearing series (Middle Huronian) of the Mar- 
quette range. 



CHAPTER III. 

A RFVISION OF THE SEQUENCE AND STRUCTURE OF THE 
""JrSvEENAWAN FORMATIONS OF THE EAST- 
ERN GOGEBIC IRON RANGE. 

R. C. ALLEN AND L. P. BARRETT. 
INTRODUCTORY STATEMENT. 
In thi. chapter there is presented the results of recent studie^of the 
geology of the Gogebic iron range from T 47 N, R 45 W, east to Gogebic 
lake, a distance of 15 miles. ^ , . • 

F;rmer interpretations of the geology of the Gogebic -n range are 
based almost entirely on early investigations o^ the Unitec States 
Geological Survey*. Prior to 1914 four years of study of the pre- 
Crmbrirn of the'region south and east of Gogebic Lake to the Iron 
River district, including 38 to^soiships adjacent m northerii A\isconsm, 
by the senior ^vriter and his assistants had failed to establish a satis- 
factory basis of correlation ^vith the rocks of the Gogebic range (Fig. 1.) 
Inasmuch as recent drilling for iron ore has supplied important informa- 
tion, unavailable to earlier workers in this field, it was though tha a 
field studv of the eastern Gogebic iron range would probably alter 
former interpretations of the geology and furnish a better basis of 
correlation with the pre-Cambrian terranes of adjacent territory and 
the Lake Superior region. The results of our studies are more satis- 
factorv than we had anticipated and we believe are of considerable 
importance to the progress of pre-Cambnan geology. 

We acknowledge our indebtedness to Dr. C. R. )^^^"Hisefc)r placing 
in our hands his early field notes and plats, to Dr. C. K. Leith for help- 
ful suggestions in field conference, to Mr. Robert Selden Rose ot Mar- 
cuette, Messrs. Luther Br(>wer and Cu-orge Rupp of Ironwood. and 
the Presque Isle Mining Company for records of drilling and un.ler- 
ground exploration. 

SUMMARY OKTllK (JKOLOCY B.\SED ON EARLIER WORK. 

V-m Hisc an<l L.itii hav.- nr.ntly i)resented the views of the United 
States Geological Surveyt which may be briefly summan/.vl. 

*MonoKrapl. 1«. Initnl States Oeolo^K-.tl Surv.-y, C H Vun IMse a.ul R. D. Irv.ng. 
tMoiioKriiph r,2. riiitPd Stales (Jc-oIokk ill Mirvf.v 



34 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

The Huronian rests on the Archean with profound unconformity and 
is represented by two unconformable sedimentary series, the Upper and 
the Lower Huronian, which are in approximate structural parallelism and 
dip steeply northward beneath the Keweenawan. 

The Archean comprises a green schist series (Keewatin) intruded by 
granite (Laurentian). 

The Lower Huronian has two members, a basal quartzite (Sunday) 
and an upper cherty dolomite (Bad River). 

The geology of the "Upper Huronian (Animikie) Group of the East- 
ern Area" is summarized in the following words: 

'Tn the eastern part of the district — that is, from about 6 miles east 
of Sunday Lake to Lake Gogebic, the Upper Huronian rocks have an 
exceptional character. In the larger part of the district the conditions 
were those of quiet sedimentation but in the eastern area throughout 
the greater part of the Upper Huronian there was continuous volcanic 
action. In consequence the rocks are lava flows, volcanic tuffs, con- 
glomerates, agglomerates, and slates, with all sorts of gradations, just 
such as one would expect if a volcano rose in a sea and volcanic action 
continued for a great period. Naturally in this area it is not possible 
to map any continuous sedimentary belts. The dominant rocks are 
greenstone conglomerates and lavas and massive eruptives. The upper- 
most formation for the extreme eastern part of the area is ferruginous 
slate. This ferruginous slate, though dominantly clastic, contains nar- 
row bands of non-clastic sediments such as chert, cherty ferro-dolomice, 
ferro-dolomitic chert. It is believed that the ferruginous slate is prob- 
ably at the same horizon as the Ironwood formation to the west and that 
its dominant fragmental character is due to the presence in this area of one 
or more volcanic mountains which arose above the water and upon which 
the waves were at work after the close of the period of active volcanic out- 
breaks.'" j 

SUMMARY OF THE WRITERS' CONCLUSIONS. 

The writers' most important conclusions, insofar as they differ from 
those of the earlier writers as summarized above, ma}' be stated briefly 
as follows: 

1. Huronian. There are three unconformable sedimentary groups in 
the Huronian series. The formations heretofore included in the Upper 
Huronian group are divisible into two groups separated by an uncon- 
formity of the first magnitude. For the superior group we propose 
the name Copps in recognition of the important exposures of its basal 
horizons near the old Copps mine. 

2. Archean. The granite heretofore described as Laurentian not only 

tWriters' italics. 



GEOLOGY OF THE GOGEBIC IRON RANGE. 35 

intrudes the Keewatin hut also cuts across two great unconformities into 
the Animikie group (Middle Huronian). 

3. The unconformity between the Copps formation and the Middle 
Huronian (Animikie) group is one of great angular and erosional magni- 
tude and is of greater importance to this district, measured by the 
usual criteria for evaluation of breaks in pre-Cambrian sedimentary 
successions, than either the unconformity separating the Keweenawan 
and Huronian or the one separating the Animikie and Lower Huronian. 

For convenience of reference and greater clearness in the discussions 
which follow we introduce the successions of the United States Geolog- 
ical Survey and the WTiters' in parallel columns. 



36 



CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 



o 

I? 
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CO '^ 

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h I fB 

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hi 

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^ s'S M 






;do 



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=3 aj • ^ 



C >- r? 



c eg o 



C « ,^ C ;^ CO 

O = C O g £ 



— < 



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^■^>> 

-- ^ 3 









53 



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o3 S 



eg m oj 

°d.S8 



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C cS ' 

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t30 Hi; Ph 









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pa) 



5 aj 



.5£ = 

o a"" 



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tr 3 ■" 
>..2 3 

o3 M 



GEOLOGY OF THE GOGEBIC IRON RANGE. 37 



ARCHEAN SYSTEM. 

The rocks of Archean age include only the fine grained chlorite 
schists in the west half of T 47 N, R 44 W, which are unconformably 
beneath the Sunday quart zite of the Lower Huronian. In the reports 
of the U. S. Geological Survey* the hornblende-mica schists and gneisses 
lying south of the Huronian sediments in T 47 N, R 43 W, (Fig. 2) are 
included in the Keewatin and the extensive area of acid intrusives, to 
which the name Presque Isle granite is here given, is placed in the Lau- 
rentian. It will be sho"v\Ti later that the Presque Isle granite intrudes 
the Middle Huronian (Animikie) and that the schists in T 47 N, R 43 
W, heretofore correlated with the Keewatin, are metamorphic phases 
of the Palms formation. The Presque Isle granite may include Lau- 
rentian rocks but it seems clear that the greater part of it is of late 
Animikie age. Farther west, however, the base of the Huronian series 
rests unconformably upon Laurentian granite. 

ALGONKIAN SYSTEM. 
HURONIAN GROUP. 

Lower Huronian Series. 

The Lower Huronian comprises two formations, viz, the Sunday 
quartzite and the Bad River limestone, exposures of which are con- 
fined to a prominent hill in the south half of sections 17 and 18, T 47 
N, R 44 W. The persistence of the Lower Huronian east of section 17 
is problematical (See map. Fig. 2 A and Fig. 2 B). 

SUNDAY QUARTZITE. 

Lithology. The Sunday quartzite is made up of a basal conglomer- 
ate from a few inches to ten feet thick overlain by about 150 feet of 
quartzite. The conglomerate contains pebbles of vein quartz, green- 
stone, antl granite, fragments of red feldspar, and occasional pebbles 
of gray chert}- siliceous material. The vein quartz pebbles are the 
most abundant. The matrix of the conglomerate is composed mainly 
of partick's of chlorite, feldspar and quartz. 

Tiie Sunday quartzite is a rather coarse grained aggregate of rounded 
grains of cjuartz and feldspar, the latter ver}' subordinate in quantity. 
Near the base it has a greenish tinge on fresh fracture l)ut grades 
rapixlly up into a grayish white or slightly pinkish rock. The formation 
dips north at angles of 30° to 40°. 

Relations to Adjacent Rocks. The Sunday (Quartzite is separated 

♦MoiiOffruplis Nos. 1'.) uiul !i'Z. 



38 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

from the underlying Keewatin greenschists by a great unconformity. 
The actual contact between the two formations (see Fig. 2) is exposed 
for a distance of 50 paces at the base of the prominent bluff near the 
north line of the SWj of SE| of section 18. The green schist at this 
locality has a nearly N-S strike and a vertical dip. It is abruptly over- 
lain by the southeasterly striking and northerly dipping quartzite. It 
is apparent from these relations that the greenstone had been folded, 
metamorphosed, uplifted, eroded and base leveled prior to the deposi- 
tion of the Lower Huronian. 

The Sunday quartzite grades upward into the Bad River limestone. 

BAD RIVER LIMESTONE. 

Lithology. This formation is described by Van Hise and Leith 
as follows:** "The formation is called a hmestone because that is 
the predominant rock. The limestone is heavily magnesian and in 
places approaches a dolomite. It commonly bears silicates of which 
tremolite is the most abundant, but chlorite and sericite are not 
uncommon. The rock is very siliceous. The coarsest varieties are 
quartz but chert is more common. In many places the siUca is 
closely intermingled with the dolomite. In other places it occurs in 
bands varying from a fraction of an inch to much greater width, and 
in one place a band of siliceous material 45 or 50 feet wide was observed. 
Thus the chert and limestone are intermingled and interstratified. The 
cherty limestone is a water deposited sediment. Whether the original 
carbonate was of chemical or organic origin we have no definite evi- 
dence, but there is no more reason to suppose that life was not con- 
cerned in the deposition of this cherty limestone than of those of later 
age. 

"The Bad River limestone has been much metamorphosed since its 
deposition. During its metamorphism the silica recrystalized. It was 
concentrated into bands. It was arranged into veinhke forms. During 
these changes a part of the silica may have been introduced from 
extraneous sources or at least from parts of the formation now removed 
by erosion. The abundant tremolite is evidence that the metamor- 
phism took place under deep seated conditions when the sihca united 
with the calcium and magnesium to form silicates, the carbon dioxide 
being released at the same time. This is an anamorphic change that 
took place with a decrease of volume." 

Middle Huronian (Animikie). 

Introductory statement. The Upper Huronian is represented on the 
east end of the range by the Palms quartzite and the Ironwood iron 

♦♦Monograph 52, U. S. Geological Survey. 



Michigan Geological and 
Biological Survey. 



Publication 18, Geology 15, 
Plate I. 










111.; SIM'.W .tl AKl/.ll'K IN < < •NI'Ai "l' WITH 



T. 47 N.. K. I I W 



GEOLOGY OF THE GOGEBIC IRON RANGE. 39 

bearing formations, volcanic agglomerates, breccias and flo^-s and in- 
trusive sranite (Presque Isle) and greenstone. , , .„ 
The IZs formation is composed mainly of qnartzite, quartz slate 
anl^r^acke; the Ironwood formation is mainly non-clast,c and .s 
comp^sTof ferruginous chert, cherty iron carbonate, iron ore and 
„c3;lphibole-magnetite rocks. Slate becomes mcreasmgly ,m- 
™wt»nt in the Ironwood formation east of Sunday Lake. 
' Coughou heTentral part of the Gogebic range the M.ddle Hu- 
ronian "ncludes a third formation, the Tyler slate, a pehte or mud 
"Z i n ofTmmense thickness. It is cut out east of Sunday Lake by 
eroln and the great Wakefield thrust fault and apparently does not 

appear on the east end of the range. ,,,..„ p_,„„ical Survey 

In the maps accompanying the reports of the U. S. Geological burvey 
the PalmTand Ironwood formations are carried as cartographic umts 
the l-alms am „ ^ ,, vr R 44 W, where, with the exception 

SltdpaTch of ion^irtion which kre correlated with the Iron- 
lodtnerunit representing ferruginous slates is introduced to carry 
Their combined strat graphic horizons to the east extremity of the range. 
AslreTu t of the field work of the Michigan Geological Survey the 
pllms and Ironwood formations have been traced in typical develop- 
m "ar east as section 23, T 47 N R 43 W, where they disappear 
not through lUhologic gradation into ferruginous slate but by ,b,upt 
ZncaLn by a super,ar unconformable ser.es. The major portion of 
the emiginous slate belt of the U. S. Geological Survey is unconform- 
aby above the Palms and Ironwood formations of the Ammikie group 
and has been separated in mapping and named Copps formaticm in 
re ognttion of the important exposures in the vicinity of tlie old Copps 
m ne The essential changes in mapping and interpretation may be 
Teen by a comparison of the map accompanying this report with those 
found 'in the reports of the U. S. Geological Survey . 

Zu^ons to adjacent forn.alrons. The /-wor Huron.an is overUam 
unconformably by the Palms formation of the Middle Huionian The 
^vi en,.e of this relationship does not refer to any one locality but is 
based largely on general fi<-ld relations, and particularly upon a lack 
^rl inuity of the Lower Huronian formations which is bclicve.1 to 
be ,lue to Lsion preceding the deposition «1 the Upper Hu^uai. 
Van Hise and Lcith" sun.inari«. the ..vidence as follows: 1 hi but 
tla wlKTc the belt of conglomerate at the base of the Palms les above 
tl„ Ha,l River limestone it bears much detritus from that limestone 
shows that the limestone aftc-r d.^position became ""'";-:^""';";'' j^";; 
eroded before Palms time. In general, the strikes and dips of the two 



40 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

formations are approximately parallel, as are those of correlated forma- 
tions in the Menominee district, but it is plain that the erosion was 
sufficient to remove the major portion of the Bad River limestone and 
also any later formations that may have been deposited in the Lower 
Huronian. The lack of marked discordance in the bedding of the 
Bad River limestone and the Palms formation is no evidence that the 
time gap between the two was not long enough to have produced a 
pronounced discordance elsewhere, for the Penokec district at this 
time may have been distant from areas of important folding and thrust- 
ing which elsewhere may have occurred." 

PALMS FORMATION. 

Distribution and exposures. The Palms formation is exposed in a few 
outcrops and many test pits south of the iron formation in sections 
17, 18 and 21, T 47 N, R 44 W. East of section 21 for a distance 
of about three and one-half miles there are no exposures. Near 
the center of section 30, T 47 N, R 43 W, and thence northeast to 
the NEj of NWJ of section 23, exposures in outcrops and test pits 
are fairly plentiful. The outcrops and test pits along the N-S quarter- 
line through the south half of section 21 and the north half of section 
28 afford an excellent cross section for studj^, especially of the pro- 
gressive metamorphism of this formation toward the contact with the 
Presque Isle granite. Near the center of the NE J of section 21 the 
upper quartzite horizon is exposed in outcrops and test pits adjacent 
to the iron formation. Recent drilling in sections 21 and 22 has served 
to define more accurately the position of the Palms formation and in 
addition has furnished data without which it would be impossible to 
obtain a clear idea of its structure in that vicinity. 

Lithology. Throughout the Gogebic range the Palms formation 
maintains a characteristic succession of members. 

The base is nearly everywhere marked by a thin conglomerate one 
to three feet thick. " Where the formation is in contact "wdth the Archean 
the pebbles are granite, gneiss, and greenschist, but where the under- 
Ijdng formation is the Bad River limestone the conglomerate also 
include fragments of chert and limestone. In a few localities the con- 
glomerate contains scattered jasper fragments. 

The central part of the formation is a graywacke quartz slate member 
from 350 to 500 feet thick. This member comprises many facies, in- 
cluding fine grained slate, novaculite, graywacke, and quartz slate. In 
the upper horizons thin layers of quartzite become increasingly im- 
portant and mark the change in conditions of sedimentation leading 
to the deposition of the upper quartzite member. 

The upper part of the Palms formation is a massive, light colored 



GEOLOGY OF THE GOGEBIC IRON RANGE. 41 

-vitreous quartzite from 50 ta 150 feet thick, of very uniform texture 
and composition. Near the contact wnth the overhang iron formation 
the quartzite is stained bro\Miish red with hematite. 

From the west hne of T 47 X, R 44 W, east to section 22 the Palms 
formation maintains its general character and merits no special descrip- 
tion. The test pits and outcrops indicate gra3n;v'acke and quartz slate 
overlain by quartzite. Its thickness in this locahty is probably in the 
neighborhood of 500 to 600 feet. 

Metamorphic Phases of the Palms Formation. In T 47 X, R 43 W, 
the Palms formation possesses an exceptional character and merits 
detailed treatment. The upper quartzite is persistent and easily rec- 
ognized but the base of the formation grades irregularh' into micaceous 
and homblendic schists toward the contact -^ith the Presque Isle granite. 

In the south half of section 21 and the north half of section 28 the 
upper quartzite member is exposed in numerous outcrops, especially 
near the center of section 21 immediately south of pits in iron formation. 
The upper horizon is massive, grayish white to pink and even grained 
but near the contact with the Ironwood formation it is dark red. Be- 
neath the massive upper horizon thin beds of slate become increasingly 
important. About 250 paces south of the center of section 21 there 
are abundant exposures consisting of alternate layers of quartzite and 
slate averaging from one to four inches thick. The rock is highly con- 
torted, the axial planes of the minor folds dipping southward. The 
slate bands exhibit well developed schistosity also dipping south. The 
formation as a whole appears to be inclined steeply northward. The 
quartzite bands are microscopically very similar to those from the 
upper member although there is a slight increase in the content of 
feldspathic material. Quartz in stringers and veins is a conspicuous 
feature of the rock in these exposures. 

Southward for a distance of about 500 paces outcrops are lacking, 
but the rocks are well exposed in exi)loration pits. The dumj^s of these 
pits contain purple and gray slate and graywacke. In some of them 
thin seams of quartzite are indicated. The coarser laj^ers are composed 
of grains of quartz, f('lds])ar, and lesser amounts of chert and biotite 
imbedded in a fine grained matrix of smaller fragments of the same 
materials and a large amount of sericite and chlorite. The more slaty 
phases are composed of a fclty mass of chlorite, sericite ami fine grained 
limpid material, probably (luartz and feldspar. A clastic texture is 
indicated only by the presence of widely scattered large rounded grains 
of quartz or fel(lsj)ar. The accessory minerals are ferrite. epidote, 
rutile, and rarely magnetite and tourmaline. (Quartz is the most 
ai)undant mineral but fel(isj)ar is common, including orthoclase, pla- 
gioclase, and microdine. The clastic biotite is more oftcMi visible 



42 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

macroscopically than microscopically. It occurs as flat rounded flakes 
on the cleavage surfaces of the slate to which it imparts a characteristic 
shimmer. In general it may be stated that these rocks do not differ in 
mineral composition or texture from similar exposures of the Palms 
formation in other parts of the range. Their composition indicates 
derivation predominantly from acid granite. The fragmental chert is 
probably derived from the Bad River formation. 

Southward from aljout 250 paces north of the south j post of section 
21 the rocks graduall}' lose their sedimentary textures and become 
hornblendic and micaceous. This change is accompanied by an in- 
creasing abundance of secondary quartz, the occurrence of pegmatite 
veins and sudden change of strike and dip. Near the Presque Isle 
granite the rock becomes a hornblende gneiss or mica schist cut by 
numerous veins of pegmatitic and granitic material. Many of these 
veins are parallel to the gneissosity while others cut across it indifferently. 

Under the microscope the change is first manifest by the appearance 
of secondary biotite, small scattered needles of hornblende, or both, and 
by the increase in importance of the epidote and titanium minerals. 
The finer grained bands are recrystalline but the coarser bands retain 
in part their sedimentary appearance in the rounded outlines of the 
larger grains. A short distance farther south the hornblende becomes 
increasingly more important and is easih^ visible in hand specimen 
w^hile the quartz and feldspar interlock in a crystalline aggregate in 
which all suggestion of sedimentary texture is obliterated. The num- 
ber of secondary minerals increases and finally includes epidote, leu- 
coxine, ferrite, sericite, chlorite, pyrite, tourmaline, apatite and rutile. 

On maps of the U. S. Geological Survey these hornblende and mica 
schists and gneisses are included in the Keewatin. It has been shown, 
however, that there are all possible gradations between them and the 
graywackes and slates readily recognizable as belonging to the Palms. 
On the N-S quarterline at the locality just described exposures are excep- 
tionally numerous and the gradational change from graywacke to horn- 
blende gneiss is unmistakable. This exceptional metamorphism of the 
Palms formation is believed to be the result of intrusion by the Presque 
Isle granite. Further evidence of granitic intrusion wdll be presented 
later. 

Exposures of the Palms formation occur at a number of other locali- 
ties in T 47 N, R 43 W, but now^here else is the section so complete 
nor can the progressive metamorphism of the Palms formation in ap- 
proaching the contact with the Presque Isle granite be so well ob- 
served. The rocks at these localities are similar to those in the typical 
section just described. 

It is not possible to determine accurately the thickness of the Palms 



GEOLOGICAL MAP OF THE EAST END OF THE GOGEBIC IRON RANGE, MICHIGAN 

BY R C ALLEN AND L P BAERETT 

R- 44 W. ,_ , n.,4fl,'ffi', , , _, , , R. 42 W. 



Fig. 2 A. 




m 



GEOLOGY OF THE GOGEBIC IRON RANGE. 43 

formation in T 47 N, R 43 W, on account of intense folding and intru- 
sion. It may be thicker here than elsewhere, but the apparent increase 
in thickness is probably to be accounted for by repetition of beds by 
folding. 

Relations to adjacent formations. The relations between the Palms 
formation and the Bad River limestone of the Lower Huronian have 
already been discussed. 

The Presque Isle granite, formerly considered as Laurentian, in- 
trudes the Palms. A discussion of this relationship will be given in 
connection with a description of the Animikie intrusives. 

The Palms formation is conformably overlain bj^ the Ironwood 
formation. In places the base of the latter is marked by a thin con- 
glomerate which, however, is not beheved to mark a break in sedi- 
mentation of any great significance. The change from fragment al 
sedimentation of Palms time to the non-fragmental deposition of Iron- 
wood time was relatively sudden although there appears" to be some 
evidence of the approach of this change in the cherty matrix of portions 
of the upper horizons of the Palms quartzite. 

IRONWOOD FORMATION. 

Distribution and exposures. In the latest maps of the U. S. Geolog- 
ical Survey* the Ironwood formation is carried as a continuous for- 
mation as far east as the northwest corner of section 22, T 47 N, R 
44 W. East of this locality small patches of iron formation are indica- 
ted in the NWj of section 15, near the W^ post of section 14, near the 
Wi post of section 23, and in the SWi of section 25, T 47 N, R 44 W. 
In T 47 N, R 43 W, iron formation is noted in the east portion of sec- 
tion 30, S half of section 20, NE^ of section 21, and in the NE^ of 
section 22. 

The recent work of the Michigan Geological Survey warrants a 
number of important changes in the mapping of this formation. On 
the accompanying map (fig. 2) we have shown the areas kno\vii to be 
underlain by iron formation and have intlicated probabilities of exten- 
sion in territory devoid of conclusive evidence. 

With regard to the patches of iron formation located on the U. S. 
Gcologicai Survey maps in sections 14 and 15, T 47 N, R 44 W, it may 
be stated that the evidence in section 15 consists of a grou]) of i)its 
ledged in black slate. The dumps of these pits contain also a small 
amount of magnetic slate and fcrniginous chert. The exact age of 
these rocks is problematical hut ou tlic basis of their jiosition and Hth- 
ologic cliaracter it is proi)al)li' that they belong to the Go])i)s (post- 
Animikie) formation rather than to Ironwooti. We were unable to 

^Monograph 52, U. S. Geologiral Survey. 



44 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

find any field ovidonco of the existence of iron formation in the east 
half of section 14, and careful search in the original field books of the 
U. S. Geological Survey failed to find any evidence of the existence 
of pits or outcrops of iron formation or associated rocks. It is there- 
fore probable that the patch of iron formation in this section is to be 
attributed to an error in printing the map. 

The Ironwood formation does not appear east of the NEj of section 
23, T 47 N, R 43 W, where it was truncated by erosion preceding the 
deposition of the Copps formation. 

Lithology. The Ironwood formation carries the iron ore bodies of 
the Gogebic range. In the central and western parts of the district it 
is made up of the following main varieties of rocks: (1) ferruginous 
chert, (2) ferruginous slate, (3) slaty and commonly' cherty iron car- 
bonate, (4) actinolitic and magnetitic schists, (5) black and gray slate, 
and (6) iron ore. Concerning the distribution of the above rocks in 
the formation Van Hise and Leith* summarize as follows: "The iron 
bearing carbonates are usually found only near the upper part of the 
formation where they have been protected by the Tyler slate. The 
ferruginous slates and ferruginous cherts are characteristic of the cen- 
tral iron-producing part of the district, and the actinolitic and magne- 
titic slates are characteristic of the eastern and western parts of the 
district. The latter also form a belt 20 to 300 feet wide bordering the 
Keweenawan rocks on the north. In the intermediate areas there are 
of course gradations between the ferruginous , slates and ferruginous 
cherts and the actinolitic and magnetitic slates, as there are also grada- 
tions between the cherty iron carbonates and the ferruginous slates and 
ferruginous cherts. Black slates form thin intercalated layers in the iron 
bearing formations. Quartzite is also found in layers up to 100 feet 
thick well up from the base of the formation near Sunday Lake." 

Interhedded slate and graywacke. With the exception of the in- 
creasing importance of interbedded clastic sediments the Ironwood 
formation east of T 47 N, R 45 W, maintains its general lithologic 
characteristics. In sections 16, 17, 18, and 21, T 47 N, R 44 W, it is 
split 200 feet up from the base by a great gray^'acke-slate member 
500 feet thick. This slate is probably continuous with the so-called 
"secondary slate foot wall" of the Brotherton, Sunday Lake, and 
Castile mines at Wakefield. The Meteor shaft in section 11 is sunk in 
this slate and recent diamond drilling in section 12 has encountered 
the same rock. At the Brotherton and Sunday Lake mines the thick- 
ness of this member varies from 40 to 140 feet. At the Castile and 
Meteor locations and the exploration in section 12, T 47 N, R 45 W, 
its thickness is unknown. In sections 16 and 17, T 47 N, R 41 W, the 



♦Monograph .52, U. S. Geological Survey, page 231. 



MAP (^1^ THE EAST END OF THE GOGEBIC IRON RANGE, MICHIGAN. 
SHO^^I]SfG 01 rCROPS, drill holes, explorations etc. by R. C. ALLEN AND L. P. BAKRETT. 

K. 41 W. , R. 43 W. 



Fig. 2 B. 




C 



GEOLOGY OF THE GOGEBIC IRON RANGE. 45 

slate has been cross sectioned in two places bj- drilling and the thick- 
ness is in the neighborhood of 500 feet. This member of the Iron wood 
formation is not exposed at the surface. 

The extensive explorations made by the Presque Isle Mining Co. has 
disclosed the character of the Ironwood formation in sections 20, 21, 
and 22, T 47 N, R 43 W. The formation in this to^oiship contains, 
especially in the upper horizon, lenses and layers of slate, some of whcih 
are of considerable thickness. Although the problem of the correlation 
of these members is complicated by folding it is believed that the slates 
are not continuous. It seems more probable from the available data 
that they are lenses and are not continuous over any great distance. 
The slates are not exposed and consequently knowledge of their char- 
acter is based almost entirely on drill cores. For the most part they 
are fine grained, gray, green or red and commonly soft but occasionally 
harder and more quartzose phases are encountered. A noticeable fea- 
lu^ '■he almost total absence of black slate. No attempt has been 
made to differentiate on the accompanying map the slate in T 47 N, 
R 43 W, from the nonclastic portion of the Ironwood formation. 

Lithology of the Ironwood formation in T 47 N, R 44 W. The total 
thickness of the Ironwood formation in sections 16, 17, 18, and 21, T 
47 N, R 44 W, is in the neighborhood of 1400 or 1500 feet. This 
great thickness is due largely to the presence of the 500 feet of gray- 
wacke-slate above described. For the purposes of this description 
the iron bearing rocks below the graywacke-slate will be referred to 
as the lower member and the iron bearing rocks above the slate as the 
up^. 1 member. 

Exposures of the lower member are not plentiful, being limited to 
three test pits in section 17 and a few outcrops on a low north facing 
ridge in section 21. All of the exposures are in the horizon immediately 
above the foot wall quartzite. The rocks on the dump of the pits in 
section 17 are soft, rich, ferruginous chert and the peculiar semi-detrital 
material characteristic of the basal horizon of the Ironwood. This 
detrital rock consists of thin layers in which red and white rounded 
grains of quartz are embedded in a chert matrix. The detrital grains 
average about one-sixtieth of an inch in diameter as a rule but occa- 
sional pcbl)les of quartz up to one-half inch in diameter are present. 
Detrital material is in general subordinate to the nonclastic chert 
matrix. It may be added that this peculiar basal horizon is character- 
istic of the Ironwood formation throughout the range. 

In section 21 the contact between th(> Ironwood and Talins t'onna- 
tions is marked by a narrow, strongly magnetic field in which the 
exj)osures are actinolite-magnetit(> sciiists. It is evident from tiie 
narrowness of the magnetic field that tiiese rocks are confined to a thin 



46 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

horizon lying directly upon the Palms. According to Van Hise and 
Irving* these rocks are identical with the actinolite-magnetite schists 
at Penokee Gap. The explanation of the occurrence of these schists in 
this locality is in doubt but it is probable that they have been formed 
through local shearing at the contact between the Palms and Tronwood 
formations. 

The thickness of the lower member is probablv in the neighborhood 
of 200 to 300 feet. 

The upper iron bearing member is made up of ferruginous chert,, 
hard jaspilite and amphibole-magnetite slate. An average of 150 
analyses of drill cores shows an iron content of 33.48 per cent. Ferru- 
ginous cherts are characteristic of the lower-middle horizon. The 
jasper and amphibole-magnetite slates are characteristic of the upper 
horizon in contact with intrusive gabbro and associated greenstone 
sills. Exposures and test pits in the upper horizon are plentiful along 
the south slope of the high ridge marking the position of the main 
gabbro mass. The amphibole-magnetite slates at this horizon are 
characterized by strong to violent magnetism. 

The Ironwood formation is in contact with the gabbro on the north 
for about four miles from the SAVJ of the SEJ of section 1, T 47 N, 
R 45 W, east to the SWf of the SWi of section 15, T 47 N, R 44 W. 
From this locality southeast to the great fault in section 25 the over- 
Ijdng rocks are effusive greenstone, agglomerates, and associated basic 
lavas. It is apparent from magnetic observations that the upper 
horizon of the Ironwood along the contact with the greenstone effusives 
carries considerable magnetite although the magnetism is not nearly 
so strong as that marking the contact of the Ironwood formation and 
the gabbro. Only two exposures of the Ironwood formation are known 
from the NE corner of section 22. SE to the NWi of section 25, but it 
is presumed to extend southwest from section 21 to section 25 mainly 
on the basis of magnetic attraction. There is a group of test pits in 
hard jasper and amphibole-magnetite chert near the Wj post of section 
23, and a small outcrop of chert}^ iron carbonate in the southeastern 
part of the NWi of the NW^ of section 25. 

The Ironwood formation in T 47 N, R 43 W. For the most part 
the Ironwood formation in this township is made up of ferruginous 
chert and slate and interbedded lenses of soft gray and green sericitic 
and chloritic slate. The exposures in section 30, T 47 N, R 43 W, and 
near the Sj post in section 25, T 47 N, R 44 W, are amphibole-magne- 
tite chert and schist. 

In sections 20, 21, and 22 the iron bearing rocks are ferruginous chert 
with a subordinate amount of ferruginous slate. An average of 130 

♦Monograph 19, U. S. Geological Survey, page 243. 



GEOLOGY OF THE GOGEBIC IRON RANGE. 47 

analyses from the drill holes in section 20 gives an iron content of 
36.87 per cent. This figure is not representative of the formation as 
a whole as only the richer beds were analyzed. 

A noticeable feature of the iron formation in T 47 N, R 43 W, is an 
abundance of white quartz veins, some of which are large. The unusual 
number of these veins is in sharp contrast to their general absence in 
the mines to the west. They were encountered with great frequency 
in the diamond drilling in adjacent territory and in the Presque Isle 
mine have rendered worthless in some instances concentrations of 
hematite that would otherwise be valuable iron ore. It is, therefore, 
apparent that these quartz veins must be taken into consideration in 
connection with the ore bearing possibihties of the iron formation in 
this territory. 

On account of the folding and post-Animikie erosion in T 47 N, R 

43 W, no accurate estimate of the thickness of the Ironwood formation 
is possible. There is no reason to believe, however, that, as originally 
deposited, it is thinner here than elsewhere on the Gogebic range. 

Relations of the Ironwood formation to adjacent formations. The 
Ironwood formation rests conformably upon the Palms quartzite. It 
is possible that the detrital horizon at its base represents a break in 
sedimentation. On the other hand it is not inconceivable that this 
horizon represents merely an abrupt change in conditions from clastic 
to nonclastic sedimentation. In restricted localities this horizon is 
conglomeratic. In a test shaft dump near the center of section 21, 
T 47 N, R 43 W, quartz pebbles up to an inch in diameter may be 
observed. 

The greenstone volcanics in sections 15, 22, 23, and 25, T 47 N, R 

44 W, are believed to lie conformably above the Ironwood formation. 
No contacts are exposed but the field relations are not indicative of 
any structural break. The exact relationship of the greenstone ag- 
glomerates to the iron formation in sections 20 and 21, T 47 N, R 43 
W, is \mknown. It is probable, however, that they occupy the same 
stratigraphic position as the greenstones adjacent on the west. 

The Copps formation rests unconformal)l>' ujxju the Ironwood for- 
mation. 

MIDDLE HURONIAN (aNIMIKIe) EXTRUSIVES. 

Greenstone Agglomerates and Associated Rocks. Effusive greenstones 
are found in three localities on the east end of the range, (1) in an area 
of about 4 s(i. miles in the vicinity of sections 23 and 24, T 47 N, K 
44 W, (2) in a narrow strip of tcnitoiv extending east from the (•('nt(M- 
of section 20, T 47 X, l{ 13 W, to in the vicinity of the Ey post and. 
(3) in a small area in the NE', of tli(> Xl*:i of section 22, and the NW, 
of tiie NWl of section 23, T 47 N, H 13 W. 



48 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

For a detailed treatment of the lithology of the effusives in these 
areas the reader is referred to section 3, chapter 8, Monograph 19, 
U. S. Geological Survey. The description to which we have referred 
is based on careful microscopic and field study. The effusives are mainly 
basic volcanic agglomerates and breccias with associated amygdaloidal 
lavas, tuffs, and some slate and conglomerate. The main area of the 
greenstone includes, near the base, coarser grained rocks largely of the 
character of diabase and diorite. These may be intrusive equivalents 
of some of the flows or they may be later intrusives with no genetic 
relation to the greenstone effusives. Certain small diabase dikes are 
without doubt later intrusives, probably Keweenawan. Amygdaloidal 
lavas are characteristic of the upper horizon of the greenstone imme- 
diately underlying the Copps formation, although an occasional bed is 
found near the center of the greenstone area. 

The effusives in section 20, T 47 N, R 43 W, are agglomeratic in all 
of the exposures. In sections 22 and 23 the greenstone includes both 
agglomeratic and fine grained basic flows. 

The greenstones at locality (1) are apparently superimposed upon 
the Ironwood formation, perhaps in part interbedded in the upper 
horizons. At locality (2) the relationship, as previously stated, is not 
clear. If the greenstone occupies the same stratigraphic position with 
reference to the Ironwood formation as it appears to occupy farther 
west, it is probable that they are in the center of a syncline overturned 
to the south, the north limb of which has been truncated by post- 
Animikie erosion. With regard to the position of the greenstone in 
locality (3) nothing definite is known, but the general field relations 
indicate that these rocks are interbedded with the lower horizons of 
the Ironwood fromation. 

The Copps series is believed to rest unconformably on the greenstone 
agglomerates. The inference is based on general field relations and 
the presence of pebbles of the greenstone in the Copps conglomerate. 
No actual contacts are exposed. Wherever the Copps formation is in. 
juxtaposition to the volcanic greenstone it invariably dips away from 
it. Volcanic rocks have not been found in the Copps formation. 

BASIC INTRUSIVES. 

Basic intrusives occur in two main areas, viz, (1) massive diabase 
in sections 22, 26, and 27, T 47 N, R 44 W, and (2) fine grained 
greenstone in sections 20, 29, and 30, T 47 N, R 43 W. Aside from 
these two localities there are many small dikes of diabase which in- 
trude the entire Huronian succession. 

We have not examined these greenstones microscopically and shall 



GEOLOGY OF THE GOGEBIC IRON RANGE. 49 

therefore rely wholly upon the petrographic studies of Van Hise and 
Irving to which the reader is referred for a detailed discussion*. 

Diabase in Sections 22, 26 and 27, T 1^1 N, R U W. Exposures in 
this locality occupy two high ridges trending north and south. The 
more prominent one extends from the center of section 27 to about 
one-quarter of a mile north of the Sj post of section 22; the other is 
located in the north central part of section 26. Both hills are char- 
acterized by a steep to precipitous west face and a more gentle east- 
ern slope. A noticeable major jointing strikes approximately N-S and 
dips east. The greater number of exposures exhibited are typical di- 
abase but in places they grade into coarse textured gabbro. 

In the west side of the hill in section 26 a narrow belt of banded 
amphilwle-magnetite chert 50 feet thick is exposed. It strikes N 7° 
W and dips 55° E. The iron formation is underlain by diabase and 
overlain by fine grained massive greenstone grading upward into diabase 
but neither the upper nor the lower contact is exposed. 

A mile west of this locality a few feet of magnetic slate striking N 
20° W and dipping 32° NE is exposed at the base of a prominent west- 
ward facing bluff of diabase. The relations indicate clearly that the 
overlying diabase is an intrusive sill. For the most part the contact 
between the slate and diabase is parallel to the banding of the formation 
but at one point a dike of diabase 4 feet wide cuts directly across the 
bedding. Small fragments of slate up to one foot in diameter are 
imbedded in the diabase close to the contact. Faulting normal to the 
l)lane of the contact has effected small displacements in both rocks. 
The thickness of the slate is unknown. It is noticeable that the most 
i:)rominent set of joints in the diabase is parallel in strike and dip to 
the })anding in the slates in section 27 and the amphil^ole-magnetite 
chert in section 26. It seems not unlikely in view of the above facts 
and the general topographic expression of the hills that the diabase is 
in the form of sills of great thickness intruded in the Huronian series. 

In describing this greenstone Van Hise correlated the fine grained 
amygdaloidal greenstones in the uoithern half of section 15 with the 
diabase to the south, but we would favor including the amygdaloidal 
greenstones in the overlying grei^nstone effusiv(>s. Their ]iosition seems 
to warrant direct correlation with the upper horizon of amygdaloidal 
lava. 

Crernstone in Sections 20, 29, SO, T J,7 N , R 4S W. (Ireenstone is 
exposed in a high ridge south of the iron formation in the southeast 
half <<f section 20, in a liill in the northwest part of s(>ction 20, and in a 
low ridge extending ahout east and west througii the center of tiie 

*M( Digraph 19. U. S (!<olo;iriil S irvey. 



50 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

NWi of section 30. Van Hise* descril)es these rocks as follows: "Petro- 
graphically this rock has all the characteristics of a surface flow. It 
is a much altered augite-porphyrite. It varies a good deal in coarseness 
of grain and degree of alteration, but the original minerals are found 
with sufficient frequency and the secondary minerals are so uniformly 
the same that there is every reason to believe that detached exposures 
are parts of a continuous belt. The original minerals are menaccanite, 
plagioclase, and augite, this being the order of crystallization. The 
secondary ones are kaolin, chlorite, epidote, leucoxine, and smaragdite." 
Van Hise also noted a mineralogical similarity between the augite 
porphyrite and the diabase in sections 26 and 27, T 47 N, R 44 W. "In 
original minerals contained in secondary products the augite-porphyrites 
are almost precisely like the diabases of the first division. The simi- 
larity is so remarkable as to make it probable that the rocks of the two 
divisions are, or once were, connected. The likeness is further rein- 
forced by the correspondence of the double angles of the feldspar as 
measured by Pumpelly's method." Macroscopically they differ. The 
augite-porphyrites are usually fine grained while the diabase is char- 
acterized by coarse cr3^stallization. 

Van Hise regards the augite-porphyrites as probably interbedded 
surface flows. The workings of the Presque Isle mine together with 
the diamond drilling show conclusively that the greenstone is intrusive 
in the iron formation at that locality. The iron formation dips south 
and the greenstone cuts directly across the bedding. The plane of 
contact between the greenstone and iron formation is nearly vertical. 

Rocks very similar in appearance to those just described occur on 
a small knoll just west of the group of old test pits and trenches in 
the S| of the NEJ of section 21. The relations of this greenstone with 
adjacent iron formation and quartzite are unknoAvn but are not pre- 
sumed to be different from the relations exhibited at the Presque 
Isle mine. 

PRESQUE ISLE GRANITE. 

Location and Extent. The Presque Isle granite is so named because 
of its typical exposures along the Presque Isle River in the -south 
half of T 47 N, R 43 W. It includes the rocks described by Van Hise 
and Irving as constituting the "Eastern Laurentian Granite area."t It 
is probable that the Presque Isle granite is to be correlated directly 
with the great granitic intrusives that cover a large area southward in 
Michigan and Northern Wisconsin, possibly including the granite of 
the Florence and Menominee districts heretofore doubtfully correlated 



*Monof,'raph 19, U. S. Geological Survey, pp. 414-1.''). 
tMonograph 19, U. S. Geologipal Survey. 



GEOLOGY OF THE GOGEBIC IRON RANGE. . 51 

with the Keweenawan. (See discussion of the Marenisco, Turtle, and 
Manitowish ranges and the Vieux Desert-Conover district). 

Lithology. The term granite is here used in a loose sense to cover 
rocks that are predominant h' acid granites, but which include here 
and there rocks of more basic types not excluding sj'enite and diorite. 
The basic varieties are possibly differentiation products of a normally 
acid magma although their predominance near the contact with the 
Huronian sediments ma}' perhaps be accounted for on the theory- that 
the marginal tendency towards basicity is an endomorphic effect of 
the intrusion of granite into the Animikie series. 

Van Hise summarizes the description of the Eastern granite as fol- 
lows: "In all essential respects the massive rocks here contained are 
Uke those found in the granite areas to the westward. The phases here 
included run from tj^pical syenites tc typical quartzose granites. Usu- 
ally in them, as in the previous areas, an alkaline feldspar, occurring 
largely in idiomorphic forms, is the chief constituent. In a few of the 
exposures quartz is as abundant as the feldspar. In some cases in this 
area the decomposition of the feldspar has gone far. Aside from the 
quartz and feldspar, hornblende and chlorite, as in the Central granite, 
are the only important minerals, and very frequently the chlorite, as 
in it, has resulted from the alteration of the hornblende or the decom- 
position of the feldspar. The typical syenite exposures are more com- 
mon than in the other two granite areas." The Presque Isle granite 
is mainly massive but marked gneissosity is not an uncommon feature. 
In general the rocks are light gray to white in color and acid in compo- 
sition. They are very uneven grained and cut by innumerable peg- 
matite dikes. Along the road leading from Marenisco to the Presque 
Isle mine predominant white varieties give the rock the appearance of 
massive white quartz or dolomite when seen from a distance. In T 
47 N, R 42 W, there are limited areas of red granite. 

Relation of Presque Isle Granite to Middle Huronian [Animikie) For- 
mations. The inference that the Presque Isle granite is intrusive into 
the Animikie formations rests on the following evidence: 

1. The exposures of the Palms formation show a gradual transition, 
in i)roportion as they approach the granite, to micaceous and horn- 
blendic schists and gneisses with accompanying obliteration of clastic 
textures. 

2. The I^iinis and Ironwood f()rniatii)ns are characterized by al)und- 
ant quartz veins and the Palms formation by veins of aplite and i)eg- 
matite which become increasingly prominent toward the granit(v 

3. The Ironwood formation becomes amj)hil)olitic, magnctitic and 
schistose in section 25. T 47 N, R 44 W and section 30, T 47 N, K 43 



52 * CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

W, where closely approached by diorite and hornblende syenite which 
is correlated with the Presque Isle intriisives. 

4. At this latter locality the Palms formation is absent and the 
amphibole-magnetite slate of the Ironwood is separated by only a short 
distance from exposures of diorite and hornblende syenite believed to 
belong to the Presque Isle granite. 

The above remarks may be illustrated by reference to certain locali- 
ties. The exposures showing the metamorphism of the Palms to best 
advantage are in sections 21 and 28, T 47 N, R 43 W, earher described. 
In no other locality are exposures sufficiently abundant to permit 
observation of the gradual change from graywacke and slate to crystal- 
line schists. Mention will be made here of a few other localities where 
the same phenomenon may be less satisfactorily observed. 

Exposures of slightly altered phases of the Palms are found near the 
center, and at the Ei post of section 29, and at the bridge where the 
township road crosses the Presque Isle River in section 28. The clastic 
texture is usually present in some of the coarser grained bands, but in 
the finer grained layers no sedimentary textures remain. In the ex- 
posures in section 29 small needles and fibres of green hornblende 
are visible under the microscope although not usually determinable in 
hand specimen. The outcrop near the east J post of section 29 is cut 
by quartz veins, some of which are feldspathic. A short distance south 
the rocks are fine grained hornblende-mica gneisses cut by bands and 
dikes of granite and pegmatite. From this point south to the granite 
exposures are numerous; the hornblende gneisses become gradually 
coarser grained and the granitic material increases in amount until 
finally the rock is contorted and banded gneiss made up of alternate 
layers of granite and hornblende schist, cut indifferently by dikes of 
aplite and pegmatite. Nowhere is it possible to draw a sharp boundary 
between the granite and the Palms formation. The base of the latter 
is doubtless involved in the former and can only be represented on the 
map by a transitional zone. 

The unusual abundance of quartz veins in the Ironwood formation 
in T 47 N, R 43 W, has already been referred to and merely need be 
recalled at this point. Although no trace of feldspathic material was 
noted in the quartz veins in the Presque Isle mine the presence of these 
veins in unusual numbers in this particular part of the range is signifi- 
cant. If correctly interpreted as originating from hot magmatic waters 
derived from the Presque Isle intrusive, the quartz veins are the only 
direct metamorphic effect of this granite on the iron formation in 
sections 20, 21 and 22. The explanation of the absence of marked 
metamorphism of the iron formation here probably hes in the com- 



GEOLOGY OF THE GOGEBIC IRON RANGE. 53 

petencj' of the massive upper quartzite member of the Palms through 
which the granite magma was not able to penetrate. 

Farther west in the SEJ of the NW| of section 30, and in the SWi of 
the SWj of section 25, there are exposures of amphiljole-magnetite 
slate and chert. In section 25 the iron formation is overlain by green- 
stone agglomerate and is onh^ about 50 feet north of exposures of diorite 
and hornblende syenite. The total thickness of iron formation cannot 
be more than 150 to 200 feet. The outcrops of amphibole-magnetite 
schist and chert in section 30 lie south of a low ridge of augite-porphy- 
rite. They are larger and more extensive than the outcrops in section 
25. Four hundred paces south of the iron formation, diorite similar 
to the exposure in the south part of section 23 is encountered. An 
intervening test pit is ledged in fine grained hornblende schist similar 
macroscopically and microscopically to metamorphic phases of the 
Palms. 

It should be stated that there is no absolute proof that the iron 
formation in section 30 is continuous as a part of the main belt in sec- 
tion 20, although such connection is plainly indicated. The apparent dis- 
continuity is the result of folding and displacement which will l)e dis- 
cussed later. 

The same statement apphes to the outcrops in section 25, and in fact 
it seems possible from consideration of the magnetic observations in 
the intervening territory that igneous intrusives maj- have eaten their 
way up through the entire iron bearing series between the exposures 
in section 25 and those in section 30. 

There can be little doubt that the diorite and hornblende syenite are 
the rocks responsible for the metamorphism of the iron formation at 
both localities. As to whether the}' should be correlated with the 
Presque Isle intrusives is not so clear. jNIacroscopicallj- the diroite 
and syenite differ from the other basic rocks in the east end of the range 
in being lighter colored and in many places banded or gneissose. The 
exposures are uneven grained and vary rapidly in composition, be- 
coming in places gradationally more acid and including phases oi 
hornblende syenite. Some of the hornblende syenite is porphvritic in 
texture; phenocrN'sts of light pinkish antl gray orthoclase up to oiu'- 
lialf inch in diameter are imbedded in a fine grained matrix of horn- 
blende and feldspar. The more l)asic varieties are commonly liadly 
altered and when examined microscopically are found to be made up 
of ragged green hornl)lende and a badl}^ altered basic feldsi)ar. The 
alteration products are gray saussurite, epidote, zoisite, anil sericite. 
Leucoxine and i)yrite are found in small (luantities. A noticealile fea- 
ture is the general absence of magnetite which is found in all tlic other 
basic rocks in sufTicicnt (|uantity to affect th(> dip needle. 



54 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

It is not possible to establish a direct areal connection of these basic 
rocks with the main mass of granite to the south but in view of their 
similarity in structural features and sudden compositional and textural 
changes it is regarded as likely that these rocks are a part of the main 
intrusive granite. The basic types may be well interpreted as a pro- 
duct of differentiation or if, as seems possible, the Presque Isle intru- 
sives have in this vicinity eaten their way up through the underlying 
Palms and in section 25 through the greater portion of the Ironwood, 
the basicity of the adjacent rocks may be a direct result of the absorp- 
tion and assimilation of the Animiki^ sediments. 

Relation of the Presque Isle Granite to the Copps Formation. The 
Copps formation rests unconformably upon the Presque Isle granite. 
The contact is marked by a great conglomerate carrying boulders 
and pebbles of the granite. 

Upper Huronian Series. 

COPPS FORMATION. 

Location. The Copps formation is the name proposed for the rocks 
which occupy most of the area which is mapped and described in Mon- 
ograph 19, U. S. Geological Survey as "Ferruginous Slates of the 
Eastern Area." 

The most westerly of the exposures which can be correlated with 
the Copps formation with certainty are those of ferruginous graywacke 
in section 14, T 47 N, R 44 W. A mile and a half farther west in the 
SWj of the NWi of section 15, there is a small group of test pits ledged 
in black slate and graywacke. The dumps of these pits include also a 
small amount of ferruginous chert and jasper. If these rocks are refer- 
able to the Copps, and this seems not unlikely in view of their position 
and lithologic character, this formation extends as far east as section 
18, T 47 N, R 44 W, where it occupies the valley lying between the 
Keweenawan lavas on the north and the high gabbro ridge on the 
south. This valley gradually narrows toward the west and disappears 
in section 6, T 47 N, R 45 W, where the gabbro is apparently in direct 
contact with the Keweenawan amygdaloidal trap. The only evidence 
of the character of the rocks occupying this depression is afforded by 
the pits in section 15 and the occurrence of a strong magnetic belt at 
the north base of the gabbro hill in section 8, T 47 N, R 44 W. 

The east limit of the Copps formation is in section 28. T 47 N, R 42 
W, where it is unconformably overlapped by flat lying red sandstones 
of Cambrian (?) age. 

Lithology. The Copps formation is essentially graywacke slate, 
highly ferruginous in its western and central parts but becoming de- 



GEOLOGY OF THE GOGEBIC IRON RANGE. 55 

creasingly ferriferous eastward from section 22, T 47 N, R 43 W^ Bands 
of ehert and jasper are characteristic of the lower horizons. The base 
of the series wherever exposed is marked by a well defined conglomerae 
The iron bearing minerals form a cement in which the fragmental 
grains are imbedded. In the westerly exposures iron oxide, largely 
hematite but including considerable magnetite, is predominant. In 
sections 16 17 and 20 the oxide and carbonate of iron are both present 
and east of section 16 the iron mineral is largely siderite. Acc^ording 
to Van Hise* the relations l^etween the iron oxide and iron carbonate 
indicate that the former is derived from the latter. Microscopic study 
reveals the presence of iron carbonate in all stages of alteration to iron 

""""The lower horizon of the Copps is a well defined basal conglomerate. 
From section 23, T 47 N, R 43 W, east to the extremity of the range 
the Copps conglomerate is in contact with the Presque Isle granite. 
It is here composed of granite boulders and pebbles, ranging up to four 
or five feet in diameter, imbedded in a matrix of arkose which includes 
also a considerable quantity of iron carbonate. In addition to the 
granite boulders are a few pebbles of greenstone, quartz, chert, and 
more rarely quartzite and jasper. Exposures of the granite cohglom- 
erate are found in section 28, T 47 N, R 42 W, and in sections 23 and 
24, T 47 N, R 43 W. 

North of the old Copps mine near the southeast corner of section 
15 T 47 N R 43 W, the Copps formation truncates the Ammikie 
formation. Here the granite pebbles have entirely disappeared giving 
place to small fragments of chert, quartz, and occasionally quartzite 
imbedded in a matrix consisting of smaller grains of quartz, chert, 
feldspar, and a large amount of iron carbonate. 

The conglomerate horizon was cut by a drill hole in the northeast 
part of section 21 and is also exposed in test pits a short distance north 
of the center of this section. In these locahties the conglomemte con- 
tains pebbl(>s of ciuartz, jasper, quartzite, and chert imbedded m a 
graywacke matrix which includes a small amount of iron oxide. 

West of section 21 conglomerates are found in test pits in section 19, 
T 47 N R 43 W. in section 14, T 47 N, R 44 W, and in fact, according 
to J m' Longvear, Jr., "all along the north and east edges of the green- 
stone area" in sections 13, 14, 19 and 24, T 47 N, R 44 W; 

It is not possible to work out in any detail from available data a 
,l,.finit(. succession of dissimilar meinl)ers r<.r the Copps formation but 
,h,. following probably expresses in a very general way the succession 
in the t(Tritory from section 13 to section 17, T 47 N, U 43 W . I'.ast, 

lA thi'sis on the Keolony i)f p:'ii <>i tin i,.()t,im< i'>"f. ■ 



li.shfil) 



56 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

of section 13 and west of section 17 information is too meager to justify 
even general statements. 

The base of the formation is conglomerate which grades up into 
hard graywacke carrying layers of chert and, in the western part of 
the formation, narrow seams of jasper. The cherty bands near the 
base of the formation are persistent everywhere. An interesting feature 
is the presence in the chert bands of considera])le detrital material and 
even in the associated clastic bands there is a subordinate amount of 
interstitial chert. 

In the eastern part of section 15 and the western part of section 14 
the ferruginous graywacke and chert are sharply overlain by a thin belt 
of black slate. Whether this member is persistent or is merely a local 
feature can not be determined. The black slate grades upward into 
coarser grained graywacke characterized by a considerable content of 
magnetite. This member can be traced magnetically and by outcrops 
from section 19, T 47 N, R 42 W, to section 20, T 47 N, R 43 W. Asso- 
ciated with the graywacke are slates and a few beds of lighter colored 
feldspathic quartzites. The magnetic graywacke is composed mainly 
of rounded grains of quartz and feldspar, the latter including ortho- 
clase, plagioclase, and microcline. The interstitial material comprises 
magnetite, hematite, carbonate, sericite, and chlorite. Magnetite occurs 
also in a later second generation of large well formed crystals which 
contain inclusions of the other minerals. 

Above the magnetic graywacke are gra^^wacke and slate, more or less 
ferruginous, containing locally considerable magnetite. In this connec- 
tion it may be said that the entire Copps formation west of T 47 N, 
R 42 W, is weakly magnetic. The rocks termed here magnetic gray- 
wacke contain important quantities of magnetite. The other gray- 
wackes and associated rocks present no unusual petrographic features. 
In the central part of the Copps formation the highest member is 
a soft slate. Its color is dark gray or grayish green excepting only a 
red phase at the top which grades downward irregularly into the darker 
colored facies. 

In T 47 N, R 42 W, exposures of the Copps formation are confined 
to the lower horizons. The cherty bands characteristic of the member 
immediately above the basal conglomerate are present and the over- 
lying rocks, insofar as it is possible to determine, are largely soft gray 
and green choloritic and sericitic slates. Iron carbonate, and to a limited 
extent iron oxide, are present but the relative importance of these 
minerals in the general make up of the rock has diminished when 
compared with their abundance farther west. 

In the western exposures in T 47 N, R 44 W, the rocks are rather 
coarse grained and highly ferruginous carrying bands n.nd layers of 



GEOLOGY OF THE GOGEBIC IRON RANGE. 57 

red chert, jaspilite and slate. The iron is almost entirely in the form 
of hematite with subordinate magnetite. The clastic grains include 
quartz, feldspar, red chert, and rarely jasper. 

The width of the Copps formation lessens rapidly going west from 
section 19, T 47 N, R 43 W, and in section 14, T 47 N, R 44 W, has 
narrowed to about 300 paces. This is believed to be due to erosion 
prior to the deposition of the Keweenawan series. This conclusion is 
strengthened by the fact that the ferruginous gra\^vackes and associ- 
ated cherty material characteristic of the lower horizons farther east 
are in section 14 in contact with the overlying Keweenawan lavas. 

The outcrops of ferruginous graywacke in the southeast part of sec- 
tion 19 and the north half of section 30, T 47 N, R 43 W, are almost 
identical with the exposures in section 14, T 47 N, R 44 W, and require 
no special description. 

Thickness of the Copps Formation. The Copps formation is one of 
considerable thickness. Throughout the greater part of its extent it 
has a surface width averaging well over a half mile. Assuming a north- 
erly dip of 65° in sections 13, 14 and 15, T 47 N, R 43 W, the thickness 
is not less than 2300 feet. It is possible that some of the beds are re- 
peated by close folding but the figure given is believed to be slightly if 
any in excess of the actual thickness of the formation. 

Relations to Adjacent Formations. The Copps formation rests un- 
conformably above the Prcsque Isle granite. The relationship is clearly 
indicated l)y the prominent granite conglomerate marking their con- 
tact. It is separated from the underlying Animikie formations by an 
imconformity of great structural and erosional magnitude. This con- 
clusion rests on the following evidence: 

1. The bottom horizon of the Copps formation is a true basal 
conglomerate containing fragments of the underlying Palms and Iron- 
wo(jd formations. 

2. The Presque Isle granite intrudes the Palms and Ironwood 
formations l)ut yields boulders and matrix to thc^ basal conglomerate 
of the Copjis formation. 

3. The Copps formation is in great structural discordance with 
tlic Animikie gnnip. 

1. The area! relationships are those of unconfoi'inity. 

5. Volcanic rocks are characteristic of the Animikie hut ar(> appar- 
• •ntiy entirely absent in the overlying Copps lorniation. 

The structural and areal relationshijis are shown on the accompanying 
ni;i|) and structure sections, (fig. 2) and arc discussed in another place. 



58 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

Keweenawan Series. 

It is not intended to discuss the Keweenawan series except with 
regard to its relations to the subjacent Huronian group. The great 
Keweenawan series of lavas, sandstone and conglomerate limits the 
Huronian series on the north throughout the (^ntire length of the 
Gogebic iron range. In addition to the overlying lavas and associated 
sandstones and conglomerates the Keweenawan series also includes 
the diabase dikes which cut the Huronian rocks and have played such 
an important part in the localization of the iron ore bodies. The 
Keweenawan is also believed to include the gabbro intrusive in the 
east part of T 47 N, R 44 W. 

Relation of the Keweenawan to Adjacent Formations. In the east 
end of the range the Keweenawan series is in contact only with the 
Copps formation which it overlies unconformal)ly. This relationship is 
clearly indicated in the exposures and test pits 450 paces west of the 
Ej post of section 15. The base of the Keweenawan in this vicinity is 
a partially indurated sandstone 50 to 75 feet thick marked by a strong 
basal conglomerate. The subjacent rocks of the Copps series are soft 
red slate and the conglomerate immediately above them contains 
abundant pebbles of this red slate and in addition pebbles of vein 
quartz, quartzite, graywacke, jasper, chert and iron ore derived from 
the various formations of the Huronian group. 

Keweenawan {f) Gabbro in T Jf.7 N, R I^J^ W . Reference has been 
made in connection with the discussion of the Ironwood formation in 
T 47 Nj R 44 W, to the high ridge of gabbro extending from section 1, 
T 47 N,' R 45 W, southeast to section 15, T 47 N, R 44 W. This ridge 
lies north of the Ironwood formation and is parallel to it. The rock 
varies from a coarse grained gabbro to fine grained diabase. Close to 
the contact with the iron formation the greenstone is aphanitic. The 
gabbro is best exposed in the southwest part of section 8 and the north- 
west part of section 17, although outcrops are plentiful in all parts of 
the ridge. The outcrops in sections 8 and 17 furnish the most favorable 
locality for observation of a well developed system of joints. The 
most prominent set strikes about east and west and dips south. A less 
marked set of joints, normal to the first, dips east. 

The gabbro is in contact with the Ironwood formation on the south; 
on the east it is limited by the greenstone agglomerates and effusive 
lavas. North of section 15 the gabbro is separated from the Kewee- 
nawan by a valley about one-half mile wide. This valley narrows 
toward the west and in section 1, T 47 N, R 45 W, the gabbro is ap- 
parently in direct contact with the trap. The gabbro thins rapidly 
westward and does not appear west of the NEj of the SE| of section 



GEOLOGY OF THE GOGEBIC IRON RANGE. 5g 

1. From section 1, T 47 N, R 45 W, to Sunday Lake the Keweenawan 
overlies the Ironwood formation. 

The exact relationship of the gabbro to adjacent rocks is not clear 
except in the case of the iron formation; actual contacts are nowhere 
exposed. The Ironwood formation adjacent to the gabbro is every- 
where characterized by highly metamorphic facies including amphibole- 
magnetite schist and slate and hard jaspilite, and this fact together 
with the occurrence of intrusive sills of fine grained dolorite in its upper 
horizon is strong evidence that the gabbro intrudes the Ironwood 
formation. The exact form of intrusion of the gabbro is not known. 
It may be either an immense dike or a sill. A prominent jointing or 
sheeting dipping southward at 65° in a plane approximately parallel 
to the northern and southern bases of the ridge suggest that this plane 
is also probably parallel to the opposite surfaces of the intrusives, in 
which case the intrusive is in the form of a dike dipping southward 
and inclined about 55° to the plane of the bedding in the iron formation. 

In section 1, T 47 N, R 45 W, outcrops of gabbro are found only 75 
paces south of the Keweenawan lavas. There is no evidence in the 
trap outcrops of contact metamorphic action and it is not possible ta 
determine the relationship between them and the gabbro. 

To summarize, — the gabbro intrudes the Ironwood formation. There 
is indirect evidence that it intrudes the rocks (Copps formation?) 
occupying the valley separating the gabbro from the Keweenawan in 
sections 7, 8, 15 and 16, T 47 N, R 44 W. There is no evidence that 
gabbro intrudes the Keweenawan. 

Van Hise makes only slight reference to the gabbro intrusive but 
apparently (correlates it with the Keweenawan. 

Structure of the Gogebic Range east of Wakefield. 

From T 44 N, R 6 W, Wisconsin, eastward to the center of T 47 N, 
R 46 W, Michigan, the (Jogebic range is a steeply northward dipping 
monocline, hut east of the Black River the structure is locally compli- 
cated. 

l-'rom the vicinily of the l-jircka min(> east to the Castile mine the 
Lower Iluroiiiaii and Animikie (Middle Huronian) formations are 
folded, faulted, and intrudcHJ by dikes and in-egular massifs of greiMi- 
stones. The most noticeahj(> feature is the great thrust fault at AN'ake- 
liel(L With the exception of the general northward tilting the folding 
;ind faulting of the liuronian rocks took place ])rior to Keweenawai\ 
litne and after the deposition and induration of the Animikie (Middli^ 
I luroni.'in ) series. 

l''roiM the Castile mine eastward to section 22, T 17 N, 1\ II W, the 
structure of the range is, so far as known, a simple nt)rthward dipping 



€0 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

monocline similar to that west of the Black River. The Lower Huron- 
ian and Animikie (Middle Huronian) series dip north at angles varying 
from 35° to 65°. The structure is illustrated by section AA accompany- 
ing the general map. In section 22, T 47 N, R 44 W, and eastward 
the structure again becomes complicated and except where diamond 
drill holes have supplied the data it is not possible to work it out in 
detail. The structural problem is complicated by great intrusions of 
granite and greenstone and by immense masses of basic volcanic ma- 
terial. In fact the igneous activity of Animikie time in the east end 
of the range is probably closely related to the exceptional local deforma- 
tion of the pre-Copps rocks. 

From section 22, T 47 N, R 44 W, eastward to section 20, T 47 N, 
R 43 W, little is known of the details of the structure. Exposures of 
igneous rocks are plentiful but the sedimentary rocks are mainly drift 
covered. The iron formation can be traced magnetically wherever the 
magnetism has developed in sufficient strength so that the iron formation 
can be clearly differentiated from the associated greenstones which are 
nearly everywhere weakly magnetic. In sections 20, 21, and 22 it is 
possible to work out the structure in some detail. 

Geologic structural sections have been drawn only in localities where 
surface data is supplemented by diamond drilling or underground ex- 
ploration. 

Structure of the Keweenawan series. The Keweenawan series has an 
approximate eastward strike and uniform northward dip. Super- 
imposed on the northward dipping monocline are gentle cross folds 
which are indicated by the gently sinuous character of the Copps- 
Keweenawan line of contact in sections 13 to 17, T 47 N, R 43 W. A 
cross anticline in the vicinity of sections 14 and 15, T 47 N, R 44 W, 
is particularly well marked. It is possible that the Keweenawan is cut 
by the cross fault in section 14, T 47 N, R 44 W, which has apparent- 
ly produced a horizontal misplacement of about 1500 feet in the under- 
lying Copps formation and Animikie volcanics. 

Structure of the Copps Formation. The trend of the Copps formation 
is about parallel to the Keweenawan and its dip is northward. South- 
ward dips are confined, so far as known, to the secondary structure, 
i. e. schistosity or slaty cleavage, while the bedding invariably dips 
northward so far as observed. The bands of chert and jasper near 
the base of the formation dip north and northward dips are exhibited 
wherever bedding is recognizal^le in the slates and graywackes. In the 
slate exposures in the SEi of the SEi of section 15, T 47 N, R 43 W, 
the cleavage dips south at angles varying from 45° to 80°, but the 
bedding is nearly vertical or steeply inclined to the north. This is 
the only locality where the dips of bedding and shistosity are opposed 



GEOLOGY OF THE GOGEBIC IRON RANGE. 61 

in the same exposure. An exception to the general northward dip of 
the Copps occurs in the exposures of ferruginous graywacke in the 
southwest part of section 19 and the NW^ of section 30, T 47 N, R 
43 W. These rocks are mainly massive and exhibit neither bedding 
nor schistosity except in an outcrop near the southwest comer of sec- 
tion 19 which contains narrow bands of chert striking northeast and 
dipping southeast. The occurrence of the Copps formation in this lo- 
cality is of considerable structural significance. The Animikie rocks 
are here believed to be compressed into an overturned syncline with 
axial plane striking NE-SW and dipping northwest. It is probable 
that this fold has passed into a thrust fault, the plane of which is ap- 
proximately parallel to the axial plane of the s\Ticline. While the posi- 
tion of the ferruginous graA'^'acke here indicates that the gra>"\vacke is 
younger than the Animikie volcanics and iron formation it may be 
equivalent to the upper horizon of the Ironwood or the lower part of 
the Tyler formation. If any part of the Tyler formation is preserved on 
the east end of the range the remnant should be looked for in sjTiclines 
such as this. There is considerable hthologic dissimilarity between the 
ferruginous graywacke at this locality and the Tyler sediments, while 
on the other hand it is identical in appearance and composition with 
phases of the Copps rocks. It is of course possible that the exceptional 
conditions which prevail in this part of the range may account for the 
deposition of material entirely dissimilar to that elsewhere simulta- 
neously laid down. 

The determination of the time at which the deformation and folding 
in this locality' took place is directh^ related to the problem of the age 
of the ferruginous graywacke. There are three possibilities. (1) If the 
ferruginous graywacke is equivalent in time to the Tyler or to the Iron- 
wood formation the deformation is probably post-Animikie and pre-Copps 
because there is no evidence elsewhere of any pre-Keweenawan folding 
in the Copps. (2) Assuming that these rocks are part of the Copps 
formation it is possible that the folding may have taken place prior to 
the deposition of the Copps and to have resulted in the formation of 
a basin or depression which Avas later filled by the post-Animikie sedi- 
ments. (3) Finally, if the ferruginous gra\-Avacke belongs to the Copps 
and has been itself affected by the folding, the age of the disturbance 
is post-Copps and probably Kewecnawan. 

Slrudurc uf the Middle lluronian Series {Animikie). It is impossible 
to obtain sufficient information on which to work out the detailed struc- 
ture of the Animikie rocks except in the vicinity of sections 21 aiul 
22, T 47 N. R 43 W. From the west line of section 21 east to section 
22, T 47 N, H 14 W, the available data warrants nothing beyond a 
general idea of the gross structure. 



62 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

The structural conditions in the east part of T 47 N, R 43 W, are 
exhibited by the geologic sections (Fig. 2) which are based to a great 
extent upon the' drill records of the Presque Isle Mining Company. 
The data for structural sections here are unusually ample because of 
the large amount of drilling and test pitting which has been executed, 
fortunately in a most critical area from the structural point of view. 

The chief fact brought out by the structure is the pre-Copps folding 
of the Middle Huronian (Animikie) and the truncation of these folds 
by the Copps formation. East of the center of the NWj of section 21 
the Copps formation is first in contact with the iron formation, then 
the Palms quartzite and finallj^ in the northeast part of the section, 
again with the Ironwood formation. In the NEJ of the section a drill 
hole pointed south entered the Copps formation, passed through its 
basal conglomerate and penetrated the Palms quartzite without en- 
countering the Ironwood formation. These relations indicate that the 
Middle Huronian (Animikie) sediments were deposited, indurated, 
folded, uplifted, eroded, and depressed prior to the deposition of the 
Copps rocks. 

The Middle Huronian (Animikie) series in sections 20, 21, and 22 
forms two major synclines, one pitching east and the other west from 
the crest of an anticline near the center of section 21. The eastward 
pitching trough is divided by a secondary anticline into two sub-troughs 
so that the iron formation in the west half of section 21 occurs in two 
parallel belts which, however, merge into the main single belt in the 
major syncline in the adjoining section to the east. Details of the 
structure of the west s>aicline are not so well known nor does the drilling 
define the limits or depth of the trough except on the east side of the 
Presque Isle river. There is some evidence that this trough is broken 
by cross faulting at the approximate location of the Presque Isle river. 
The drilling just east of the river indicates a relatively shallow syncline 
but a short distance west at the Presque Isle mine drill holes to a depth 
of 850 feet have failed to reach the bottom of the trough. The sudden 
termination of the greenstone belts north and south of the iron forma- 
tion is also suggestive of faulting. If there is a fault at this locality 
it is evident that the rocks west of the river are on the do\\ai thrown side. 

Nothing is known concerning the attitude of the wide belt of iron 
formation continuing east through section 20 except that the rocks 
along the north border dip south. The iron formation in the Presque 
Isle mine and in the old exploration in the NWj of the SWj of section 
20 dips south. 

T\w, stratigraphic position of the greenstone agglomerate in section 
20 with reference to the Ironwood formation is uncertain. The plane 
of schistosity in the greenstone dips south parallel to the bedding in 



GEOLOGY OF THE GOGEBIC IRON RANGE. 63 

the adjacent iron bearing rocks. If the greenstone is stratigraphically 
above the Ironwood formation as in T 47 N, R 44 W, the volcanics 
probably occupy the center of an overturned syncUne whose axial 
plane dips south. 

It has been said that little is known regarding the structure of section 
20 westward to section 22, T 47 N, R 44 W. The following is merely 
a statement of kno^\^l facts of structure with such interpretations as 
seem best fitted to the facts. 

The prominent feature of this territory is the great belt of greenstone 
agglomerate and other extrusives in the east part of T 47 N, R 44 W, 
and the occurrence of thick diabase sills in sections 26 and 27 of the 
same township. The relations and structure of the diabase sills have 
already been discussed. The greenstone agglomerates are in general 
massive in structure, less commonly schistose. Structural problems in 
a volcanic formation such as this are hopeless of solution because it is 
not possible to identify everywhere any of the flows or ash beds which 
might, if continuous, serve as horizon markers. Detailed petrographic 
and field study of the greenstones might throw some light on structure, 
but the chances are that even this would be of slight assistance. 

Information regarding the Animikie sediments is also meager. There 
are no rocks susceptible of correlation with the Palms east of section 
22 in T 47 N, R 44 AV, and the iron formation is exposed only at wide 
intervals. 

The definite magnetic line immediately south of the greenstone 
volcanics indicates that the Ironwood formation is continuous from 
section 22, T 47 N, R 44 W, southeast to the exposure of cherty iron 
carbonate in the northwest part of section 25, although actual ex- 
posures in the intervening territory are found only near tiie Wj i)ost of 
section 23. 

Reference has been made to the amphibole magnetite chert and slate 
associated with the diabase in sections 26 and 27. If these rocks are 
part of the Ironwood it is evident that this formation is distributed 
here over a surface width of about a mile, this being accounted for by 
the occurrence within the formation of the great diabase sills possibly 
comljinetl with a tlattenting of the dij). 

The general strike of the iron formation west of the agglomeratic 
greenstone is southeast. An excej^tion is the exposure in the northwest 
part of section 25 in which th<' strike is slightly east of north. The 
strike of the iron fonnalion hclt east of the greenstone in this territory 
is NE. The most westerly outcrop of iron formation is in the south 
l)art of section 25, T 47 \, K 44 W. The magnetic attractions in the 
south half of section 2(1 arc prol)al)ly caused by iron formation tlirectly 
connected with (he exposure in section 25. 



64 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

The difference in strike of the iron formation east and west of the 
greenstone extrusives is one of the chief structural features in this part 
of the range. The question at once arises, (1) are the two belts of iron 
formation one and the same formation or (2) if not now actually con- 
nected were they originally deposited as a continuous formation? The 
evidence on this point seems conclusive. It has been shown that the 
iron formation in section 21 overlies rocks which are identical in suc- 
cession and lithologic features with the type exposures of the Palms 
formation in the central part of the range. The structure in sections 
25 and 26 is an overturned fold, the western limb of which has been 
faulted and thrust eastward and northward above the lower or western 
limb. There is considerable field evidence in favor of this interpreta- 
tion (1) in the attitude of the iron formation and greenstone in section 
25, (2) the absence of diabase south of the fault line, (3) the location 
of the magnetic attractions in section 25 and (4) finally in the fact 
that the indicated fault line follows low ground in both sections. For 
the most part the above facts show clearly on the map. 



Michigan Geological and 
Biological Survey. 



Publication 18, Geology 15, 
Plate II. 




A. CilAKACTEUISTIC EXl'U.SUUES UF I'KKSt^l K ISLE GRANITE IN T. 47 N. 

R. 43 W. 




I'.. i\i.\ii.\i-\Kn\sin- •nii'u(;i{.\i'in (ii- Tin; (;ui:i:.\si'»ink.^ in i". 4T n. r. 44 w. 



Michigan Geological and 
Biological Survey. 



Publication 18, Geology 15, 
Plate III. 




A. ALLIi;.\T()K I'dlN'r. 



I.(M»KIX(; NORTH 
<;OGEBIC. 



FROM SOITII END OF LAKE 




I'.MJ.S »».\ 
32. T. 4ti N. 
STONE. 



■'Ill-; Mllil>I.i: I'.KANCll (IF TlIK UNT(»NA(iON UIVKU. SlX'llUN 
i:. 41 W.. SlIoWlNi; .HUNTING IN AMYGDALOIDAL GREEN- 



CHAPTER IV. 
GEOLOGY OF THE :MARENISC0 RANGE. 

R. C. ALLEN AND L. P. BARRETT. 
INTRODUCTORY STATEMENT. 

The Marenisco Range extends from the northwest corner of T 46 N, 
R 41 W, Michigan, where it is overlapped by the Keweenawan Series 
of the South Trap Range, southwest ward through Marenisco to T 44 
X, R 2 E, Wisconsin, and an unkno"s\Ti distance beyond. It was pros- 
pected for iron ore in the 80's near Gogebic Lake in Michigan and in 
the vicinity of Long and Pine lakes in Wisconsin. At the latter locality 
the village of ^Magnetic Center was laid out at the north end of Pine 
Lake but all traces of it have long since disappeared. Among the early 
explorers this range was knoA^^l as the South Range but few today 
recall its existence. 

Only about a half dozen outcrops are knowai on this range from the 
vicinity of ]\Iarenisco southwestward. In the opposite direction there 
are a considerable number of exposures but these alone furnish an in- 
sufficient basis for geologic mapping. The range is characterized by a 
strong to violent magnetism which furnishes a ready means and, for 
the greater part of its extent, the only means of determining its posi- 
tion. That this strong magnetism attracted the early explorers is 
evidenced by traces of their i)rospects in the most violently magnetic 
localities. Fragments of diamond drill cores properly arranged in core 
trays, were found on the site of the Magnetic (Vnter ex])loration in a 
dense forrst of hardwood and hemlock in the SWj of NEj, section 28, 
T 44 X, K 3 E, Wisconsin. These cores furnish the only direct evidence 
of the nature of the magnetic rock southwest of Marenisco. Although 
there are many pits and an old shaft in this vicinity there is no evidence 
that bed rock was opened in any of them. In T 46 N, R 42 W, Michigan, 
outcrops are more abundant, and in this locality only is it possil)le to 
work out a definite succession. Whether the succosion luTC is n^p- 
resentative of the range as a whole can not be detennined. 
9 



66 



CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 



Succession on the Marenisco Range. In T 46 N, R 42 W, Michigan. 



Keweenawan ? 



Huronian 



Archean 



Igneous contact 



Middle Huronian 
(Animikie) 



Unconformity 



Diabase. 

Intrusive granite 
(Presque Isle) 
Intrusive greenstone 
Slate 

Extrusive lavas 
Iron formation 
Quartzite and graywacke 



Northern area. Granite and greenstone. 
Southern area. Mica schist, greenschist and 
amphibolite. (May be Huronian) 



ARCHEAN SYSTEM. 



NORTHERN AREA. 



The Marenisco range is separated from the Gogebic range to the 
north by a territory from three to twelve miles wide, underlain for the 
most part by granitic rocks but including subordinate areas of green- 
schist. The granite in this area was formerly considered as entirely 
Laurentian but, as has been shown in the discussion of the east end of 
the Gogebic range, there is ample evidence that, in part at least, it is 
intrusive into the Middle Huronian. On the other hand, the relations 
between the Huronian rocks and the granites and greenschists through- 
out the major part of the Gogebic range clearly indicate the Archean 
age of the latter. 

On' the basis of present knowledge it is not possible to differentiate 
the granites representing the two or more different periods of intrusion 
except in limited localities where the relations to adjacent Huronian 
rocks are exhibited. The facts, however, seem to warrant the general 
statement that the granite from about the vicinity of the Little Presque 
Isle river east to Lake Gogebic is preponderantly Middle Huronian. 

In section 4, T 46 N, R 42 W, the northern granite intrudes the 
Middle Huronian slates. Elsewhere the relations between the northern 
granite and the Huronian sediments may only be inferred. 



SOUTHERN AREA. 



Location and General Statement. The Marenisco range is separated 
from the Turtle range by a non-magnetic area two to seven miles wide. 
In this territory, in sections 4 and 9, T 43 N, R 3 E Wisconsin, there 



GEOLOGY OF THE MARENISCO RANGE. 67 

are several outcrops of granite gneiss and hornblende schist. North- 
east for a distance of approximate!}^ 24 miles there are no rock expo- 
sures. In T 46 N, Rs. 41 and 42 W, Michigan, the territory between 
the Marenisco and Turtle ranges is occupied by a complex of mica 
and hornblende schist and coarse grained amphibolite with a large 
amount of intrusive granite, greenstone and fresh diabase. 

There is no certain evidence that any of these rocks are Archean 
and it is probable that the intrusives are Algonkian. The schists and 
gneisses are placed tentatively in the Archean because they appear on 
what seems to be an anticline separating the Marenisco and Turtle 
ranges and are plainly older than the other rocks which are igneous 
and intrusive in them. The Huronian rocks are also extremely meta- 
morphic and therefore correlation on the basis of metamorphism or 
lithologic similarity with the Archean of other localities is not warranted. 

Mica Schist. The most interesting rocks in the southern area are 
the mica schists. In T 46 N, R 41 W, particularly in sections 13, 14, 
15, 18, 29 and 30, outcrops of mica schist are associated with massive 
greenstone. Mica schist also occurs in association with greenstone in 
one exposure in section 30, T 46 N, R 42 W. Near the south quarter 
post of section 8, T 46 N, R 41 W, there is a small exposure of biotite 
schist intruded by granite. This is the only exposure knowTi where 
granite is in contact with mica schist. 

The mica schists are generally of light gray color; some are coarse 
and others are fine grained. A common feature of all of the exposures 
is the occurrence of an abundance of milky white quartz in blebs and 
stringers parallel to the schistosity. Here and there these stringers are 
minutely folded. Other quartz veins cut across the schistosity in- 
differently, apparently filling joint planes. At two localities in the 
south part of section 29 the mica schist shows a banding in addition 
to the ever present schistose structure. In many cases the plane of 
schistosity is inclined to the banding, but in general the two struc- 
tures are i)arallel. At one locality' the l)anding has a strike of N 25° E, 
the schistosity N 62° E. The dip of banding and schistosity is about 
vertical. 

Tlie mica S(;hists arc coiuposcd i)rincipally of (luartz, biotite and 
limpiil feldspar, with subordinate muscovite magnetite, sericite, garnet, 
("pidolc and apatite. Th(> (luartz, biotite and feldspar, which make uj) 
the ma.ss of the rock, constitute ati interlocking crystalline mosaic 
Locally the mica is muscovite but in general, biotite pi-e\ails. The 
biotite is definitely oriiMited, but the interlocking grains of (juartz and 
fel(ls[)!ir do not, in the coarser grained rocks, paitake.of the schistose 
strucliire. In the liner' jirained wirieties there is ;i tendencx- for elon- 
gation of many of the (|u;iit/, ;in(l feldspar grains |);ii-all<"l to the biotite 



68 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

flakes. Quartz, the most abundant mineral, exhibits strain effects and 
inchisions in the coarse grained schists, but the alisence of these fea- 
tures is characteristic of the finer grained facies. The feldspar, except 
in case of the biotite schists associated with the granite in section 8, 
T 46 N, R 41 W, occurs in small, limpid, unstriated grains and is very- 
difficult to distinguish from the quartz. However, certain of the weak 
doubly refracting grains show brightly polarizing alteration specks, 
presumably sericite, and grains of this type have a uniformly lower 
index of refraction than the other grains which are clear and free from 
all alteration products. In addition, biaxial interference figures may 
be observed in some of the larger grains. The schist where intruded 
by granite shows considerable striated plagioclase feldspar, and is also 
fresher in appearance and rather coarse grained. Biotite alone exhibits 
parallel orientation; the quartz and plagioclase (about albite-oligoclase) 
form an allotrimorphic crystalline mosaic. Accessory minerals in the 
granitized schist are zircon, pyrite partly altered to red ferric oxide, a 
few grains of magnetite, epidote and apatite. The zircon, pyrite and 
magnetite are included in the biotite. 

Origin of the mica schist. While we incline strongly to the view that 
the feldspathic mica schist is an altered sediment, the evidence is in- 
conclusive. Rounded grains of quartz and feldspar are not uncommon, 
but in all cases they fit perfectly into the crystalline mosaic and can 
not be regarded as detrital with any degree of certainty. One or two 
doubtful indications of secondary enlargment were noted, but little 
reliance is placed on them. In the field, the schist is associated in 
almost every exposure with either diabase or massive greenstone. This 
association lead early to consideration of the idea that the schist might 
be a sheared phase of the massive rocks. Microscopic study of the 
greenstone and diabase shows, however, that these rocks are much 
more basic in composition than the schists. They are composed, 
dominantly, of green hornblende and basic plagioclase. Some speci- 
mens of the fresher diabase show traces of original pyroxene and the 
characteristic ophitic or lath shaped development of feldspar is in 
every instance preserved. The metamorphism of a rock of this type 
under conditions which would induce schistose structure would produce 
an amphibolite or hornblende schist rather than a typical quartz-mica 
schist. Furthermore it is possible to say with certainty that the mas- 
sive greenstone intrudes the schist. The schist in fact seldom occurs 
except in association with massive greenstone. As a whole the schist 
is soft and easily eroded and therefore is more apt to be exposed in those 
places where it is partially protected by intrusive masses of greenstone 
of much greater erosive resistance. Consequently, it is probable that 
observation is confined to the most severely metamorphosed parts of 



Micliigan Geological and 
Biological Survey. 



Publication 18, Geology 15, 
Plate IV. 






s 



^ 




i;.\Mii:i> MICA sciiisi. si;i'i'i(t.\ ;'.(i. v. u; \.. i;. ij \v, 



GEOLOGY OF THE MARENISCO RANGE. 69 

the schist wherein evidences of sedimentary textures and structures 
have been obUterated. It is well known that contact metamorphism, 
even without addition or subtraction of material from extraneous 
sources, can change a pelite or psammite formation into a crystalline 
mica schist. jMention was made earlier in the discussion of the presence 
in certain outcrops of a banding at variance with the prevailing re- 
gional schistosity. 

Intrusives. In addition to the mica schist the rocks of the southern 
area comprise amphibolite, hornblende schist, and intrusive greenstone, 
granite and diabase. These are certainly younger than the mica schist, 
and are probably to be correlated with similar intrusives in the Huronian 
sediments in T 46 N, R 42 W. The amphibohte and hornblende schist 
are in part derivatives of massive intrusive greenstone. Certain iso- 
lated outcrops may, however, be older and have no connection mth 
the massive greenstone. The intrusives will be described later. For 
the present it should l)e ]:)orne in mind that intrusives are extremely 
abundant in the southern complex. 

RELATIONS TO ADJACENT ROCKS. 

Relations to Keiceenawau. In section 13, T 46 N, R 41 W, Michigan, 
the steeply dipping and highly metamorphosed micaceous schists are 
overlain by the almost flat lying unaltered basal sandstone and con- 
glomerate of the South Traj) range. The conglomerate contains pebbles 
derived from the underh'ing schists. 

Relations to Huronian. The relation of the southern schists to the 
Huronian rocks of the ^larenisco range may only be inferred. The 
schist series lying south of the range is referred to the Archcan for the 
following reason. These rocks lie at the eastern extremity of the 
territory between the !Marcnisco and Turtle ranges. The magnetic 
belts of these ranges mark the location of folded sedimentaries and 
lavas, a conclusion amply sustained by their linear character and the 
nature of the underlying rocks wherever exposed or opened in explora- 
tion. The gross structure between the Marcnisco and Turtle ranges 
is believed to be anticlinal with the Huronian series exposed on either 
flank of the major fold. 

The exposures along the Turtle rang(> in T 46 X, H 41 A\', Michigan, 
are effusive lavas with sul)ordinate intrusive greenstone and dial)ase. 
At no place are these rocks in contact with the inica schist. 

.MII)|)I,K IHKOXIAX SKRIES (AXIM I Kli:). 

The full succession of the Middle Huronian is exposed only in T 46 
N, K 42 W, Michigan. The succ(\ssion here includes basal graywacke- 
quartzite, iron formation and a vast thickness of slate occui)ying a 



70 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

belt not less than two miles wide between the iron formation on the 
south and the Presque Isle granite of the Ciogebic range on the north. 
The entire succession with the exception of the central part of the slate 
is highly metamorphosed. 

GRAYWACKE-QUARTZITE . 

The base of the Middle Huronian series is highly metamorphosed 
quartzite and graywacke. The quartzite is exposed in a large outcrop 
in the NWi of SE^ of section 7, T 46 N, R 41 W, and also occurs on the 
dump of a pit in section 12, T 46 N, R 42 W, where it is apparently in 
association with the iron formation. The quartzite is a hard gray, 
vitreous, fine grained aggregate of interlocking, crystalline, quartz 
grains, containing rounded grains of orthoclase and plagioclase, abund- 
ant chlorite, and a small amount of sericite as an alteration product 
of feldspar. In thin section, in ordinary light the outlines of original 
clastic quartz grains are made conspicuous b}' fringing shreds of chlo- 
rite, but under crossed nicols a crystalline mosaic appears in which the 
only indication of sedimentary origin are the rounded grains of feldspar. 

In sections 22 and 23, T 46 N, R 42 W, are a number of outcrops 
of fine grained, banded graywacke associated with intrusive diorite 
and diabase. The bands vary from mere laminae up to an inch or more 
in width. In places differential weathering of the bands has given the 
rock a ribbed appearance. Near the contact with the intrusives the 
graywacke is usually greatly contorted. Under the microscope the 
graywacke reveals a crystalline aggregate of quartz, epidote and zoisite, 
with some secondary feldspar and a few shreds of green hornblende. 
The rock is thoroughly recrystalline and in some cases a schistose 
structure is produced by parallel aUgnment of the hornblende crystals. 
The banding is caused by alteration of zones of finer and coarser grained 
material. The least altered of these rocks are made up of finely crystal- 
line quartz, secondary feldspar, badly altered to sericite, scattered 
needles of green hornblende, and a few specks of magnetite, epidote 
and chlorite. In general, the texture is that of a recrystalline rock, 
but there are many rounded grains of quartz which seem to be caught 
in a feldspathic cement. 

The occurrence of the graywacke immediately south of the iron 
formation in sections 22 and 23, as well as its position on the strike of 
the quartzite exposed in section 12, T 46 N, R 42 W, and section 7, 
T 42 N, R 41 W, its mineral composition, and its structure are the 
basis of our belief that this formation is part of the basal detritus of 
the Middle Huronian series. The exposures are in all cases intruded by 
greenstone and diabase, or located a short distance from outcrops 
of igneous rocks. To the presence of these igneous intrusives must be 



GEOLOGY OF THE MARENISCO RANGE. 71 

ascribed the recrystalline character and the obhteration of the clastic 
texture of the graywacke when viewed under the microscope. 

Neither the quartzite nor the altered graywacke were observed in 
contact with the Archean or any formation of the Huronian, but that 
both are stratigraphically below the iron formation is apparent from 
the field relations. 

IRON FORMATION. 

The iron formation of the Marenisco range is exposed in a number 
of pits and outcrops in sections 12 and 13, and in a few pits in section 
22, T 46 N, R 42 W. The formation is also exposed in section 21, 
T 46 N, R 43 W, along the C. & N. W. R. R. track one half mile south 
of the station of Marenisco. From Marenisco southwest along the 
strike of the range there are no exposures, but diamond drill cores of 
iron formation were found at the Magnetic Center exploration in 
section 28, T 44 N, R 3 E, Wisconsin. 

Lithology. The drill cores from the Magnetic Center exploration 
are a banded quartz-amphibole-magnetite rock. The banding is con- 
spicuous on the polished sides of the core but on fractured surfaces 
close examination is necessary to distinguish the structure except 
where a wide band of light colored chert or quartz is present. 

Proceeding from this locality northeast, exposures are first encount- 
ered on the C. & N. W. R. R. in section 21, T 46 N, R 43 W, Mich- 
igan, one half mile south of Marenisco station. The most striking 
characteristic of these outcrops is a conspicuous fine banding of various 
shades of gray, green and brown alternating across the face of the 
exposure in the plane of a well developed schistosity which is vertical 
and strikes in the general direction of the magnetic belt, N 55° E. 
Here and there are bands of dull gray to white chert, less persistent 
than the others, and broken into lenticular shaped masses resembling 
flattened fragments in an exceedingly schistose conglomerate. Blebs 
and stringers of quartz are abundant. Some of the bands are highly 
ferruginous, weathering to a rusty brown color. 

The microscope reveals the presence of rounded clastic grains of 
(luartz and feldspar embedded in a groundmass of finely crystalline 
chert, dirty greenish brown l)i()tite, magn(^tite, carbonate (probably 
siderite) with subordinate amphibole and chlorite. The fine bands, 
so conspicuous on exposures, are caused by the alteration of bands of 
ainpliil)()le and l)i<)tite with bauds of chert, magnetite and carbonate. 
I'Vagmental grains of quartz and feldspar are for the most part oriented 
with the long dimensicm of the particles parallel to the banding, but 
sonir of tlicni show no tcndcncN' toward orientation. This rock is evi- 



72 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

dently a highly metamorphosed lean iron formation intermixed with 
clastic material. 

The best localities for observation of the iron formation are the pits 
in section 22 and the pits and outcrops in sections 12 and 13, T 4G N, 
R 42 W, Michigan. A careful examination of the rocks at these lo- 
calities has been combined with the study of a large number of thin 
sections. In general, the rock is a lean iron formation, intensely meta- 
morphosed and, for the most part, typically l^anded, the bands vary- 
ing in wddth from mere laminae up to one to two inches. The wider 
bands are mainly light colored, crystalline cjuartz. The darker and 
narrower bands contain quartz and iron bearing silicates and oxides. 
These bands are not uniformly continuous, the smaller ones often 
pinching out and dovetailing with the larger ones in an intricate man- 
ner. Thin veins of pyrite and carbonate cement innumerable fractures 
across the banding. Pyrite and perhaps pj^rhotite have been con- 
centrated along some of the darker bands. Certain of the dump speci- 
mens are not banded, and as a rule iron amphiboles are abundant in 
this type. The rocks may be classified as follows: (1) Chloritic-sideritic- 
magnetitic schist, (2) manetitic-sideritic-quartz rock, (3) actinolitic- 
magnetitic schist, (4) griineritic-magnetitic schist, and (5) chloritic- 
biotitic-magnetitic schist. 

The prevalent type in the pits of this township is the magnetitic- 
sideritic-quartz rock. In hand specimens the most pronounced feature 
is the alternate gray and white banding mentioned before. It is made 
up of quartz and magnetite with subordinate siderite and considerable 
chlorite in occasional bands and small flakes. Hematite and pyrite 
are present but not abundant. The banded structure, which is such a 
marked macroscopic feature, is due to alternate zones of fine and 
coarse grained quartz. The banding is heightened by the concentra- 
tion of magnetite in the zones of finer grained quartz. This mineral 
occurs in two forms; (1) as small grains with a tendency towards 
crystal development, and (2) in large irregular patches that commonly 
show considerable fracturing with development of hematite and siderite 
in the fracture planes. The siderite and hematite in these situa- 
tions appear to be alteration products of the magnetite. In gen- 
eral, the siderite occurs in small patches, and occasionally in bands 
or zones not directly associated with the large irregular patches of 
magnetite although the carbonate, with the exception of an occasional 
small crystal, is confined to the bands in which magnetite is concen- 
trated. It is probable that the siderite is original except in the cases 
noted above, where it appears to be formed along the cleavage cracks 
of magnetite. In by far the larger number of cases the tendency is 
for the development of magnetite along the cleavage cracks and around 



GEOLOGY OF THE MARENISCO RANGE. 73 

the edges of the carbonate. Where the chlorite occurs in sufficient 
quantity to be a marked feature of the rocks as a whole it may be de- 
fined as a chloritic-magnetitic-sideritic schist. 

The actinolite-magnetite-schists are usually dark gray and may or 
may not show banding. The bands vary from one inch to one eighth 
of an inch or less in width. The wider bands are predominantly finely 
crystalline quartz, magnetite and fibrous amphibole being concentrated 
in the narrower bands. Where the rock is not banded it is dark gra}- 
and shows the metallic luster of innumerable minute crystal faces of 
magnetite. In this rock the greenish yellow actinolite crystals seem to 
be collected in masses and rosettes. The banded types are made up 
of zones of finely crystalline quartz, alternating with zones of actinolite, 
magnetite and siderite. The quartz grains exhibit no trace of detrital 
origin such as rounded outlines or zones of secondary growth. While 
quartz is the most abundant mineral in the so-called quartz bands 
they contain a considerable amount of scattered siderite associated 
with the minute crystals of magnetite and occasional needles of acti- 
nolite. Alternating with the quartz are bands of magnetite, actinolite 
and siderite. The latter mineral occurs in ever}^ stage of alteration 
to actinolite, from pure crystals surrounded b}' needles of amphibole, 
through early stages of alteration in which actinolite occurs in the 
centers of the siderite grains along cleavage cracks, until, finally, the 
crystals of siderite can barely be discerned through the mass of actin- 
olite fibres. In general, the center of these bands is occupied by un- 
altered iron carbonate, althougii in some of the narrower bands the 
alteration to actinolite has been complete. The amphibole is in its 
customary fibrous development. There is no alignment of the fibres. 
In the center of the bands and on the edges near the contact Math the 
quartz zones the fil^res lie at right angles to the banding and penetrate 
into the adjacent quartz grains. The magnetite is in well developed 
octahedra and is most abundant in the siderite-amphi]H)le bands 
although it is present in much smaller quantiti(^s aiul smaller crystals 
in the quartz bands where it is closely associated witii scattered grains 
of carbonate. 

Thin sections of the iioH-lxnidcd <uiiiioJi(c rocks present a much dif- 
ferent appearance. Siderite is not obs(>rv('d in rocks of tills tyi)e. 
Upon a dull gray mosaic of small intiM-locking (piartz grains there 
appear, as if painted, rosettes of actinolite through which are scattered 
idiomorphic crystals of magnetite. In the spaces Ix'lwccn tlu^ rosettes 
of actinolite the finely crystalline mass of interlocking ([uartz grains 
appears. The actinolite occurs in rosettes of long, faintly grecMi, almost 
colorless needles ;in<i in minute liundlcs between the (lunrtz grains. 
the ne(vll(vs p(>net r.-iting the ;i(lj;iceiit (|ii:irtz inosjiic. Small individu.-d 



74 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

needles are also included in single individuals of quartz. Crystals of 
magnetite are scattered haphazardly through the rosettes of actinolite 
although there is sometimes a tendency towards concentration at the 
center and at the junction of two rosettes. In areas where the quartz 
predominates, the magnetite, while not entirely absent, is represented 
only by occasional and usually small crystals. 

Grunerite-mdgnetite-schist occurs in a small outcrop near the pits in 
section 12. The variety cummingtonite has not been identified but 
as this mineral is rather common in the metamorphic iron formations 
of the Lake Superior country it is probable that a more careful search 
would result in its identification. 

So far as it is possible to determine from the very limited data avail- 
able, the iron bearing rocks of the Marenisco range do not differ from 
the metamorphic phases of the iron formation on the other Lake 
Superior ranges and there is no reason to believe that the Marenisco 
iron formation differs from them in origin. The abundance of car- 
bonate and its apparent close genetic relationship with the magnetite 
and iron amphibole together with the bands of finely crystalline chert 
point to the primary deposition of cherty iron carbonate. There is 
evidence that locally, at least, the deposition of non-clastic ferruginous 
material was accompanied by clastic sedimentation. 

Ore hearmg possibilities of the iron formation. With regard to the ore 
bearing possibilities of the iron formation of the Marenisco range atten- 
tion should be once more directed to a statement made earlier in this 
discussion, viz., that the range is everywhere characterized by strong 
to violent magnetism. The iron bearing rocks throughout the entire 
range are highly metamorphosed. The possibilities for discovery of 
valuable ore bodies are not promising. It is of course possible that 
some portions of the iron formation may have escaped the general 
intense metamorphism, and that in such places, ore bodies may have 
been concentrated. 

Igneous intrusives in the iron formation. The iron formation exposed 
in the pits and outcrops in T 46 N, R 42 W, is closely associated with 
massive igneous intrusives. Considerable igneous material occurs on 
the dumps of some of the pits in iron formation. Many outcrops of 
altered diorite and diabase are in close proximity to the pits and a 
short distance southeast of the pit in section 22 is an area of granite, 
probably intrusive. Intrusives were nowhere observed in contact with 
the iron formation, but there can be no doubt that the latter has been 
severely altered by igneous intrusion. The presence of iron silicates, 
the completely crystalline character of the quartz, abundant magnet- 
ite, almost total lack of hematite, and igneous injection, described 



GEOLOGY OF THE MARENISCO RANGE. 75 

below, all point to contact action as the cause of the extreme meta- 
morphism of the iron bearing rocks. 

Specimens from the dump of a pit near the center of section 22 are 
of special significance inasmuch as they show intimate injection of 
igneous material. These rocks are typicall}- quartz-magnetite schists, 
dark gray to black and strongly magnetic. The noticeable feature is 
the presence of thin veins and streaks of red feldspar which cut across 
the banding of the rock in some places and in others are parallel to it. 
A slide made from one of these specimens cut one of the red veins and 
served to confirm the field determination of its mineral composition. 
The rock is made up of bands of finely crystalline quartz and magnetite 
alternating with bands composed of small interlocking grains of altered 
feldspar with subordinate quartz and considerable chlorite. The 
feldspar is filled with fine red dust and shows considerable alteration 
to sericite. The contrast between the clear, fresh quartz and the cloudy 
feldspar is marked. The feldspar in the bands is in a crystalline ag- 
gregate and the individual grains are small and unstriated. The 
uniformly lower index of refraction of the feldspar compared with the 
quartz, the observation of biaxial interference figures, and the char- 
acter of the alteration product, leaves no doubt that these small grains 
were correctly determined. There is a tendency for chlorite to concen- 
trate between the quartz-magnetite and the feldspathic bands, although 
it is scattered in flakes throughout the slide. Magnetite is present 
throughout the rock in well developed octahedra but is concentrated 
for the most part in the feldspar free bands. 

No intrusive igneous material was found in or near the exposure of 
iron formation at Marenisco or at the Magnetic Center locality at the 
western extremity of the range, but the character of the rocks in these 
localities points toward metamorphism through igneous intrusion. In 
section 4, T 45 N, R 44 W, Michigan, there is a knob of granite pro- 
jecting above the drift. This outcrop is a short distance north of the 
line of maximum attraction in the south magnetic belt and there can 
be little doubt that it is intrusive into the adjacent magnetic rocks. 

THE SLATE FORMATION. 

The slate formation in T 46 N, R 42 W occupies a belt about two 
miles wide north of the iron formation. Outcrops are not plentiful 
but their distribution together with th(> character of the tojiograpiiy 
and the absence of exposures of other rocks warrant the assumption 
that the slate underlies practically the entire area. The formation 
strikes northeast and the secondary clcavagj' is. on the whole, inclined 
steeply soutliward. The slate is })r()l)al)ly as thick as the Tyler forma- 
tion of the Gogebic range with which it is apparently lithologically 



76 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

identical, but as in the case of the Tyler, its apparent thickness of 
about 9,000 feet may be in considerable measure accounted for by 
close folding. 

Lithology. The main types are gray, grayish green and black slate 
and fine grained grayAvacke. Exceptional phases of local importance 
are chlorite, mica, and hornblende schists. The hornblende schists 
are confined to the borders of the slate belt near the contacts with the 
southern intrusives and the northern Presque Isle granite. In the 
central portion of the belt the rocks show no evidence of extreme 
metamorphism. Microscopic examination shows that the slate and 
graywacke are composed mainly of fine fragments of orthoclase, plagi- 
oclase and quartz. The central part of the formation is cut by veins 
of quartz which become increasingly important near the borders of 
the belt. In the neighborhood of the intrusives these veins are often 
noticeably feldspathic. 

The most favorable localities for study of the unaltered slate of the 
central part of the belt are the exposures near the quarter post between 
sections 8 and 17 at the locality known as Nelson canyon, the extensive 
outcrops at Judson Falls on the slate river and the exposure at the 
outlet of the Slate river into Lake Gogebic. At Nelson canyon, a tri- 
butary of the Slate river has cut a gorge 375 feet long, 20 feet \nde, 
and from 5 to 25 feet deep, parallel to the strike of the slate. At Judson 
Falls the river tumbles in a series of low cascades over the upturned 
edges of the slate for a distance of 500 feet. The rock at both locah- 
ties is gray slate, very uniform in appearance, ^viih. here and there 
coarser graywacke phases. At Nelson canyon the slate contains red- 
dish stained breccias which apparently caught the ej^e of the early 
explorer for iron ore and led to the sinking of a few shallow pits in 
the vicinity. At the mouth of the Slate river the rock is predominant^ 
a fine grained graywacke with subordinate slaty phases. A peculiar 
feature of the coarser varieties are ellipsoidal cavities from one to three 
inches long, one half inch wide and from one half to one inch in depth. 
The cavities are in all cases oriented in the direction of schistosity. 

The southern exposures of the slate series are more or less meta- 
morphosed although their clastic character is commonly recognizable 
in the field. In the north half of section 20 there are two small outcrops 
of fine grained granitic material injected parallel to the schistosity. 
The schistosity is pronounced but the cleavage is less perfect than that 
of the northern exposures. As determined in thin sections this rock is 
composed of quartz, biotite, orthoclase, plagioclasc and chlorite with 
scattered small crystals of apatite and tourmaline. The original clastic 
structure is plainly apparent in the rounded character of the larger 
grains of feldspar. The biotite is plentiful in small grayish brown 



GEOLOGY OF THE MARENISCO RANGE. 77 

flakes with parallel alignment. The space between the larger frag- 
ments is filled with an interlocking mosaic ox quartz, secondary limpid 
feldspar and biotite. 

About 650 paces north of the south quarter post of section 15 is an 
outcrop of dense massive graywacke, the clastic structure of which is 
still plainly evident under the microscope, although a partial recrystal- 
lization has taken place. In the same section there is a small outcrop 
of the slate series in contact with diabase. Near the contact all traces 
of sedimentary textures in the slate has been obhterated by recrystal- 
hzation. 

Relations of the slate to adjacent formations. So far as kno^\m the 
contact between the slate and the underlying iron formation is not 
exposed but the relations are beheved to be those of conformity. 

The slate formation is intruded by the Presque Isle granite. The 
actual contact is exposed in two locaUties in section 4. Sheared chlo- 
ritic schist occurs adjacent to the contact, but a short chstance awa^' 
the slate differs but little in appearance from the typical exposures in 
the center of the belt. The schist adjacent to the contact is cut by 
abundant stringers and veins of granitic material. In one place blocks 
of the schist are imbedded in the granite. 

Dark colored fine grained quartz-biotite schists occur along the east 
harder of the great granite ridge that forms the west shore of Lake Go- 
gebic in section 33, T 47 N, R 42 W. The biotite schists, some of which 
are hornblendic, are intimately injected by granite and pegmatite 
and present the appearance of banded and contorted gneiss. Whether 
these biotite schists are a part of the slate series along the south border 
oi the Presque Isle granite is entirely conjectural but their occurrence 
east of the granite is of some interest and furnishes the only clue to 
the nature of the rocks occupying the low ground in the southeast 
l)art of T 47 N, R 42 W. 

IGNEOUS ROCKS. 

The entire Huronian succession is intruded by both acid and 1)asic 
rocks and the iron formation and basal (luartzite are associated with 
extrusive flows and possibly tuffs. 

Extr\isii>es. 

Altered Porphiinlcs. In scM-tion 7, T 4(1 N, I{ \\ W , ;il)out 200 paces 
north of an outcrop of the basal (luartzitc and in the strike of tlie iron 
forniMtion in T Hi N, i{ 12 \V, there is an exposure of schistose por])hy- 
ritie lava. 'I'he (»uterops occur for a distance of 30 ])aees in a narrow 
gorge on the west branch of 'i'rout ("reek. The l:i\:is present sonu> pe- 



78 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

culiar and interesting features and will, therefore, be described in some 
detail. They are highly metamorphosed and schistose parallel to the 
prevailing strike. The dip of the schistosity is nearly vertical or in- 
clined at a steep angle southward. The least altered rocks are plainly 
porphyritic but those that have suffered the greatest deformation are 
beautifully crenulated schists in which the phenocrysts may be detected 
only by careful search. The crenulated structure is caused by minute 
folding of the fibrous amphibole of the ground mass which has pro- 
duced perfect false cleavage*. In the schistose specimens phenocrysts 
of red feldspar, showdng in may cases good crystal form and attaining the 
size of one fourth inch in the longer dimension, are embedded in an 
aphanitic dull gray ground mass. At one place for a space of a few^ 
feet the rock has been brecciated and recemented by calcite which 
gives it the appearance of dolomite or limestone containing scattered 
nodules of chert. Under the microscope, however, these fine grained 
gray cherty appearing fragments are seen to be porphyrite differing 
in no respect from other porphyries in this locality. The porphyrites 
are cut out b}^ a narrow' dike of fresh diabase. 

Microscopic examination of the porphyrites discloses badly altered 
and mashed phenocrysts of feldspar and quartz, feldspar predominat- 
ing, embedded in a ground mass, which, in the schistose type, is com- 
posed of a fine grained fibrous mass of green hornblende or actinoUte 
with abundant minute grains of epidote. Lesser quantities of magne- 
tite, biotite, chlorite, quartz, limpid feldspar and calcite are present. 
Where the small fibers of amphibole are thrown into a series of close 
minute folds the slide shows beautifully the crenulated or helicoidal 
structure, while the rock as a whole has a well developed false cleavage. 
Where the rock is more massive in character the ground mass is made up 
of a very fine grained interlocking mosaic of unstriated feldspar, sub- 
ordinate quartz, grains of epidote, a few scattered flakes of biotite, 
green hornblende and occasional grains or laths of magnetite. The 
phenocrysts are orthoclase with considerable subordinate plagioclase. 
In one specimen coarse grained granulated areas of quartz were noted, 
probably representing original quartz phenocrj^sts. Except in the 
freshest types the phenocrysts show crushing and granulation around 
the edges and where the ground mass is crenulated the large crystals are 
often bent and fractured and the cracks filled with a mosaic of quartz 
grains. In all cases the feldspar crystals are badly altered and some 
of the orthoclase is filled with fine red dust. Traces of carlsbad twining 
are common but the laminations characteristic of albite are rare. 
Alteration of the orthoclase produces sericite, of the plagioclase, epi- 
dote, biotite zoisite and albite. A pecuUar and persistent feature of 

*Leith, C. K. Bulletin 239, U. S. G. S., page 49, PI. XIV B. 



GEOLOGY OF THE MARENISCO RANGE. 79 

the porphyrites is the occurrence of micropegmatitic phenocrysts of 
quartz and orthoclase. quartz always subordinate. The structure is 
emphasized by the simultaneous extinction of the angular quartz areas 
and the contrast between the clear quartz and the badly altered ortho- 
clase. Phenocrysts of this type have been noted by Iddings in rhyolite 
from the Eureka district of Nevada* and in obsidian from Yellowstone 
Park**. The ground mass of the non-schistose variety presents some 
suggestive features in the arrangement of epidote grains when observed 
in ordinary light. Fairly clear traces of flow structures appear and 
also less distinct indications of original perlitic parting. 

There can be no doubt that the rocks in this vicinity are metamor- 
phosed effusive lavas of composition near trachite. The determination 
of original character is rendered difficult by extreme alteration. 

In section 29, T 46 N, R 43 W, on the south edge of the magnetic 
belt there is an exposure of similar altered effusive porphyrites intruded 
by granite. 

Hornhlende Schist. Closely associated with the iron formation in 
section 15, 21 and 22, T 46 N, R 42 W, Michigan, are a number of 
exposures of hornblende schist. They are dark gray to greenish black 
and are extremely fine grained and schistose. The schistosity controls 
a platy parting, in many cases so perfect as to give the rock the appear- 
ance of slate. Rarely a faint banding may be observed. Under the 
microscope the schists are seen to be composed almost entirely of a 
matte of small prisms and needles of compact green hornblende. Filhng 
the interstices between the amphibole fibers are minute clear grains 
of limpid feldsj)ar (albite) with subordinate quartz. Magnetite in 
small lathshaped individuals, oriented parallel to the schistosity, 
is abundant. 

No traces of original structure or texture remain and it is therefore 
impo.ssible to assign a definite origin to these schists. It is regarded 
as possible that they are metamorphic tuft's or squeezed basic lavas, 
an inference based upon the fact that in mineral composition and 
structure they are almost identical with the ground mass of the altered 
porphyrites found in .section 7, T 40 N, R 41 W and at other locaUties 
on the Marenisco and Turtl(» ranges. 

In .section 15 the hornblende schists are intruiled by massive green- 
stone, and in sections 21 and 29 by granite. 

Relation of the Ejj'tisirc.s to the Hiironinn Rocks. The outcrops of 
fine grained hornblende schist, thought to be meiamorphic lavas or 
tuffs, are closely associated with the magnetic belt marking the posi- 
tion of the iron formation in T 16 X, l\ 12 W. Michigan. Outcrops 

♦.MoiioKruph 20 V. S. CJ. S.. p. .17.5. Plate \-2. 
♦*SfV(*:itli Aiiiiuai Itepurt. V. S. G. S., pp. 274-27(). I'latfs .\V and X-2. 



80 CONTKIBl'TIONS TO PRE-CAMBRIAN GEOLOGY. 

of this rock were found ])oth north and south of the pits in iron forma- 
tion in section 13. If these schists are altered lava flows it is probable 
that they are interbedded with the lower portion of the Huronian 
series. 

The porphyrites exposed along the west branch of Trout creek in 
section 7 are located about 250 paces north of a large exposure of basal 
quartzite. The relation between the lavas and quartzite is unknown. 
About all that can be said is that the lavas dip under the quartzite 
but whether they are stratigraphically above or below it can not be 
determined. The attitude of the regional schistosity and the general 
succession of the Huronian series indicate that they are above the 
quartzite. 

Intrusives. 

The following discussion is applicable only in T 46 N, Rs. 41 and 42 W, 
Michigan. Southwestward, intrusive rocks are found in contact with 
the greenstone porphyries in section 29, T 46 N, R 43 W. IMichigan. 
It was noted in connection with the description of the iron formation 
that a large exposure of granite occurs in the middle of the magnetic 
belt in section 4, T 45 N, R 44 W, Michigan. The position of this 
outcrop with reference to the magnetic belts in that locality suggests 
that the granite is intruded into the Huronian series. No other Huronian 
intrusives are knowTi southwest of T 46 N, R 42 W, Michigan. How- 
ever, the strong magnetism together with the metamorphic character 
of the exposed rocks is presumptive evidence that igneous intrusion 
has played an important role in metamorphism throughout the entire 
range. 

Inasmuch as the intrusives form the most abundant exposures in 
T 46 N, Rs. 41 and 42 W, Michigan, and are believed to have been 
the most important agents in the metamorphism of the Huronian 
rocks they will be dicussed in considerable detail. 

The following description is not a thorough treatise of these rocks. 
From data at hand regarding the field relations and the limited number 
of thin sections available for study we can attain no more than a broad 
view of the field as a whole with the hope of bringing out the more 
general relations and characteristics of the intrusives. 

From the standpoint of composition, the intrusives fall, roughly, 
into three general classes; viz., Jiornhlende hearing greenstone, granite, 
and fresh massive diabase. All three types are found in contact with 
both the Huronian rocks and the southern schist series. Whether the 
intrusives in the southern schist belong to an earlier period of igneous 
activity, or are contemporaneous and to be correlated, in whole or in 
part, with similar rocks intrusive in the Huronian series can not be 
determined with certainty. However, consideration of their petro- 



Plate V. 



(A). (Without aiializer. X 10). Amphibole-magnetite-quartz rock from 
test pits in the NWH of NWV4 of section 12, T. 46 N., R. 41 W.. Michigan. 
The rock is typical of the metamorphic iron formation found on this part of 
the Marenisco range. The black specks are magnetite, and fibrous mineral is 
griinerite and actinolite. the light areas are finely crystalline quartz and iron 
carbonate. The intimate association of the amphibole and magnetite with 
the siderite, while not apparent in the plate, is a marked feature of this rock. 



(B). (Without analizer, X 16). Iron formation with intermixed detrital 
material from exposure along the Chicago and Northwestern Railway one- 
half mile south of the station of Marenisco, Michigan. The clastic grains of 
quartz and feldspar are easily seen. The remainder of the rock is composed 
of magnetite, chert, carbonate, amphibole and chlorite. 



Michigan Geological and 
Biological Survey. 



Publication 18. Geology 15, 
Plate Y. 




lAj 




(15) 



Plate VI. yi 



(A). (Without analizer, X 16). Greenstone porphyrite from SW14 of 
NWi/4, section 7, T. 46 N., R. 41 W., Michigan. This is one of the more basic 
varieties from this locality showing the development of false cleavage in 
the arrangement of the hornblende needles of the groundmass. Near the 
center of the plate is a phenocryst of plagioclase showing saussuritic altera- 
tion in the center. Under crossed nicols the plagioclase phenocryst exhibits 
granulation around the edges. 



(B). (With analizer, X 16). Feldspathic biotite-schist from Archean (?) 
area lying south of the east end of the Marenisco range. This is one of the 
coarser grained and thoroughly crystalline types. It is composed of plagio- 
clase (albite and albite-oligoclase) quartz and biotite. Accessory minerals 
are sericite, epidote, apatite and zircon. These rocks are believed to be 
altered sediments. 



Michigan Geological and 
Biological Survey. 



Publication 18, Geology 15. 
Plate VI. 





'wm 



(H) 



GEOLOGY OF THE MARENISCO RANGE. 81 

graphic character and, more especially, the relation of the various 
types to each other in both areas renders it likely that in the case of 
the diabase and granite, and probably in large part the greenstone 
also, most of the similar intrusives in both the Huronian and Archean 
are of about the same age. 

That the three main tj'pes, greenstone, granite, and diabase, repre- 
sent three fairly definite periods of igneous activity seems to be estab- 
lished from a consideration of the field evidence. A more detailed 
study would undoubted modify this statement to a considerable extent, 
and would probably furnish evidence of a greater number of periods 
of intrusion, but it is believed that these would be largely in the nature 
of substages under one or more of the three major divisions. 

The earliest intrusives are the greenstones. These rocks comprise 
altered diabase, diorite, and possibly gabbro. They are regarded as 
older than the fresh diabase and granite because where the relations 
were observed, the granite and diabase intrude the greenstone. Fur- 
thermore, the greenstones are in all cases badly altered and largely 
converted into schists in which nearly all, and in extreme cases all, 
traces of original textures and structures are obliterated. On the other 
hand, the granite, while in places rendered gneissose, is generally mas- 
sive and fresh in appearance. The characteristic lath-shaped feldspar 
is a i)r()n()unced feature of the diabase. Thin sections exhiliit fresh 
feldspar and the augite shows httle uralitization. 

While the evidence that the diabase and granite are younger than 
the greenstone is conclusive, the relation between the diabase and 
granite is ol)scure, as no contact of the two were ol)served. The ex- 
treme freshness of the dial)ase cou])led with the fact that the granite 
nowhere intrudes the Kewcenawan, whereas the diabase does, lends 
presum[)tion in favor of the older age of the granite. 

The most abundant types of inti'usive rocks are hornblende hearing 
green~'<tone and green schists. The only mini'ral that can be universally 
recognized in hand specimen is green hornl)lende. Occasionally, in 
the more massive varieties, feldspar, or rather gray saussurite, can hv 
recognized in blotches between the green li()rnl)leiide crystals. In 
general, the structure is massive l)ut where the rocks are in contact 
with granite they are charactcM-izc^d by a markedly schistose structure 
and devel(»|)nient of considerable biolite wiiich ma>' entirely' replace 
the hornblende. Near some contacts the schists are cut l)y many 
stringers of granitic material and i)egmatiti<' veins and here and there 
blocks of schist are enclosed by granite, in olhei' cases the contact 
is sharp and about parallel to the strike (»f the i)revailing schistosity. 

The most striking and constant features of the greenstone are th(> 
universal pl-esence of green hornblende and the ba(ll\' altered condition 
II 



82 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

of the rock as a whole. The minerals are green hornblende, plagio- 
clase, quartz, chlorite, biotite, epidote, magnetite, leucoxine, calcite, 
zoisite, saussurite, sericite, rutile, anataze and apatite. Badly altered 
original feldspar with a tendency toward idiomorphic development in 
the prism zone is ordinarily present in the massive types. Traces of 
ophitic and granitic textures are not uncommon but more often, even 
in the massive types, original textures and structures are nearly obli- 
terated. The feldspar is almost destroyed, but obscure twinning lamellae^ 
are occasionally visible in a mass of alteration products. The common 
alteration products of the feldspar are quartz, albite, epidote and 
calcite. Cloudy saussurite is less often observed. The hornblende is 
in large green fibrous individuals generally showing a tendency towards 
crystal development in the prism zone but fraying out at the ends. 
In some cases this mineral is compact but in general it is sedgy and 
fibrous. Where the hornblende is compact it is commonly cellular. 
Alteration of the hornblende to chlorite may proceed to such extent 
that chlorite predominates over the amphibole. No trace of original 
augite occurs although hornblende is occasionally pseudomorphous 
after the pyroxene. Biotite is a minor constituent of the massive 
greenstone and occurs in part as a parallel intergrowth with horn- 
blende, and in part as an alteration product of feldspar. Irregular 
grains of epidote are abundant. Magnetite is an important accessory 
mineral, largely associated with clusters of rutile or surrounded by leu- 
coxine. Irregular blotches and small grains of calcite are very conamon. 

In summary, the greenstones are all badty altered; green hornblende 
is a universal constituent; wherever traces of original texture or struc- 
ture remain they are those of igneous rocks. Owing to the badly 
altered condition of the feldspar, and the entire absence of primary 
augite it is extremely difficult to define the exact original character of 
the rock from which they were derived but the majority are probably 
altered diorites and diabase. Hornblende pseudomorphs after augite 
probably means that gabbro is included in the parent rocks. However, 
the common idiomorphic development of feldspar in the prism zones, 
and the development of secondarj^ albite and quartz rather than saus- 
surite favor the assumption that diorite or diabase was the prevailing 
original rock. On the other hand, the tendency of the pyroxene to pass 
over into green hornblende or uralite may account for the present 
apparent absence of rocks of the gabbro type. 

Granite. Granite is exposed in two areas in the southwest portion 
of T 46 N, II 42 W, and the west central portion of T 46 N, T 41 W. 
The rocks are light gray and pink and for the most part massive and 
fresh in appearance. Near the contact with the greenstone the granite 
is usually gneissose. In texture it varies from fine grained normal 



I 



GEOLOGY OF THE MARENISCO RANGE. 



83 



granite to coarse pegmatite. The minerals include orthoclase, plagi- 
oclase, microcline, quartz, biotite^ sericite, chlorite, hematite, pyrite 
and apatite. The orthoclase and iDlagioclase show considerable altera- 
tion l>ut the microcline is verj^ fresh. The granite exhibits no other 
petrographic features of importance. 

Diabase. Fresh massive "diabase occurs in abundance in dikes and 
small knobs. It is dark greenish gray to black with an occasional 
outcrop shoA^^ng a reddish tinge. The rock is easily recognized by the 
universal presence of lath shaped feldspar. ]\Iany of the dikes cut the 
greenstone at wide angles to the prevailing schistosity. Under the 
microscope the diabase appears to be very fresh or only slightly altered. 



j^POST BETWEEN 
SECTIONS 11 AND 14 




s s- 



-^M 



ARCHEANf?) 

wm 



LEGEND 

KEWEENAWAAJ 



PLEISTOCENE 



T~ 



s 



MICA SCHIST 



SANDSTONE 



DIABASe GLAC!Al DRIFT TEST PIT 
, 30O Fi. 



Figure 3. Diagram sliowiiig relations between Keweenawan sandstone, trap and intrusive dia- 
base in sections 11 and 14, T 46 N, R 41 W. (After Seaman). 

The minerals are plagioclase, augite, olivine, uralite, serpentine, magne- 
tite and hematite. The texture varies from fine grained basaltic to 
coarse grained. The felds))ar is idiomorphic giving the rock a well 
develoiK'd opjiitic texture. The interstitial material is augite, some- 
what altered to uralite and f)livine showing alteration to serpentine, 
hematite, and magnetite. 

Rddlions of the fnlnisiirs Id the AUjonkian and Archcan. Dialja.se 
is intrusive into (he lower sandstone member of the Keweenawan in 
section 14, T 4() X, I{ 41 \V. This relationship was noted by A. K. 
vSeaman* in 1891 duiing the examination of the South Ti;ip r,ing(\ 

♦Unpubllshw! field noti-. Midiigaii Geological Survey. Notebook 8.". 



84 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

There are a number of pits on the slope of the rise leading to a steep 
southwest facing bluff of Keweenawan trap (see Fig. 3, reproduced from 
drawing in Seaman's notebook). Diabase, showing the characteristic 
lath shaped feldspar, occurs in a shaft 35 feet deep about half way up 
the slope of the hill. About 35 feet southwesterly from the shaft and 
15 feet down the slope is a shallow pit of sandstone (quartzite). Seaman 
describes this occurrence as follows; "The rock is quite hard and 
appears to be indurated by the diabase. The dip is under and toward 
the diabase at an angle of about 12°. The rock is composed of finely 
rounded grains of c^uartz and minute reddish specks of feldspar material 
evenly distributed through the mass with what appears to be a silice- 
ous content." Twenty five feet southwest of the pit in sandstone and 
about 20 feet lower is another pit "in a somewhat closely consolidated 
aggregate of rounded grains of quartz and feldspar with a cement of 
iron oxide and silica. The dip here is the same as in No. 2 and the rock 
bears no evidence of disturbance. The strike as indicated by the dip 
is a little south of east." The above facts establish with certainty 
that the diabase is later than the lower sandstone member of the Ke- 
weenawan series. Correlation of the many diabase dikes and knobs 
south of this locality wdth the rock here exposed must rest on litho- 
logical similarity alone. Fresh diabase intrudes sedimentarj^ members of 
the Huronian as well as the greenstone effusives of probable Huronian 
age in section 7, T 46 N, R 41 W. It also intrudes the mica schist on 
the southern Archean area, and the northern Laurentian granite. 

The southern granite was not observed in contact with the Upper 
Huronian sediments, but certain of the specimens from the test pit 
dumps, section 22, T 46 N, R 42 W, show intimate injection of felds- 
pathic material and the slate formation in section 20 of the same town- 
ship is cut by narrow pegmatite dikes. In sections 29 and 30, T 46 N, 
R 42 W, granite is intrusive in fine grained hornblende schist described 
as metamorphic Huronian lava or tuff. The mica schists of the southern 
complex are intruded by granite in section 8, T 46 N, R 42 W. Aside 
from the above occurrence granite was found in contact only with 
greenstone. 

The predominating intrusive in T 46 N, Rs. 41 and 42 W, as before 
noted, is the greenstone. Actual contacts between this rock and the 
micaceous schists of the southern complex are plentiful and in all 
cases the greenstone is intrusive in the schists. In section 22 the green- 
stone intrudes the graywacke at the base of the Upper Huronian. 
Although the greenstone is not known to intrude the iron formation, 
abundant outcrops of the former in close proximity to the test pits, 
the finding of igneous material mixed with the iron formation in some 
of the pits, and the general metamori)hi(' character of the iron forma- 



GEOLOGY OF THE MARENISCO RANGE. 85 

tion are evidence that the greenstone is a'so intrusive into the iron 
bearing rocks. The coarsening of grain and the tendency toward 
recrystalHzation shown ]\y the southern part of the slate series is pre- 
sumptive evidence that this formation has also been intruded l)y 
igneous material, although it is apparent from the character of the 
central exposures and general lack of outcrops of igneous rocks inside 
the slate area that this formation has been the least affected by the 
tremendous igneous activity which wrought such extreme metamor- 
phisni in the two lower formations. 

There can l)e no doubt that the metamorphism of the Huronian 
rocks, both sediments and lavas, is mainly the result of igneous intru- 
sion and injection. The type of metamorphism is that characteristically 
induced and affected by igneous action. Abundant iron amphibole 
and magnetite, the completely recrystalHne character of the quartz, 
the occurrence of intimately injected fe!dspathic material in the iron 
formation and the entirely recrj^stalline character of the basal quartzite 
and graywacke are evidence of metamorphism induced by igneous 
intrusion. 



CHAPTER V. 
GEOLOGY OF THE TURTLE RANGE. 

R. C. ALLEN AND L. P. BARRETT. 

INTRODUCTION. 

The Turtle Range extends from T 41 and 42 N. R 1 W, Wisconsin, 
northeast about 76 miles to the central portion of T 46 N, R 40 W, 
Michigan, where it is cut off b}^ the overlapping Keweenawan rocks 
of the South Trap Range. The range has been traced by magnetic 
survey about six miles farther east underneath the Keweenawan sand- 
stones and trap to the vicinity of Barclay in T 46 N, R 39 W. (?) Its 
position is marked, except for short breaks at each extremity, by a 
continuous magnetic field. In many places there are double, triple, 
and even cjuadruple parallel belts. The Turtle range is separated from 
the Marenisco range north of it l)y a territory- from two to seven miles 
wide in which there is little magnetic distortion and, except in the 
northeastern i^ortion, no rock exposures. The geology of the territory 
between the two ranges was described in connection with the Marenisco 
range under the heading, "Southern Archean Area." 

The Turtle range is separated for the greater part of its extent from 
the Manitowish ranges south of it by a similar territory almost devoid 
of rock exposures and characterized by absence of magnetic belts. In 
places, however, the Turtle and Manitowish ranges are apparently 
connected by magnetic belts bridgi^ig the stretch of non-magnetic 
territory, although in no instance can a magnetic belt of either range 
be traced to a dire^ct connection with a magnetic l)elt of the other. 
This relationship is shown on fig. L In T 45 N, R 41 W, Michigan, 
the magnetic belt which has an east west strike through the south 
half of the township to the east takes a sudden turn to the north and 
can !)(' traced as far as section 2 of the former townshij). There is an 
apparent connection of the Turtle and Manitowish ranges in T 43 N, 
R 6 E, Wisconsin, in tiie vicinity of sections 14 and 15. In T 41 and 
42 N, Rs. 2 and 3 K, Wisconsin, there are nine i)arallel magnetic belts 
separated b>- iianow intervals of non-magnetic territory. Of these we 
include the three south Ix'lts in the Manitowish range and the six north 
belts in tlie Turtle r;mge. Hock expo.-^ures are known in the area be- 



88 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

tween the two ranges only at the northeastern extremity in T 46 N, R 
39 W, and T 45 N, R 39 W, Michigan. 

The Turtle range comprises rocks of both the Middle and tlie Lower 
Huronian series. There are exposures and explorations on its opposite 
ends, but its middle section of 25 miles from the Banner locality in 
section 1, T 45 N, R 43 W, Michigan, southwest to Mercer, Wiscon-sin, 
is entirely drift covered. The succession and character of the sediment- 
ary rocks are determined mainly from data obtained from explorations 
for iron ore. Natural exposures of the sediments are almost entirely 
lacking but in some localities there are abundant exposures of the 
associated intrusive and extrusive igneous rocks. 

Explorations for iron ore were conducted many years ago in Wis- 
consin at the old Michigan mine in section 22 and at the Broomhandle 
location in section 29, T 42 N, R 1 E; on the Lucas-Ford and White- 
side farms in sections 5 and 3 T 41 N, R 1 E, at other localities in this 
township, and in T 42 N, R 2 E. In Michigan the only exploration 
of importance is at the Banner mine in section 1, T 45 N, R 43 W. 
Kecent diamond drilling by R. B. Whiteside and associates in section 
4, T 41 N, R 1 E, Wisconsin; by the F. L Carpenter syndicate in the 
vicinity of Mercer and Winegar, Wisconsin, and by the E. J. Longyear 
Company at the Banner locality, has added definiteness to some of 
our earlier conceptions of the geology of the Turtle range and has 
furnished the only evidence of the occurrence of the Lower Huronian 
series. As in the case of the Marenisco range, we know of no reference 
to the existence of the Turtle range in geological literature. In 1877, 
F. H. King* mentioned the occurrence of granite, hornblende, and mica 
schist on the Turtle river in the southern part ofT41 N, R 1 E, and 
the occurrence of greenstone at Turtle Falls in section 6, T 42 N, R 
3 E, Wisconsin. 

The following expresses the succession of formations so far as known : 

*Geol. of Wisconsin, Vol. IV, Part IV, Geology of the Flambeau Valley, by F. H. King. 



GEOLOGY OF THE TURTLE RANGE. 



89 



SUCCESSION ON THE TURTLE RANGE 
Keweenawan (?) 



Intrusive diabase, granite 
and greenstone. 







Granite 






Effusive, agglomeratic and 






ellipsoidal greenstone. 






Basic tuffs (•?). 




' Middle < 


Black slate and graphitic 


Algonkian 


Huronian 


schist. 




(Animikie) 


Iron formation. 
^ Quart zite and mica schist. 




Unconformity (?) 




Huronian ^ 




Mica schist (^lay be Mid- 




Lower 


dle Huronian.) 




Huronian 


Dolomite and dolomitic 
quart zite. 




. 


^ Quart zite. 




— Unconformity 




Archean 


Keewatin (?) 

ARCHEAN. 


Mica schist and green 
schist. 



With the exception of those described in connection with the Maren- 
isco range, exposures of Archean rocks are confined to the territory south 
of the east extremity of the range. 

The mica schists and banded mica and hornblende gneisses exposed 
along the Flambeau River in T 41 X. R 1 E; T 41 N, R 2 E; and T 
42 N, R 2 E, Wisconsin, are assigned doubtfully to the Archean merely 
because there is no evidence of later age. These were described by 
King in 1877. Garnetiferous feldspathic biotite schist occurs also in 
test pits near the Nj corner of section 11, T 41 N, R 1 E. The strike 
of the gneissose and schistose structures of these rocks is in general 
N 45° to 60° E. The schist from the pits in section 11 is very similar 
to the mica schist from tlie Soutlicrn .\rchean area of the Marenisco 
range. 



90 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

HUROXIAX CROUP. 
LOWER HURONIAN SERIES. 

Dolomite and quartzite. The Turtle range has been cross-sectioned 
by recent diamond drilling at Winegar, Wisconsin, by the F. I. Car- 
penter syndicate, and at the Banner location, Michigan, by the E. J. 
Longyear Company. At both localities dolomite underlies a slate iron 
formation series. The dolomite is massive, light colored and recrystal- 
line, very similar in general to the Lower Huronian dolomite forma- 
tions of the Marquette, Menominee, Crystal Falls, and Gogebic dis- 
tricts but apparently lacks the characteristic chert bands of these 
formations. The dolomite in the Winegar section is almost pure cal- 
cium magnesium carbonate and contains few other minerals or impuri- 
ties. Cores from a drill hole on the south end of the Winegar section 
show a dolomitic quartzite with abundant development of tremolite; 
the hole on the extreme opposite end of this section shows a vitreous 
ciuartzite and mica schist intruded bj' granite and greenstones. It is 
therefore possible that the dolomite is underlain by quartzite in which 
event the analogy between the Lower Huronian here and on other Mich- 
igan ranges is practically complete. 

There is evidence in the drill hole sections at Winegar that the dolo- 
mite is strongty developed; it can hardly be less than 1000, and it may 
be as much as 2000 feet thick. 

The drilling does not determine an unconformit}' between the dolo- 
mite and the overlying slate-iron formation series for none of the holes 
cross the contact horizon. On the basis of analogy with other Mich- 
igan ranges unconformable relations may be assumed pending definite 
determination. 

MIDDLE HURONIAN (aNIMIKIE) SERIES. 

At the west end of the range the Middle Huronian is represented by 
quartzite, iron formation, and slate, the succession characteristic of 
this series in the greater part of the Lake Superior region. In the 
east central portion of the range, at the Banner mine and at Winegar, 
black slate apparently underlies the iron formation, and at both locali- 
ties the rocks are compressed in close synclinal folds and the formation 
overlying the iron bearing rocks has been removed. The Middle Hu- 
ronian series is associated with effusive greenstones showing in many 
places ellipsoidal and agglomcratic structures and porphyritic textures. 
Rocks of this type are abundantly exposed from Mercer, Wisconsin, 
southwestward to the vicinity of the old Michigan mine. On the 
opposite end of the range from the Banner location northeastward 
outcrops of effusive greenstones are plentiful. In general it may be 



GEOLOGY OF THE TURTLE RANGE. 



91 



said that basic effusives are characteristic of the range in all locahties 
where rocks are exposed, and that they occur throughout the range is 
a reasonable assumption. In certain localities the Huronian rocks 
have been intruded by diorite, diabase, and granite. 

The sedimentarj^ rocks of the Middle Huronian ma}^ be studied to 
best advantage in townships 41-42 N, R 1 E, Wisconsin, where many 
old exploration pits furnish a basis for a definite determination of the 
series. Natural exposures are limited to a single outcrop of iron forma- 
tion found at the Michigan mine locality in section 22, T 42 N, R 1 E. 
At this end of the range the Huronian rocks are apparentty throwai 



^\J-f Comr 






o 


/I 


/ 
k 




A 

\ 

\ 




( 


1 


^^^\^^ - ^ 






-^ 


.1 








•\ 


^^ 








,^^ ^ 




--^ 


-^ ■ • \ sio.1* 








1 

s 


PLAT AND SECTION 


fl 




n 


of 








LUCAS-FOKD 




m 




EXPLORATION 

SECTION 5. 

T.41.N.R.1E. 

WISCONSrN. 

SCALE. 






iSib W. 



iMgure 4. 



into a number of close parallel folds wliose position is marked by parallel 
magnetic belts. 

I'>om the present information description of a general succession 
applicable to the entire range is impossible. We will therefore describe 
in detail th(> rocks from a number of dilTerent localities. 

Ford-JAicax Section (Fig. 4)- At the Fortl-Lucas exploration (NE^ 
of the NIOJ and SEJ of the NEi sec. 5, T 41 N, R 1 E, Wisconsin) 
the Mi(l(ll(> Huronian rocks were cross sectioned many years ago by 
a number of lest pits. The material thrown out on the dumps of 
these pits furnishes al)undant evidence of the character of the under- 
lying rocks. The successi(jn is ciuartzitc-iron formation-slate forming 
the north limb of a syncline, both limbs of wiiich are indicated in the 



92 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

Whiteside exploration one and three quarters miles east. No other 
rocks are exposed at this locality. The accompanying figure shows 
the location of the pits. There are a number of other pits in the vicinity 
but their dumps give no indication that bed rock was opened in them. 
No natural exposure of the Middle Huronian sediments is known at 
this locality. 

The basal quartzile is exposed in the two most northerly pits while 
additional information as to its character is furnished by a recent drill 
hole put down by R. B. Whiteside near the northwest corner of section 
4. This hole is reported to have passed through 400 feet of muscovite- 
quartz schist. The rock exposed in the test pits is a dense, fine grained, 
faintly banded, gray quartzite through Avhich are scattered numerous 
small cubes of pyrite. It is composed of quartz, pyrite, and innumer- 
able particles of black dust. The quartz is entirel}^ recrystalline and 
in thin section is arranged in a finely grained mosaic of roughly hex- 
agonal individuals. Pyrite is plentiful in small cubes, slightly larger 
than the individual quartz crystals, and the black dust is scattered 
throughout as inclusions in quartz. Metamorphism has obliterated 
all traces of clastic texture but the mineral composition and the pres- 
ence of definite though faint banding admit of no doubt that the rock 
is an altered sediment. The thickness of this formation is not less than 
150 feet and is probably greater than 300 feet. 

The iron formation is exposed in a number of pits south of the quartz- 
ite. In general, it may be described as finely banded, dark gray to 
black, siliceous amphibole-magnetite schist carrying a little residual 
carbonate. Narrow bands of bro^^^l amphibole and evader bands of 
rather coarsly crystalline quartz are abundant. The wider, light colored 
bands are mainly ciuartz, the magnetite and amphibole being concen- 
trated in the narrower and darker colored layers. The important 
minerals are quartz, griinerite, and magnetite with subordinate car- 
i)onate, (probably siderite) and brown iron oxide. In general, the 
minerals are arranged in definite layers or bands. The quartz is entireh' 
recrystalline and appears in thin section as a typical interlocking 
mosaic. The coarser grained bands are nearly pure silica, while those 
of finer grain contain abundant magnetite. The amphibole is, in all 
specimens examined microscopically, griinerite, but it is very probable 
that the varieties actinolite and commingtonite are present. Griinerite 
commonly occurs in fairly large individuals oriented parallel to the 
banding; to a less extent as small twinned individuals and in compact 
bundles of needles. In the larger griinerite individuals alteration to 
brown iron oxide occurs along cleavage cracks and around the borders. 
Many irregular shaped crystals of magnetite are included in griinerite 
and the contact between the griinerite individuals is often marked by 



GEOLOGY OF THE TURTLE RANGE. 93 

concentration of magnetite and carbonate. Certain bands are made 
up of small, well formed, twinned crystals of griinerite with considerable 
magnetite, the latter partly in parallel intergrowth with the former. 
The grunerite has a well developed parallel alignment. 

Unaltered ferruginous chert and cherty iron carbonate are not 
found in this locality. The formation is highly metamorphic and offers 
httle promise of containing workable bodies of iron ore in the immediate 
vicinity. The thickness is not less than 650 feet, and probably not 
more than 700 feet. 

The overlying formation, exposed in the two southerly pits, is soft 
black spotted slate that w^eathers yellow. Under the microscope small, 
well crystallized, colorless garnets appear imbedded in an exceedingly 
schistose ground mass which under high magnification is resolved into 
a felty mass of minute flakes of biotite between which can be dimly 
seen limpid grains of dull gray quartz and feldspar (?) filled with car- 
bonaceous dust. The ciuartz and feldspar (?) are barely discernible 
because of the abundance of the mica and carbonaceous inclusions. 
The garnets, which give the rock the spotted appearance in hand 
specimen, are colorless and usually well crystallized. The schistosity 
curves around them producing a characteristic augen structure. 

No data is available upon which to base a determination of the thick- 
ness of the slate. In Fig. 4, the dip given to the formation in the cross 
section is entirely theoretical. The drill cores from the hole shown in 
the NE corner of the section indicate a very steep dip. Inasmuch as 
the structure in this region, as revealed a short distance east, is probably 
synclinal, a southerly dip was assumed in constructing this section. 

Whiteside Exploration (Fig. 5). The Upper Huronian series here 
apparently forms a syncline with opposite limbs expos(>d in the explo- 
ration pits. The north limb of the syncline is continuous with the 
Lucas-Ford exploration to the Avest. The succession at the latter lo- 
cality is repeated in the Whiteside pits. 

The basal rjunrtzitc so well exposed at the Lucas-Ford locality is 
represented here l)y a few large angular fragments of hard, gray, vitre- 
ous quartzite thrown out on the dump of the southernmost pit. 

The slate is almost identical in character with that in section 5. It 
is a typical, |)yritic, black slate. 

The iron formation is well cxjxtscd in the dumps around the old shaft 
and pit on the north liml) and on the dunii>s of several i)its on the south 
liinl) of the syncline. On the north limh it is for the most part a rather 
lean; linionitic chert containing considerable magnetite and a littl(> 
unaltered iron carbonate. The rock is typically banded, yellowish- 
l)rown limonitic bands alternating with other bands ('omi)osed pre- 
dominantly of while, gi-ayish-black. and occasionally rechlish chert. 



94 



CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 



Iron amphibole is noticeably absent at this locality. In general, the 
formation is soft, somewhat porous, and shows no evidence of having 
been subjected to severe metamorphism but it is lean and shows little 
concentration of iron oxide. 

The formation of the south limb of the fold, so far as known, con- 
trasts sharply with that of the north hmb just described. It is an 




Quartziie^f^ 



IDEAL SECTION. 

SECTION 3. 
T.41.N.R.1.E. 
WISCONSIN. 

SCALE. 



Figure 5. 



amphibole magnetite schist similar to that at the Ford-Lucas explora- 
tion. The chief minerals are amphibole (mainly grunerite with sub- 
ordinate actiholite), quartz, magnetite, carbonate and broAvn iron 
oxide, the latter apparently the result of surface weathering of the 
grunerite. It is needless to add that the formation in this vicinity is 
unpromising from an economic standpoint. The thickness here is 
apparently the same as at the Ford-Lucas locality, namely, in the 
neighborhood of 650 feet 



GEOLOGY OP THE TURTLE RANGE. 



95 



The cross section, (Fig. 5), is in part ideal. It is probable that the 
fold is much deeper than represented in the draw-ing. The boundary 
between the iron formation and quartzite on the north limb of the 
syncline is placed just north of the shaft on the basis of statements 
made by residents of the locality who worked here at the time explora- 
tion was active. Such information is not considered relialile. 









e 


















N 

i 








Mvoa. 


&Am< 




\ 



























B 

a 
a 
D 


















B 


k \ 








a 


1 












a 














^ = 












? — 


a 


GreensUrm 
A 




^ S. 1-4. Cor. : 


a- 








-A 


PI^T OF TEST PITS 

AND 

SECTION 


^*^ 


li^ 


rtiHnJrVVni'rU' 


%\ '^'^''^ 




m 


M ^ 




AT 
MICHIGAN MINE. 

8EC.22.T.42 N.R.I. E.WIS. 
SCALE 


voaWa 


''■.VX\ 




660.FT. 



Figure 6. 

Michiqan Mine. {Fig. 6.) The old Michigan mine is located in the 
SEi of the SWl of section 22, T 42 N, R 1 E. The workings consist 
of a number of test pits and a shaft. The dump adjacent to the shaft 
contains about 2,000 tons of iron formation but none of the other 
workings show very extensive exploration. 

The rock exposed in the northernmost pits is a soft, gray, rather fine 
grained tnvscovitc schist which in thin section is observed to be composed 
mainly of (luartz and muscovite with considerabh^ grajihite; accessory 
minerals are biotite, cpidote and hematite. The nuiscovite, biotite 



98 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

and graphite flakes possess a decided schistose structure, and even the 
individuals of quartz forming the crystalHne mosaic filling the spaces 
between the micaceous minerals show a tendency toward elongation 
parallel to the alignment of the muscovite crystals. The epidote seems 
to have formed after the development of the schistosity as it occurs in 
fairly large, fully formed crystals whose growth has pushed aside the 
schistose ground mass. Although no traces of sedimentary structures 
or textures remain there can be little doubt from consideration of the 
mineral content, and especially the presence of graphite, that the 
schist is a metamorphic sediment. 

As to the question of the position of the schist with reference to the 
iron formation no direct evidence is available. The dip of the rocks 
in this vicinity, as revealed by the single known outcrop of iron forma- 
tion, is toward the south at an angle of about 85 degrees. If the schist 
is regarded as belonging to the upper slates, overturned folding is 
indicated. The presence of graphite in the schist renders it probable 
that this rock should be placed in the overlying slates rather than in 
the basal member of the Upper Huronian which is ])redominantly a 
hard vitreous quartzite. On the other hand the rocks underlying the 
iron formation are in places mashed to micaceous schist as shown by 
the drill holes in section 4, T 41 N, R 1 E. 

The iron formation is typical, lean, banded "hard ore jasper." In 
general the bands are white, dark gray or black and occasionally red. 
The iron is in the form of magnetite and hard blue and specular hem- 
atite. Amphibole and chlorite are sparingly developed. It is possible 
with considerable search to pick up small pieces of ore that contain 60 
per cent metallic iron, but in general the formation is lean and cherty. 
In thin section, quartz, magnetite, hematite and siderite, with occa- 
sional needles of amphibole and flakes of chlorite, appear as the con- 
stituent minerals. 

The thickness of the iron formation is probably in the neighborhood 
of 650 feet, about the same as at the Whiteside and the Ford-Lucas 
localities 'in the township adjacent on the south, although the exact 
thickness cannot be determined from the pits. 

Ellipsoidal greenstorie occurs a short distance south of the exploration 
in section 27. Its relation to the iron formation is unknown. It is 
probable t"hat effusive greenstone is interbedded in the Middle Huronian 
series. The petrographic character of these rocks will be considered 
later. 

Broomhandle exploration. In the NWj of the NEj of section 29, T 
42 N, R 1 E, there are five pits on the magnetic belt that marks the 
position of the iron formation at the Michigan mine in section 22. 
Highly metamorphic, lean, iron formation and a coarse grained, altered 



Plate VII. 



(A). (Without analizer, X IG). Black slate from test pits of Ford-Lucas 
exploration, SEi/4 of SE14 of section 5, T. 41 N, R. 1 E., Wisconsin. Second- 
ary garnet imbedded in a schistose groundmass composed of quartz, feldspar 
(?), carbonaceous material and biotite. Note that the garnet during growth 
has pushed aside the groundmass. 



(B). (Without analizer, X 16). Griinerite-magnetite-schist from drill 
hole 17, SE14 of SW14 of section 26, T. 48 N., R. 3 E., Wisconsin. The sec- 
tion shows only griinerite and quartz but magnetite is abundantly developed 
In this rock. The concentration of the amphibole in thin bands is well shown. 



Michigan Geological and 
Biological Survey. 



Publication 18. Geology 15, 
Plate VII. 







(A) 




(15) 



Plate VIII. 



(A). (With analizer, X 16). Altered porphyrite from section 22, T. 42 
N., R. 1 E., Wisconsin, near Michigan mine. The plate exhibits a large 
phenocryst of plagioclase showing granulation and fracturing. The ground- 
mass is a schistose matte of green hornblende needles with scattered grains 
of epidote and a few irregular patches of magnetite. 



(B). (With analizer, X 16). Altered porphyrite from east end of Turtle 
range near south quarter post, section 27, T. 46 N., R. 41 W., Michigan. The 
normal composition of this rock is essentially similar to that in (A). The 
groundmass is coarser grained but is predominantly green hornblende with 
a small amount of magnetite, epidote, limpid feldspar and quartz. The large 
phenocrysts are plagioclase. 



Michigan Geological and 
Biological Survey. 



Publication IS, Geology 15, 
Plate VIII. 





iTw 



»>4^ 




(B) 



Plate IX. 



(A). (Without analizer, X IG). Fine grained liornblende scliist from 
SE14 of NE14 of section 33, T. 46 N., R. 41 W., Micliigan. This plate shows 
the appearance of the fine grained hornblende schist of the Turtle range 
which is regarded as metamorphic basic lava or tuff. Mineralogically, this 
rock is identical with the groundmass of the altered porphyrite. 



(B). (Without analizer, X 16). Altered basic amygdaloidal lava from 
near the center of section 4, T. 42 N., R. 3 E., Wisconsin. This rock is one 
of the effusives from the Turtle range west of Mercer, Wisconsin. The 
amygdaloidal structure is well shown in the figure. The filling is epidote, 
zoisite and quartz. The groundmass is largely composed of compact needles 
of green hornblende. 



Michigan Geological and 
Biological Survey. 



Publication IS. Geology 15, 
Plate IX. 




(A) 




Plate X. 



(A). (With analizer, X 16). Diabase from dike cutting basic lavas in 
the NW14 of SW14 of section 34. T. 43 X., R. 3 E., Wisconsin. This plate 
is introduced to show the extremely fresh appearance of the diabase dikes 
found on the Turtle and Marenisco ranges. Minerals present are plagioclase, 
augite, magnetite and olivine. The olivine is generally more or less altered 
to serpentine but the other minerals are fresh and show only incipient altera- 
tion. 



(B). (With analizer, X 16). Altered diabase in the SW^ of section 15, 
T. 42 N., R. 2 E., Wisconsin. This is one of the more readily recognizable 
massive greenstones abundant on the Turtle and Marenisco ranges. Contrast 
the appearance of this rock with the diabase of (A). Consideration of the 
mineral composition of the altered diabase shows that it was probably identi- 
cal with that in (A), with the exception that the rock in (B) is apparently 
lacking in olivine. It is made up of altered feldspar, the original lath shapes 
of which are barely discernible, magnetite, leucoxine and green hornblende, 
the latter probably derived from augite although no trace of pyroxene now 
remains. 



Mielugan Geological and 
Biological Survey. 



Publication IS. Geology 15, 
Plate X. 




I A) 




(H) 



GEOLOGY OF THE TURTLE RANGE. 97 

diorite appear on the dumps. The formation is mainly dark gray, 
recrystallized chert with occasional bands containing considerable 
magnetite and a few narrow bands of green amphibole. The major 
part of the dump material is dull gray chert. In thin section, banded 
rocks of the iron formation disclose the presence of quartz and mag- 
netite, with many small needles of actinolite, a few flakes of chlorite, 
specks of hematite and small irregular areas of carbonate and pyrite. 
The quartz constitutes a very fine, crystalline mosaic and in places 
shows the irregular grains and suture lines characteristic of chert. 
Magnetite is roughh' concentrated in streaks and bands and associ- 
ated wdth this mineral are minute needles of actinolite usually visible 
only under high magnification. 

Closely associated with the iron formation in this locality is a coarse 
grained, massive greenstone. One of the pits shows both iron formation 
and greenstone on the dump, while the two south pits are in greenstone 
alone. The greenstone is composed mainly of large individuals of 
allotriomorphic, ragged, green hornblende. The interstitial material is 
mainly secondary albite, quartz, epidote, zoisite, magnetite-leucoxine, 
and occasional flakes of biotite. Obscure traces of large striated plagi- 
oclase are still to be seen, but the original textures and minerals have 
been largely destroyed by alteration. 

The highly metamorphic character of the iron formation in this 
locality' together with the sudden termination of the magnetic belt 
just west of the pits is evidence that the greenstone is intrusive in the 
iron formation and perhaps representative of a large body of igneous 
material which cuts out the Huronian rocks west of this locality. 

Mercer Section. Proceeding northeast along the Turtle range from 
townships 42 and 41 X, R 1 E, Wisconsin, nothing is known regarding 
the character of the ^Middle Huronian sediments except the data obtained 
from recent diamond drilling. A number of drill holes were put down 
by the F. I. Carpenter syndicate in the vicinity of Mercer, T 43 N, 
R 3 E, Wisconsin. At this locality the Turtle range is marked by 
two belts of magnetic attraction separated by narrow strips of normal 
territory. 

Diamond drillhole No. 17 is located on the maximum magnetic line 
of the south belt a short distance north and west of the south quarter 
corner of section 26, T 43 N, R 3 E. After j)assing through 174 f(H^t 
of overl)ur(len this hole was ledged in a steeply (lipi)ing handed rock 
showing wider hands of light gray quartz alternating with narrower 
bauds of light brown anii)hil)ol(' and others nearly black in color 
containing magnetite. In thin section the rock ai)i)ears to be composed 
of (juartz, magnetite, grunerite and a small amount of carbonate. The 
minerals are arranged in bands, some consisting of (luartz alon(>, some 



98 CONTRIBUTIONS TO PRB-CAMBRIAN GEOLOGY. 

of quartz and magnetite, and others of grunerite and quartz with 
subordinate magnetite and a httle carbonate. The grunerite fibers 
in the center of the grunerite bands are oriented parallel to the band- 
ing but on the borders the amphibole needles are at right angles to the 
banding and penetrate into the adjacent quartz grains. The larger 
crystals of grunerite are beautifully twinned and generally idiomor- 
phic in cross section The rock is a gnmerite-magnetite-schist typical 
of highly metamorphic iron formations. Analysis of drill core at a 
depth of 208 feet shows Fe., 29.1 per cent. 

Drill hole No. 19 on the maximum line of the north belt in the NWj 
of the NEi of section 26, T 43 N, R 3 E, was discontinued after pene- 
trating 22 feet of rock. After passing through about one and one-half 
feet of biotite-hornblende schist the drill entered a dark colored, cherty 
rock containing reddish garnet, green fibrous amphibole, and pyrite 
in crystals of sufficient size to be macroscopically visible. Toward 
the bottom of the hole the rock has a pronounced banding caused by 
alternating layers of pure black chert and cherty bands containing 
amphibole and garnet. Magnetite is present in sufficient quantity to 
affect the magnetic needle although not developed in crystals of suffi- 
cient size to be macroscopically visible 

A thin section cut from one of the amphibole-garnet bands exhibits 
a fine grained, cherty mass of quartz in which the other minerals are 
embedded. Amphibole, biotite and large irregular crystals of garnet 
are scattered throughout the rock. Small grains of magnetite are in- 
cluded in the garnet and larger amphibole crystals. The amphibole is 
mainly dark green and strongly pleochroic but associated with it is a 
colorless amphibole. In thin section the latter commonly forms a light 
border around the edges of the former with which it is in perfect optical 
orientation, a feature which is also retained when, less commonly, the 
two varieties are in parallel intergrowth. Finally, both the colorless 
and green varieties occur in separate crystals, the latter in greater 
abundance. A very small optic angle giving an interference figure 
nearly uniaxial in character is a peculiarity of the green hornblende. 
The optical sign is negative. The colorless amphibole has a large 
optic angle and is apparently positive. Where it is not associated with 
the green hornblende it possesses twinning characteristic of grunerite. 

That this rock is a metamorphic phase of the iron formation is ap- 
parent from its cherty character, abundance of magnetite and iron 
amphibole, and pronounced banding. The presence of garnet is inte- 
resting. A somewhat similar type of alteration, described by Leith, 
occurs in the Biwabic formation of the Mesaba range at the contact 
with the Embarrass granite* and Van Hise, Bailey and Smyth describe 

*Leith, C. K. Monograph No. 4.3, U. S. Geological Survey, p. 162. 



GEOLOGY OF THE TURTLE RANGE. 99 

similar occurrences in the Negaunee iron formation of the Marquette 
range at the Republic and Magnetic mines**. At the latter locality 
grunerite and green hornblende occur in parallel intergrowi^h and bio- 
tite and garnet are abundantly developed in the iron formation adjacent 
to the greenstones and at low horizons in the formation. 

Four other drill holes in this vicinity penetrated altered basic intru- 
sives belonging to the diorite and diabase family. The highh' meta- 
morphic iron formation found on both belts renders it very probable 
that the latter are intrusive in the Huronian series at this locality. 

About three miles southwest of Mercer a drill hole (No. 14) located 
on the north border of the magnetic belt passing through section 11, 
T 42 N, R 3 E, penetrated a slate formation in the form of a garnet- 
iferous, pyritic and graphitic schist. In hand specimen and in thin 
section this rock is very similar to the slate at the Ford-Lucas and 
Whiteside localities. In mineral composition the two rocks are identical. 

Winegar Section. {Fig. 7). The Turtle range was recently cross- 
sectioned in the vicinity of Winegar, Wisconsin, T 44 N, R 6 E, by eight 
diamond drill holes. The section indicates a synclinal fold carrying 
Lower Huronian quart zite and dolomite on the opposite flanks with 
the Middle Huronian slate-iron formation series occupying the middle 
or trough. The quartzite and dolomite have been described above. 

In general, the Middle Huronian has suffered extreme metamorphism 
and there is abundant evidence in the drill cores that igneous intrusion 
has played an important role in this connection. As near as it is pos- 
sible to judge from the drilling the iron formation is underlain and 
perhaps in part interbedded with badly altered slate. 

Hole 39 penetrated 34 feet of dark gray, fine grained carbonaceous 
schist cut by many stringers of intrusive granitic and pegmatitic ma- 
terial, especially in the upper 10 feet. The contact between the schist 
and the granite is marked by narrow veins and seams of pyrite. The 
cleavage ajiproaches in perfection that of slate and in part of the core 
a well defined narrow banding is exhibited. Biotite is the most abund- 
ant mineral. The interstices between the biotite crystals are filled 
with a dull gray, weakly })olarizing substance not separa)3le even under 
the raicroscoi)e. It is probably secondary quartz and feldspar. The 
only other noticeable features are long, lens shaped bodies of i\vrite 
oriented parallel to the schistosity and the occasional develoi")ment of 
large j)orphyritic chlorite individuals at variance with the schistosity. 
The latter are rich in inclusions of pyrite and carl)onaceous material. 
As will be shown later, the Middle (Animikie) Huronian s(>riesat thr Ban- 
ner locality, about five miles norlheasl, is repres(>nted l)y iron formation, 
slightly anamorpliosed, nnderlain by a considerable thickness of black 

**V'un Hise, C. R.; Bailey, W. L.; aiul Snivtli, M. L. U. S. d. S. MoiH)i,'rapli No. 2S, pp. 
300-91. 



100 



CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 




GEOLOGY OF THE TURTLE RANGE. 101 

slate. There can be no doubt that the carbonaceous schist found in hole 
39 is an altered sediment although original textures and structures are 
now largely destroyed. From a consideration of its mineral composi- 
tion and its position ^^^th reference to the holes in dolomite and in 
iron formation it is probable that this rock is to be correlated with 
the slate below the iron formation at the Bamier exploration. 

Hole 33, located about 100 paces south of No. 39 encountered rock 
at 215 feet and drilling was continued to a depth of 307 feet. The core 
from 215 to 239 feet is fine grained, dark colored gra^^'acke. Here 
and there the rock has a silk}^ lustre due to local development of mus- 
co\'ite. ]Microscopic examination reveals small rounded and angular 
grains of quartz embedded in a schistose ground mass of micaceous 
minerals, mainly chlorite, biotite, and musco\dte. Accessory minerals 
are epidote, black dust (carbonaceous?), a few small garnets and octa- 
hedrons of magnetite. The core from 239-244 feet is lean ferruginous 
chert made up of alternating bands of various shades of gray and to 
less extent red or green. The iron is in the form of magnetite with 
subordinate hematite and pjTite. 

The lean iron formation described above grades downward into a 
dark, greenish black, garnetiferous schist containing quartz, pyrite, 
garnet, biotite, and considerable black dust,^ probably carbonaceous 
material. In thin section most of the garnets are altered around the 
edges to a fine grained, green colored matte. Under high magnifica- 
tion this green matte appears to be a chaotic assemblage of innumer- 
able minute doubly refracting needles of a mineral resembling serpentine. 
The origin of this rock is indeterminate, but the presence of carbonaceous 
dust, the apparent absence of feldspar, either original or secondar}-, 
and the fact that the rock from 215-244 feet is plainly sedimentary 
are strong presumptive evidence that the schist from 244 feet doA\ni- 
ward is also a sediment. 

Hole 28, about 250 paces south of hole No. 33, penetrates 26 feet of 
typical, banded, biotitic-sideritic-magnctitic-schist cut by a dike of fine 
grained dialxise. The constituent minerals in the order of their relative 
abundance are quartz, biotite, magnetite, siderite. i)yrite. chlorite, 
sericite, and amphibole. The carbonate in this rock was tletermined 
as siderite l>y microchemical tests. Although in some respects this 
rock is peculiar, when the abundance of iron is taken into considtM-ation 
there can l)e no doubt that it belongs to the iron bearing series. The 
presence of consi<leral)le l)iotite and chlorite and tiie almost total lack 
of typical iron amphiboles such as grunerit<' or actinoHtc may in'rhaps 
be accounted for on the theory that the iron formation at this locality 
was originally deposited with considerable^ inteiinixcd clastic material. 
No trace of clastic textures now remain, but in mineral composition 



102 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

this rock is very similar to the iron formation largely intermixed with 
fragmental grains of quartz and feldspar exposed south of the Maren- 
isco station. 

Hole 25, about 130 paces south of No. 28, cut 15 feet of dark, greenish 
colored, fine grained schist, full of narrow wavy bands or veins of white 
quartz. Locally there is an abundant development of orthoclase and 
pyrite in the quartz bands and this suggests the idea that the (juartz 
bands are pegmatitic material injected parallel to the schistosity. The 
greenschist is, in general, very fine grained and near the bottom of the 
hole is characterized by abundant develo})ment of light red garnets. 
Near the top of the hole the schist contains many small oval sha})ed 
spots of lighter green color than the ground mass and oriented parallel 
to the schistosity. Under the microscope these green spots appear as 
large crystals of green hornblende embedded in a fine grained ground 
mass consisting of biotite, feldspar, and quartz, and a few specks of 
leucoxine; the large hornblende individuals are full of small inclusions 
of limpid feldspar and quartz. Small grains of limpid feldspar 
and quartz also fill the spaces between the flakes of biotite 
which is the most abundant mineral. The number, distribution 
and arrangement of the inclusions of quartz and feldspar in the 
large green hornblende crystals simulate exactly their distribution 
in the ground mass and under crossed nicols there is little 
difference in appearance between the large hornblende crystals and 
the micaceous ground mass. Close examination reveals, however, that 
whereas in the ground mass the biotite is made up of many small indi- 
vidual flakes with nearly simultaneous extinction due to crystallo- 
graphic parallelism, each green spot represents a large individual of green 
hornblende. The ahgnment of the hornblende and biotite and the 
abundance of ferro-magnesian mineral makes the thin sections nearly 
dark when the direction of schistosity is parallel to one of the cross 
hairs of the microscope. The small limpid feldspar grains are more 
abundant than quartz and show incipient alteration to sericite. In 
general, both the quartz and feldspar grains are arranged with their 
longer dimensions parallel to the schistosity. A few rounded grains of 
both minerals are to be seen, but they are of small size and fit perfectly 
into the mosaic. In addition to the minerals described, magnetite, 
pyrite, epidote, and rutile are present in small amounts. This rock 
is now in the most complete sense a crystalline schist and no direct 
evidence of its original character may be obtained. The presence of 
abundant basic material and particularly the green hornblende are 
somewhat suggestive of an igneous rock, perhaps of the nature of a 



GEOLOGY OF THE TURTLE RANGE. 



103 



tuff. In some respects it resembles the ''spilosites" produced bj^ con- 
tact metamorphism of the ]\Iansfield slate by intrusion of dolerite*. 

The interpretation placed upon the data obtained from drilling at 
Winegar may be seen by reference to Fig. 7. The highly metamorphic 
character of the Middle Huronian adds greatly to the difficulty of 
interpretation and no definite succession can be determined with cer- 



N 



A 











GEOIX)GICAli PLAT — 

A3iJi 
CROSS SECTION 
BANNEB LOCATION 

6.E.I4. SEC.l. 

T.45.N.R.43.W. MICH. 

SCALE 


Oolamile ^ 

~~~ ~~ ~~. o \ O 

Blaek Sat* 


400. FT. 




l'ii;iire 8. 

tainty. It seems highly ])r(>l)al)le that the d(>))()siti()n of the iron bearing 
rocks was accompanied by a large intermixture of nuid and i)robal)ly 
volcanic tuff. Later the rocks were folded and intruded by both acitl 
and basic igneous material with tlie production of a complex series of 
schists, the origin of i)arts of which are in many cases extremely diffi- 
cult of determination. 

Banner Location. (Fig. S). Recent drilling by the K. ,1. Longyear 
Co. in the SIOj of section 1, T 45 N, R 43 W, in the neighborhood of 

*.I. M. Clements, H. I>. Siiivtli. ami \V S Hailcy, Moii. AO, V. S. Geologkal Survey, p. 20(>, 
plate 37, fig. A. 



104 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

an old shaft and group of test pits knoAMi as the Banner mine, has 
given a very clear idea of the character and succession of the Huronian 
rocks of this portion of the Turtle range. At this locality the rocks 
are much less metamorphic than any heretofore described. As may 
be seen on the accompanying geological maps and sections the Huronian 
series occupies a southwestward pitching synclinal trough. 

Dolomite, the lowermost known member, is overlain by gray schist, 
apparent^ not less than 150 feet thick. Between the schist and iron 
formation is a thickness of not less than 500 feet of black jnjritic slate. 
The iron formation which occupies the center of the trough, is probably 
from 700 to 1000 feet thick. It is, for the most part, rich ferruginous 
chert or "soft ore jasper." Near its southern margin it contains some 
hard blue hematite and unaltered iron carbonate. In places the iron 
formation carries ore bodies, but, so far as known, these are too 
small to warrant mining. 

The succession here may be compared with that at Winegar. Dolo- 
mite appears to miderlie a slate-iron formation series in both localities. 
At Winegar there is apparently considerable mud and volcanic debris 
in the iron bearing rocks but at the Banner exploration they are com- 
paratively free from intermixed detrital material. The two localities 
are comiected by a continuous magnetic belt and it is probable that 
the two successions are representative of one and the same series. 

IGNEOUS ROCKS. 

With the exception of the single outcrop of iron formation at the 
Michigan mine the known natural exposures on the Turtle range are 
igneous, and comprise both intrusives and extrusives. 

Intrusives. In general, the intrusives are basic in composition al- 
though granite occurs in drill holes at Winegar. At the Broomhandle 
exploration near the west end of the range, greenstone is closely asso- 
ciated with and prol^ably intrusive in the iron formation. Northwest 
from this locality, on the rock ridges southwest of Mercer, dikes of 
diabase occur in association with effusive lavas. The intrusives are 
highly altered and are composed mainly of large individuals of sedgy 
green hornblende magnetite-leucoxine and alteration products of feld- 
spar such as epidote, quartz, ziosite, and limpid albite. In most in- 
stances the microscope reveals some traces of original, lathshaped 
plaglioclase. In addition to the altered types, dykes of fresh diabase 
are occasionally seen. The fresh diabase is characterized by its content 
of primary augite. 

Two drill holes near Mercer penetrate fresh diabase, a third, mas- 
sive greenstone conforming more closely to altered diorite. 



GEOLOGY OF THE TURTLE RANGE. 105 

Granite and greenstone apparently intrude both the Middle and 
Lower Huronian series at Winegar. 

Extrusives. The most abundant rock outcrops on the Turtle range 
are effusive greenstone. Rocks of this type are plentifulh" exposed in 
Ts 46-45 X, R 41 W, Michigan; and in T 42 N, Rs 1, 2, 3 E, and T 43 N, 
R 3 E, Wisconsin. 

At the former locality, the magnetic belt marking the position of 
the Turtle range is closely associated with porphj^itic green schist and 
a number of exposures of ellipsoidal greenstone occur a short distance 
south of the belt in section 3, T 46 N, R 41 W. Associated with the 
porphyrites and elUpsoidal greenstones are fine grained green schists 
and aphanitic greenstone. 

The greenstone porphyrites were found on several traverses just 
south of the maximum magnetic line. They are light green, fine grained 
and exhibit on weathered surfaces small white phenocrysts of feldspar, 
generally rounded but occasionally lathshaped or roughly rectangular 
in outline. The phenocrysts are in general from 1-32 to 1-4 of an inch 
in diameter, although larger ones are occasionally found. On fresh 
fractured surfaces the feldspar phenocrysts are not easily seen. Schist- 
ose structure with an average strike of N 60° E and southerly dips is 
conspicuous. 

Microscopic examination shows that these rocks are very similar in 
many respects to the lavas in the north portion of this townaship des- 
cribed in connection with the Marenisco range. They are, however, 
more basic in composition. Phenocrysts of plaglioclase are embedded 
in the ground mass consisting mainlj^ of pale green weakly pleochroic 
needles of amphibole. Associated with this mineral are lesser amounts 
of magnetite, pyrite, limpid feldspar carbonate and probably quartz. 
Occasionally the main mineral in the ground mass is chlorite instead of 
amphibole. Epidote in small granules is rather common in the chlorite 
areas. In one observation the ground mass has the ai^pearancc of 
whorls suggestive of an original perlitic structure. In general, the 
needles of amphibole follow the periphery of the feldspar phenocrysts 
but occasionally penetrate them ])roducing a micropoecilitic structure. 
The magnetite is scattered throughout the ground mass in irregular 
patches and is apparently unassociated with Icucoxine although rutile 
is a common associate. The j^henocrj^sts of jilaglioclase are in general 
saussuritizcd; less commonly they arc altered to biotite and muscovite. 
The twinning lamellae are practically destroyed but the .small angle of 
the feldsjmr phenocrysts in general indicates a rather acid plaglioclase, 
so far as we can determine, approaching oligoclase in composition. 
The green porphyritc appears to have been, originally, an andesite. 

Ellipsoidal (jreenstoncs occur just south of the north line in section 3, 



106 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

T 45 N, R 41 W. The outlines of the eUipsoids are in some cases 
plainly discernible, but in the majority of instances they are imperfect 
and only faintly suggested. Some parts of th(> rock are made up almost 
entirely of the ellipsoidal forms resting in material having the same 
apparent composition as the ellipsoids themselves. The ellipsoids vary 
in size from a few inches up to four feet in major axis. The longer 
ones he in the general direction of regional schistosity. The outUnes 
of the ellipsoids are marked by the deflection of the lines of schistosity 
around them, the matrix between the ellipsoids being plainly more 
schistose than the rock within the ellipsoidal boundaries. Amygdaloids 
occur in outcrops close to the ellipsoidal greenstones. No minerals are 
recognizable with the naked eye in either the ellipsoidal or amygda- 
loidal greenstone. The entire series has been intensely metamorphosed 
with a development of schistosity striking about N 70° E and dipping 
south. 

Closely associated with the rocks described above are fine grained, 
green schists which under the microscope appear almost identical in 
mineral composition with the ground mass of the greenstone porphyrite 
which is composed of small green hornblende needles. Accessory 
minerals are feldspar, quartz, epidote, carbonate and magnetite- 
leucoxine. 

Similar fine grained hornblende schists are found in connection with 
the effusives southwest of Mercer, Wisconsin. These will be described 
in greater detail in connection with that locality. 

From the vicinity of Mercer, Wisconsin, to the southeastern part of 
T 43 N, R 3 E, to section 9, T 42 N, R 2 E, there are many exposures 
of greenstone and greenschist on a series of rather pronounced ridges 
having a general strike parallel to the magnetic belts. For the most 
part the rocks are effusive lavas of agglomeratic and occasionally amyg- 
daloidal structures and porphyritic textures. Associated with the 
effusives there are lesser amounts of basic intrusives, mainlj^ diabase. 

The agglomeratic structures are not abundantly developed, but in 
places they are a conspicuous feature of the exposures. In section 34, 
T 43 N, R 3 E, and in section 4, T 42 N, R 3 E, this structure is beau- 
tifully developed in many exposures. At the former locality, near the 
center of the section the outcrops present the appearance of typical 
breccia. Angular and rounded fragments of fine grained greenstone, 
ranging from less than an inch to a foot or more in diameter, are em- 
bedded in a dark green, schistose matrix generally of the same apparent 
mineral composition of the ground mass but of coarser grain. However, 
the composition of the matrix is somewhat variable and at one locality 
it appears to be mainly calcite. In section 4 many of the fragments 
of the breccia are amygdaloidal but an exposure in section 32, T 43 N, 



GEOLOGY OF THE TURTLE RANGE. 107 

R 3 E, is characterized bj^ a porphyritic texture. At the latter locaUty 
the fragments are of large size and in some cases oval or ellipsoidal in 
outline. Some of the ellipsoidal boulders measure two or three feet in 
the longer dimensions. The material which cements them together is, 
for the most part, coarse grained, white, vein quartz with considerable 
feldspar. In some respects the structure of the greenstone at this 
locality resembles the ellipsoidal structure described by Clements* as 
characteristic of the Ely greenstones of the Vermilion district of Minne- 
sota. However, in the Vermilion district the ellipsoids are set in a 
matrix described as not greatly different from the greenstone itself, 
whereas in the present case the matrix is radically dissimilar. Por- 
phyritic texture is a very prominent feature of most of the outcrops 
of greenstone. The agglomeratic structures are only occasionally seen. 
Closely associated -with the porphyritic types are fine grained aphanitic 
greenstones and green schists occasionally exhibiting amygdaloidal 
structures. They resemble in texture the ground mass of the porphy- 
ritic varieties and in many cases are almost identical in mineral compo- 
sition. In a single exposure the rock may change from porphyritic to 
non-porphyritic and there is no doubt that these fine grained schists 
and aphanitic greenstones are basic lavas differing but little in com- 
position from the porphyritic varieties. Both types of rock exhibit 
in places a well developed schistosity striking about N 60° E. 

The porphyritic greenstones are in many respects similar to those 
already described but are, in general, more basic in composition. They 
are composed mainly of phenocrysts of highly altered plaglioclase 
embedded in a ground mass of small, pale green, weakly pleochroic 
needles of green hornblende. The intensity of pleochroism is apparently 
governed by the size of the individual crystals. The smaller crystals 
arc very weakly pleochroic,- but this phenomenon is strongly developed 
in the larger ones. The ground mass also contains secondary unstriated 
plaglioclase and prol^ably quartz, many scattered grains of epidote, 
and considerable magnetite with associated leucoxine. Small patches 
of carbonate, pyrite, and occasional crystals of rutile and brookite are 
present in some specimens. The iiluMiocrysts of feldspar most com- 
monly show alteration to a coarse grained mosaic of quartz, albite 
and biotite. The alteration is rarely complete; as a rule much of the 
original miiieral remains. Saussuriti/ation and sericitizatiou t)f the 
feldspar is less often observed. Granulation around the eilges of the 
crystals is a common feature. In the schistose varieties the pheno- 
crysts exliil)it only jiartial parallel orientation, some of them lying with 
their major axes at right angles to the schistosity in which i)osition 
many of them are completely fractured and broken in halves cemented 

♦Clements, J. Morgan. Vermilion Iroti HfiirinK District. Monograph 45, U. S. Geological 
Survey. 



108 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

by a coarse crystallization of secondary quartz and albite. The horn- 
blende needles of the ground mass are oriented in all directions in 
the massive types while in the schistose varieties they exhibit a perfect 
parallelism. 

The plaglioclase phenocrysts are too highly altered for exact determi- 
ination by optical methods, but the prevailing alteration products and 
a rather large extinction angle showTi by the remnants of the original 
crystals are indicative of a basic composition, which, in consideration 
with the basic character of the ground mass, points to an original 
composition near that of andesite. 

The microscope readily reveals the effusive nature of the aphanitic 
non-schistose greenstones, especially of the fresher types. Traces of small 
lathshaped feldspars and the presence of serpentine indicates that the 
rock is basalt, probablj^ oli\dne-bearing in phases characterized by 
serpentine. Even the fresher basalts are now largely composed of 
secondary products of which green hornblende feldspar and quartz are 
the most important. Magnetite, epidote, zoisite, carbonate, chlorite, 
apatite, rutile, pyrite and sericite are present in considerable quantity. 
The occurrence of the first named three is practically universal, of the 
others sporadic. Magnetite, as usual, is generally coated with leucoxine. 

The more altered aphanitic greenstones are characterized by a large 
amount of amphibole and absence of all traces of original structures 
and textures except where they are amygdaloidal. Their general 
character is almost identical with the ground mass of the altered ande- 
sites. In some instances the hornblende individuals are shghtly larger 
and more pleochroic than ordinarily in the porphyrites. 

Amygdules may be observed in rocks of this type in at least two 
localities. The outcrops near the center of section 4, T 42 N, R 3 E, 
exhibits the best development of this structure. The rock here is a 
dark green, aphanitic greenstone sho"v\dng many small w^hite amygdules 
from 1-20 to 1-10 of an inch in size. In thin section the vesicular 
filling appears to be coarse grained interlocking crj^stals of zoisite, 
epidote and quartz. The matrix of this rock is identical with that of 
the rocks just described. 

By parallel orientation of the hornblende the rocks pass into green 
schists which in mineral composition do not differ from the more 
altered massive variety. As a rule they are coarser grained, but ordi- 
narily the only difference consists in the parallel orientation of the 
amphibole fibres. In the schistose varieties no traces of original min- 
erals, structures or textures are left, and were these rocks found apart 
from the less altered phases it would be impossible to prove their 
origin from basic lavas. 

Ellipsoidal lavas are exposed in sections 27 and 23, T 42 N, R 1 E. 



GEOLOGY OF THE TURTLE RANGE. 109 

The outcrops in section 23 are also characterized by the parallel align- 
ment of the ellipsoids. However, the rock itself possesses no parallel 
orientation of the constituent minerals and therefore cannot be correctly 
termed a schist. Microscopic examination indicates that the rock is 
an altered andesite. 

The close association of the effusive greenstones with the magnetic 
belt is characteristic wherever they are exposed. In many cases the 
greenstone forms long low ridges parallel to and lying just in the out- 
side Umits or immediately north or south of the belts of attraction. In 
other cases, notably from Mercer northwestward, the greenstone occu- 
pies non-magnetic territory between the areas of magnetic attraction. 
At the Michigan mine the greenstone occurs just south of the iron 
formation and a mile east. Exposures of this rock are immediately 
north of the belt marking the position of the iron bearing formation. 

No contacts between the greenstone and the sedimentaries are known 
anywhere on the range but the distribution of the greenstone and its 
close association with the magnetic belts marking the position of the 
Middle Huronian (Animikie) iron formation amply justify, in the ab- 
sence of definite proof to the contrar}-, the conclusion that the effu- 
sives are flows interbedded in the Huronian series. 



CHAPTER VL 
GEOLOGY OF THE MANITOWISH RANGE. 

R. C. ALLEN AND L. P. BARRETT. 

The ]\Ianitowish range extends from Watersmeet. ^Michigan, south- 
west into T 41 X, R 2 E, Wisconsin, a distance of approximately 55 
miles. Its position is marked by a series of weak parallel magnetic 
belts although locally, as in the vicinity of Watersmeet, strong mag- 
netism is characteristic. In general the magnetic belts are narrow but 
in some localities they exceed a mile in width. This is true of the belt 
of strong magnetism near Watersmeet. 

The region is hea\'il3' drift covered and aside from a number of 
exposures of a peculiar kyanite-mica schist in the southwest portion of 
T 42 N, R 4 E, Wisconsin, only three outcrops are kno\\'n. Diamond 
drilling in a number of localities by the F. I. Carpenter s\Tidicate shows 
clearly the general characteristics of the rocks in AVisconsin. In ]\Iich- 
igan the nature of the rocks underlying the ]\lanitowish magnetic 
belts may only be surmised. 

Both the outcrops and drill cores exhibit coarse grained schists and 
gneisses and granite. At no locality is it possible to work out a succession 
of sedimentary formations and in fact it is extremely difficult in manj^ 
cases to determine even a close apjiroximation to the character of the 
original rock from which the crj^stalline schists were derived. It seems 
perfectly clear that the Manitowish rocks have been metamorphosed 
to a degree more extreme than that of any other known range or con- 
siderable area in the Lake Superior country. There is ample evidence 
that this intense metamorphism is due to the intrusion of granite on 
a grand scale. The facts seem to be ex])lainable only on the theory of 
sul)-crustal fusion, i. e. the original sediments have been completely 
engulfed and assimilated by the intruding magmas, the resultant 
schists and gneisses deriving their constituents in part from the magma 
and in part from the fus(<(l sediments. 

Owing to their limited nuinlici- llic outcrops and tlif drill records 
will. be discussed in detail and such coiiclusioiis drawn as sccni war- 
ranted from the meager facts. 

(iranite is exj)ose(l at two localities, viz., in sections 1^4 and :-?5, T 4.S 
N, H 7 E, Wisconsin. In both localities the rock is light gray biotite 



112 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

granite cut by many pegmatite veins, the latter showing in places well 
developed graphic intergrowths of quartz and feldspar. The granite 
is composed of orthoclase, quartz, biotite, muscovite, sericite, apatite 
and ferrite. These rocks differ from the granites previously described 
in connection with the other ranges in the absence of microcline. Under 
the microscope the larger minerals show characteristic granulation and 
the feldspar has undergone considerable alteration around the edges, 
although the centers of the crystals are usually very fresh. The ex- 
posures lie a little south of the magnetic belts of the Manitowish range 
but they are probably to be correlated with the great mass of granite 
intrusive into the sediments of the Manitowish range and the Vieux 
Desert-Conover district. 

The only other exposures on the Manitowish range are coarse grained 
crystalline schists and gneisses marked by an abundant development 
of biotite, garnet and kyanite. In sections 28, 29, 31 and 33, T 42 N 
R 4 E, Wisconsin, there are a number of outcrops of these rocks. In 
section 13, T 42 N, R 5 E, there is a doubtful exposure of the gneiss on 
the narrows between Spider and Island lakes. At this locality there 
are groups of large angular boulders and several large blocks of gneiss 
that possess a common strike of schistosity of about N 70° E. The 
blocks may be in place. , It is certain that none of the material has 
been transported any great distance. 

The rocks in these outcrops are coarse grained and vary from light 
gray to grayish black; a bluish tinge is noticeable wherever kyanite 
is abundant. The kyanite is more resistive to weathering than the other 
minerals and stands out in prominent relief. Gneissose structure, 
marked by alternate layers of shghtly different texture or mineral 
composition, is nearly always present. In all of the larger outcrops 
contortion of the gneissose bands is a conspicuous feature. The gneiss 
is cut by innumerable pegmatite dykes and stringers and veins of quartz. 

The chief minerals of the gneiss are quartz, biotite, garnet, kyanite 
and plagioclase. Muscovite, magnetite, zircon, rutile, apatite, chlorite 
and pyrite are of subordinate importance. Quartz, biotite, garnet and 
kyanite are easily recognized in hand specimen, but the other minerals are 
usually detected only under the microscope. The coarser grained bands 
are composed mainly of quartz, biotite, kyanite and garnet; the finer 
grained layers lack the kyanite and usually show a considerable devel- 
opment of feldspar. The texture is very coarse and thoroughly crystal- 
line but the only minerals which exhibit idiomorphic forms are kyanite, 
apatite and in some instances garnet and pyrite. Quartz is the most 
abundant mineral and, in larger individuals, exhibits abundant strain 
shadows and inclusions of liquid and gas. Biotite is the second most 
important mineral and is deep brown and strongly pleochroic except 



GEOLOGY OF THE MANITOWISH RANGE. 113 

where bleaching accompanied by separation of magnetite has taken 
place. The garnets are light bro\\Ti or reel in hand specimen, colorless 
in thin section. Good crystal development is rare and they are com- 
monly rounded or irregular in outline. The kyanite crystals show the 
characteristic long development and good crystal form in the prism 
zone. The individuals average one half inch in length parallel to the 
C axis and shows no tendencj^ to orientation in the plane of schistosity 
or in anj^ other plane. Both the garnet and kyanite are filled with 
inclusions of quartz, biotite and magnetite; this, together with lack of 
orientation, is evidence that these minerals formed under conditions of 
static metamorphism after the crystallization of the other minerals had 
taken place. Plagioclase is the common feldspar and occurs in consi- 
derable abundance in the finer grained kyanite-free bands. It is in 
all cases very fresh, showing only slight alteration. In composition it 
appears to be oligoclase and albite-oligoclase. 

The main clue to the origin of the biotitic, garnetiferous, kyanitic 
gneiss lies in its mineral composition. The universal presence of the 
aluminum silicate, kyanite, is the most suggestive feature. In dis- 
cussing the occurrence of the aluminum silicate group Van Hise* says: 
"The special homes of the aluminum-silicate minerals are the meta- 
morphosed argillaceous sedimentary rocks. As is well known, kaolin 
is one of the chief constituents of such rocks, and doubtless it is from 
this mineral, in larger part under deep seated conditions, that the alu- 
minum silicates are formed." The same author in discussing the criteria 
to be used in discriminating between the metamorphic igneous and 
sedimentary rocks, says,t "Staurolite and andalusitc, sillimanite and 
cyanite are very characteristic minerals. Therefore, where certain 
single minerals are dominant in the schists and gneisses it seems to be 
a fairly safe conclusion that the rocks are sedimentary' in origin." The 
mineral kyanite, because it has the highest specific gravity of the alu- 
minum silicate group, is significant also of the most profound anamor- 
phism and the imiversal presence of this mineral coupled with the 
entire absence of andalusite or sillimanite is a measure of the intensity 
and cliaracter of the metamorphism ])revalent on the Manitowish 
range. The regional schistosity and contortion of the gneissose l)ands 
together ^vith the occurrence of numerous pegmatitic dikes cutting the 
gneiss is proof that the original sediments have been metamorphosed 
not only by folding, l)ut l)y igneous intrusion as weil. The thoroughly 
recrystalline character of tiie gneiss furnishes the filial proof of the 
intensity of the metamorphic changes to whicli these rocks liave been 
su))jecte(l. 

♦Van Hise, C. R. A Treatise on Metiuiiorphisni, Moiio. -17, I'. S. C .'>. j). .''.IT. 
tVan Hise (citd) p. 91fi. 

15 



114 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

RESULTS OF DIAMOND DRILLING. 

Hole 16 is on the northeast extremity of the maximum magnetic hne 
of the narrow magnetic belt which extends from section 14, T 41 N, 
R 2 E, to section 24, T 42 N, R 3 E. The rock was tested for a depth 
of 40 feet. The upper 25 feet of rock in this hole (Uffers so remarkably 
from that of the lower 15 feet and the rock in both zones is so unique 
that separate detailed descriptions are demanded. 

The rock in the upper 25 feet of the hole is of reddish color, contains 
abundant dark colored crystals of garnet, is cut by many bands of quartz 
and is finely banded. The main mass of the rock is composed of dark 
colored irregular shaped crystals of garnet which are imbedded in a crys- 
talline ground mass of quartz, carbonate, limonite and magnetite. Lay- 
ers of nearly pure white quartz and of red and white banded chert alter- 
nate with the garnet bearing zones. In thin section the rock is observed 
to be composed of the minerals garnet, quartz, carbonate, limonite and 
magnetite with a mall amount of biotite and chlorite. The garnets 
are colorless, irregular in outline, and full of inclusions of other minerals. 
The carbonate is closely associated with hematite and is probably sider- 
ite. This rock is probably an intensely metamorphosed iron formation. 
This conclusion is warranted from a consideration of its iron content, 
the occurrence of thin seams or bands of ferruginous chert and its 
relation to the maximum magnetic line of a linear magnetic belt which 
has been traced continuously for a distance of 8 miles. Garnetized iron 
formations are elsewhere found near contacts with intrusive igneous 
rocks. 

The lower 15 feet of core is a feldspathic biotite schist microscopically 
somewhat similar to that of the upper 25 feet. It differs mainly in the 
absence of siderite, iron oxide, and the presence of feldspar. It contains 
abundant small red garnets imbedded in a finely grained, dark, mas- 
sive ground mass. In thin section the garnets exhibit irregular outlines 
and lie in an interlocking crystalline mosaic of quartz, biotite and feld- 
spar. The biotite shows a slight tendency toward parallel alignment. 
With the exception of an incipient sericitic alteration of the feldspar 
the minerals in the rock show no alteration. The rock is completely 
recrystalline and no trace of original texture remains. This rock could 
be derived from a sediment of graywacke tj^pe but its essential simi- 
larity to the rock in the upper part of this hole, which seems to be with- 
out a doubt a metamorphic iron formation, suggests a similar derivation 
through extreme metamorphism by granitic intrusion, the feldspathic 
material being a direct contribution from the granite magma. This 
conclusion is further supported by evidence, in other parts of this range, 
of the assimilation l)y granite of the sedimentary rocks, the original 



GEOLOGY OF THE MANITOWISH RANGE. 115 

positions of which are now occupied by schist and gneisses and are 
preserved only by linear faint magnetic belts which have been traced 
continuously in some cases for as much as 20 miles. 

Beginning in section 10, T 41 X, R 3 E, a continuous belt of mag- 
netic attraction has been traced in a northeasterly direction a distance 
of 20 miles to an apparent comiection with a magnetic belt of the 
Turtle range in sections 14 and 15, T 43 X, R 6 E. Three drill holes 
were put cIoa\ti at widely separated localities on this belt. 

Hole 22 was located in section 26, T 42 X. R 4 E, a short distance 
north of Powell on the C. & N. W. R. R. This hole was sunk on the 
line of maximum magnetic attraction and, after passing through 122 
feet of drift, drilling was continued for a depth of 12 feet in a coarse 
grained garnetiferous mica schist showing injected veins of granitic 
material. The rock is composed of quartz, biotite and garnet with 
considerable chlorite and pyrite, small scattered grains of magnetite, a 
few needles of rutile and fine flakes of carbonaceous (?) material. This 
rock is very similar in mineral composition to the kyanite free bands 
in the kj-anitic garnetiferous biotite gneiss that outcrops just north of 
the magnetic belt two miles west of section 26. It is thoroughly cry- 
stalline schist and the only clue to its origin is its present mineral 
composition. The predominant minerals are biotite and quartz while 
garnet is occasionally found in considerable abundance. Schists which 
are composed mainly of quartz and mica are regarded by Van Hise* 
as probably metamorphic sediments. The general resemblance of this 
rock to certain layers in the associated kyanite-bearing gneisses is 
strong presumptive evidence of a sedimentary origin. The intimate 
injection of granitic material is one, if not the most important, cause 
of the extreme metamorphism of this rock. 

Hole Xo. 35 in section 2, T 42 X, R 5 E, about 7 miles northeast 
of hole 22 along the strike of the magnetic belt was put down on the 
south edge of the magnetic field. The core exhibits dark gray to black, 
coarsely grained rock heavily impregnated with pyrite and carrying 
an abiHidance of small light reddish garnets, for the most part about 
l-K) of an iiicli in diameter. Xear the top of the hole are many grains 
of cjuartz and jnrite. In thin section the rock is almost identical in 
texture and comi)osition with the core in the bottom of Hole 33. of the 
Winegar section of the Turtle range, which is of particular interest in 
view of the fact that there is an apparent comiection l)et\veen the 
Turtle and Manitowish ranges along the line of this magnetic iielt. 
The rock is compo.sed of colorless garnets rich in inclusions of fine 
black particles of carbonaceous material imbedded in a groundmass of 
interlocking (juartz and biotite. Pyrite and small patches of minute 

♦Van Hise, C. R. Treatise on Metamorpliism, Mono 47, U. S. G. S. p. 916. 



116 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

green fibres .similar to those in the core from hole 33 are also present. 
There can be little doubt of the sedimentary origin of this rock, the 
best proof of which is the abundant carbonaceous material. 

In section 28, T 43 N, R 6 E, about four miles northeast of hole 35, 
the underlying rocks were tested again by drill hole No. 41. The over- 
burden at this locality extends to a depth of 129 feet and drilling was 
continued in the ledge for 19 feet. The rock is coarsely grained, gar- 
netiferous biotite-quartz-schist similar to that found in hole 22. 

The magnetic belt lying south of the one tested by holes 22, 35 and 
41 was tested by three holes. In the vicinity of Powell this belt ex- 
hibits two lines of maximum attraction, the southernmost of which is 
separated from the main belt in section 36, T 42 N, R 4 E, by a short 
interval of normal territory. Hole 20 (section 26) was sunk on the 
north line of maximum magnetic variation; hole 24 (section 36, T 42 
N, R 4 E) is a short distance north of the maximum line but within 
the limits of the magnetic field. Seven miles northeast along the strike 
of the magnetic belt a third drill hole (No. 38) was put do^vn just 
south of the maximum magnetic variation in section 7, T 42 N, R 6 E. 

The rock in hole 20 is coarse grained biotite-muscovite-quartz schist 
thoroughly impregnated with granitic material. The granite cuts the 
schist in a series of veins of pegmatite and aplite. In general, the 
schist is composed of quartz and biotite, with considerable feldspar, 
pyrite and magnetite. Near the pegmatite veins muscovite is abund- 
antly developed. 

Hole 24 penetrated 15 feet of fine grained feldspathic biotite schist 
cut by veins of pegmatite. The bottom three feet is coarsely grained 
massive white biotite muscovite granite. 

Hole 38 is in feldspathic biotitic quartzose gneiss. A definite band- 
ing is produced by alteration of zones of the same apparent mineral 
composition but of slightlj^ different texture. Occasional narrow 
pegmatite veins running parallel to the schistosity are to be seen. 
Like all the other rocks found on this range the gneiss is thoroughly 
recrystalline and in its present condition contains no direct evidence 
of its original character. 

In the south part of T 42 N, R 5 E, and the north portion of T 41 
N, R 5 E, there are four small disconnected areas of magnetic attrac- 
tion. The two easterly belts have a strike almost at right angles to 
the strike of the Manitowish ranges. 

Hole 32 was put down at the north end of the most easterly belt, 
which trends a little west of north through section 35, T 42 N, R 5 E. 
It is just a mile in length and a little less than one-half mile wide. The 
hole is in the projection of this belt about 300 paces from its north- 



GEOLOGY OF THE MANITOWISH RANGE. 117 

eastern extremity and penetrates coarse grained white muscovite- 
biotite granite. 

GENERAL SUMMARY. 

We can briefly sum up the known facts regarding the ^Nlanitowish 
range in three statements, i. e. (1) its position is marked by a series of 
parallel narrow linear magnetic belts broken here and there by gaps 
of normal territory; (2) the underlying associated rocks are mainly 
crystalline schists and gneisses; (3) the schists and gneisses are almost 
universally associated with large amounts of injected granitic material 
and pegmatite dikes. 

The narrow and linear characters of the magnetic belts admit of 
but two inferences in regard to the original character of the underlying 
rocks, viz., they were (1) either folded sedimentaries or (2) basic lavas. 
At the present time the rocks underlying the magnetic belts are thor- 
oughly crystalline acid schists and gneisses, which fact effectually 
eliminates the second inference. It is not eas}' to grasp the signifi- 
cance of such profound metamorphism on such a grand scale. This 
extreme metamorphism is not confined to the Manitowish range but 
also occurs throughout the Yieux Desert area, on the western extremity 
of the Conover slate belt, and doubtless over a large unexplored terri- 
tory the limits of which can only be conjectured. The original sedi- 
ments seem to have been permeated, injected, and probably over large 
territories, absorbed by the intruding granite magma. The rocks now 
occupying the space originally filled by sediments are crystalline schists 
which have derived their constituents from both the sediments and 
the granitic magmas. 



CHAPTER VII. 
GEOLOGY OF THE VIEUX DESERT DISTRICT. 

R. C. ALLEN AND L. P. BARRETT. 

The Vieux Desert district includes a belt of country about four to 
five miles wide and about twenty miles long extending E-W through 
Lake Vieux Desert across the boundary between Michigan and Wis- 
consin. (See Fig. 1). West of the lake there are two parallel faint 
magnetic double belts trending E-W across flat sand plains. In Mich- 
igan there is a single magnetic belt traversing wooded hilly country 
from the lake to the middle of T 47 N, R 37 W. 

The rocks in the Vieux Desert district in Wisconsin are buried be- 
neath 125 to 200 feet of glacial drift. The thickness of the drift in 
Michigan is unknown but, as in Wisconsin, there are no rock exposures. 
Eleven diamond drill holes were sunk in 1912 by the F. I. Carpenter 
syndicate in the magnetic territory in the vicinity of Lake Vieux Desert 
and westward. These holes are so located as to give a general idea of 
the geology of the Vieux Desert district which is essentially similar to 
that of the Manitowish range. The rocks are mainly coarse grained 
schists and gneisses, granite and highly metamorphic but recognizable 
slate and graywacke. A description of the drill cores from each of 
the eleven holes follows: 

Hole 1 is on the north line of maximum magnetic attraction of the 
northern douljle belt in the SWi SW| section 35, T 43 N, R 10 E. 
The drill penetrated four f(H't of pink biotite granite. 

Hole 2 rSWi SWi of section 35, T 43 N. R 10 E, Wisconsin) is a 
short distance north of No. 1. It cut 50 feet of granite and, below it, 
al)0ut 50 feet of dark gray schist cut in many plac(>s l)y pegmatite and 
granite dikes. The granite is gray and ))ink. medium grained, and is 
composed of orthoclase, (juartz. microcline, plagioclase, chlorite, biotite, 
fcn'itc, nniscovite. zircon, apalile. and a few little grains of niagnetite- 
leucoxine. In thin section only a slight t(>ndencv toward granulation 
and the development f)f mortar structure is to be noted. The ortho- 
clase is full of inclusions of hematite and exhibits considerable altera- 
tion, to rather large Hakes of nniscovite. The microcline is fresh and 
does not ap])ear to be an alteration |)roduct of the orthoclase as in the 
granites of the WOlf Lake aica. Plagioclase feldspar is very subordi- 
nate and like tln' microcline is fresh. Hiotite is badl>' altered to chlorite, 



120 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

in many cases the alteration is practieally complete. This feature is 
especially noticeable in the upper 15 feet of core. 

The schist from the lower 50 feet of core is fine, even grained and 
made up jinncipally of quartz, biotite, feldspar, j^yrite, graphite and 
magnetite with occasional grains of ei)idote and small crystals of apa- 
tite. In thin section the feldspar is limpid, unstriated and probably 
near albite in comi:)osition. The content of graphite is a marked feature 
and gives the rock its dull gray color. Pyrite is very abundant in seams 
and streaks running parallel to the schistosity. The abundant graphite 
and pyrite in this rock is strong evidence of its sedimentary origin. 

Hofe 3 (NWi NWi, section 2, T 42 N, R 10 E, Wisconsin) is about 
300 paces north of hole No. 1. The drill i)enetrated 34 feet of biotitic 
garnetiferous kyanitic gneiss similar to that on the Manitowish range. 

Hole 6 (NEJ NEi, section 2, T 42 N, R 10 E. Wisconsin) is about 
three fourths, of a mile east of hole No. 1 and on the same maximum 
line of magnetic attraction. It is bottomed in light gray banded gneiss 
composed of quartz, feldspar, pyrite muscovite, biotite and graphite. 
The banded structure is produced by alternating layers of coarser and 
finer texture. The coarser and finer textured bands are apparently 
identical in mineral composition with the exception that the coarser 
layers are lacking in graphite. 

Hole 4 (SWi NWl section 2, T 42 N, R 10 E, Wisconsin) is a short 
distance south of the south maximum line of the north belt. It was 
bottomed at a depth of 206 feet in granite and dark gray, rather fine 
grained rock made up principally of quartz, garnet and biotite. The 
granite is coarse grained and pink and appears, in hand specimen, to be 
composed almost entirely of quartz and feldspar. The gray rock is 
exceedingly hard and has a well defined schistose structure although in 
thin section the biotite exhibits a rough parallel alignment. In addi- 
tion to the minerals mentioned, microscopic examination reveals con- 
siderable pyrite and magnetite and a small amount of chlorite, sericite 
and apatite. The cjuartz, garnet and biotite are arranged in an allotri- 
omorphic crystalline aggregate. The garnet is filled with inclusions of 
quartz, pyrite and innumerable particles of magnetite and graphite (?). 
Some small flakes of sericite are developed along the fractures in the 
garnet. The larger magnetite crystals are coated with leucoxine. Both 
pyrite and magnetite are in close association with biotite and as said 
above are also included in garnet. 

Hole 9 (NEi NEi, section 3, T 42 N, R 9 E, Wisconsin) is at the 
extreme west limit of the north magnetic belt. It is bottomed in granite. 

The south magnetic belt was tested by four holes, numbers 5, 7, 10 
and 12. These holes were put down at widely spaced localities. In 
each hole the rock is crystalline coarse grained micaceous schist and 



GEOLOGY OF THE VIEUX DESERT DISTRICT. 121 

giieiss cut by dikes of pegmatite and granite. For the most part the 
schists are composed of quartz, biotite and muscovite. Garnet and 
feldspar are locally important. The core from hole 5 is characterized 
by abundant greenish white talc. PjTite, magnetite, chlorite, rutile, 
zircon, and apatite are everywhere present in small quantities. 

CONCLUSION. 

The rocks of the Vieux Desert district are granite, gneiss, and schist, 
the gneiss and schist predominating. The gneiss and schist are char- 
acterized by the minerals graphite, kyanite, garnet, magnetite, and 
pjTite, and are related to parallel linear sj'^mmetrical magnetic belts. 
It is believed that the schist and gneiss have formed through injection 
and assimilation of sedimentary rocks by granite magma. The meta- 
morphic effect of granitic intrusion in the Vieux Desert district is 
apparently similar to that prevalent on the Manitowish range. 



CHAPTER VIII. 
GEOLOGY OF THE CONOVER DISTRICT. 

R. C. ALLEN AND L. P. BARRETT. 

The boundaries of the Conover district are ill defined. A series of 
magnetic belts with a general east-west trend extend through T 41 N, 
R's 9, 10, 11 and 12 E, Wisconsin, almost exactly parallel to similar 
magnetic belts in the Vieux Desert district about eight miles north. 
The geology of the Conover district is known only from exploration 
by diamond drilling made by the F. I. Carpenter syndicate in 1913. 
Fifteen holes were drilled with particular reference to the magnetic 
belts. As a general statement it may be said that the magnetic belts 
mark the position of a slate series covered by from 150 to 200 feet of 
glacial drift striking east-west parallel to the magnetic Hues. The 
bedding in the slates is everywhere steeply inclined. As in the Vieux 
Desert district these slates are intruded by granite by which they are 
apparently entirely replaced west of the center of T 41 N, R 9 E. From 
thispoint eastward granite was not enc ountered under or near the max- 
mum magnetic lines and the slate shows little or no effect of meta- 
morphism by igneous intrusion. Granite occurs to the south in section 
33 and vicinity, T 41 N, R 10 E. A drill hole near the north \ corner of 
section 8 of the same township penetrated slate. This slate is much 
harder and more highly recrystalline than that which underlies the 
magnetic belt of the Conover district and probably indicates approach 
to a contact with the intrusive granite which is known to occur through- 
out the Vieux Desert district and very ])robably occupies the greater 
part of the intervening area. 

It is very probable that the slate series of the Conover district is 
continuous eastward with the slate-iron formation series of the Iron 
River district. This connection is indicated by the general structural 
features of the two districts, the similarity of the slates, and the pene- 
tration of slate in numerous drill holes in section 4, T 42 N, R 36 W, 
Michigan, and in a single drill hole in section 9. T 41 N, R 13 E, Wis- 
consin, thus practically bridging the interval between the eastern 
extremity of the magnetic belts of the Conover district and slate exi>o- 
sures in the Iron l{ivcr district. About one and one-ciunrter miles soutii- 
west of the latter locality in the NE^ of section 17 of th(> same town- 
sliij) another diill hole is ii\ granite. 



124 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

From the meager information available the Conover slate^i may be 
considered as a tongue of the slate-iron formation series of the Iron 
River district j)rojected westward into the area which has been invaded 
by the great granite batholith and its outliers of Northern Wisconsin. 
This granite, as we have seen, terminates the Conover slate belt near 
the center of T 41 N, R 9 E, Wisconsin, but the main magnetic belt 
which is underlain b}- slate throughout the Conover district continues 
on without break into the granite area for several miles where it is 
proved by drilling to mark the position of quartz-mica schist and gneiss 
cut by dykes and stringers of white biotite granite exactly similar to 
the rocks which underlie the magnetic belt of the Vieux Desert district. 
There can be no ciuestion that this mica schist and gneiss is a meta- 
morphic equivalent of the comparatively unaltered slate underlying 
the same magnetic belt east of that localit}-. This relationship is a 
further evidence of the origin of the mica schists and gneisses of the 
Yieux Desert district wherein the relationship to the original unaltered 
sediments is less clear. 

The rocks underlying the eastern three-fourths of the Conover 
district are soft, red, gray, and black slates containing seams of ferru- 
ginous chert and carrying abundant iron carbonate. Oxidation has 
extended to varying depths up to 75 feet and in a couple of drill holes 
material rumiing as high as 40% metallic iron was encountered on 
and extending a few feet below the rock surface. It is entirely possible 
that the Conover slate is associated with productive iron formation 
which has, however, not been penetrated in drilling. 

The Conover slates are exactly similar to those associated with the 
Vulcan iron formation in the Crystal Falls and Iron River districts. 

PETKOGRAPHIC DESCRIPTION OF THE CONOVER SLATES. 

For the sake of convenience the petrographic description of the 
rocks penetrated by the various drill holes in the Conover district 
will be referred to the different magnetic belts wherein these holes 
are located (see Fig. 1). 

The most easterly magnetic belt is short, narrow and ill defined. It 
extends through sections 13, 14 and 15, T 41 N, R 12 E. The rocks 
underlying this belt have not been explored by drilling. 

Belt A extends from section 9, T 41 N, R 12 E, westward three miles 
into sections 11 and 14, T 41 N, R 11 E. This belt was tested near 
the south border by hole 48 located near the west \ post of section 
13, T 41 N, R 11 E. Rock was encountered at 168 feet and drilling 
was continued to a depth of 65 feet in soft banded ferruginous slate, for 
the most part red, or grayish red in color but with some gray and green- 



GEOLOGY OF THE CONOVER DISTRICT. 125 

ish gray phases. Banding and cleavage are parallel and steeply in- 
clined to the vertical. Xo other holes were put do^^^l on this belt. 

Belt B extends from section 24. T 41 X. R 11 E, 18 miles westward 
into section 7, T 41 X, R 9 E. It has an average width of about three- 
ciuarters of a mile and with the exception of the westernmost five or 
six miles is characterized by a single continuous maximum magnetic 
line. 

Belt B was tested at several different localities. In the central and 
eastern part the underlying rock is slate. On the west end where the 
magnetic field is variable and uneven the s'ates have suffered extreme 
metamorphism by the intrusion of granite and are now represented by 
mica schist. A description of the holes put doA\-n on this belt follows: 

At the eastern extremity, hole 46, XEJ of the XEj. .section 23, T 41 
X, R 11 E, penetrated ferruginous slate, grading do^^^lward into un- 
oxidized greenish gray carbonate slates. 

Hole 27, XEi of XWf section 22, T 41 X, R 10 E, near the south 
border of the belt, penetrated gray slate. The rock has a good cleav- 
age and a well defined banding. Both structures are parallel and indi- 
cate a nearly vertical dip. 

In the eastern part of sections 17 and 20. T 41 X, R 10 E, Belt B 
has two lines of maximum attraction and is also characterized by 
strong disturbance of the needle. The belt was cross-sectioned at this 
locality l)y three drill holes. Hole 8, XEj of X'Ej, section 20, is located 
on the south line of maximum magnetic attraction. The rock in this 
hole, which was tested for a depth of 25 feet, is not duphcated else- 
where in the C'onover area and presents some features of exceptional 
petrographic intere.'^t. 

The lower ten feet is a soft light grayish green rock composed essen- 
tially of carbonate, talc and magnetite. Carbonate forms the body of 
the rock. Magnetite occurs in disseminated grains although there is 
a tendency for this mineral to be concentrated in irregular and also 
roughly rectangular areas. Talc occurs abundantly in irregular masses, 
thin veins and scattered flakes. This rock has ])rol)ably resulted from 
the metamorphism of a ilolomite containing a large amount of iron 
carl)onat(". Heat and pressure has converted the iron carlionate to 
magnetite and the dolomite to talc. It is ])n)l)al)Ie that the metamor- 
])hism is due to igneous intrusion rather than to dee)) l)urial becau.se 
the rock is not schistose. The igneous intrusives furnished the silica 
necessary for tiie formation of talc. 

. The core from the uj)j)i'r part of \hv hole exhibits a massive grayish 
green rock very similar niicroscoj)ically to that in the lower ten feet. It 
differs mineralogically in being compo.sed mainly of serpentine with 



126 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

subordinate magnetit(> and carbonate. The appearance of this rock 
under the microscope is ilkistrated in Plate XII. In ordinary- 
light a well defined granular texture is apj)arent in the arrangement 
of the magnetite and carbonate but under crossed nicols this 
texture is obscured by the doubly refracting lattice work of interlock- 
ing fibres of serpentine. The serpentine is colorless and has a 
low birefringence and index of refraction. It is probably the variety 
antigorite. The carbonate and magnetite are closely associated and 
their peculiar distri})ution is their chief feature of interest. The ser- 
pentine fibres are arranged without regard to the granular texture 
outlined by the magnetite and carbonate areas. 

The origin of the serpentine rock is not entirely clear. Three possi- 
bihties are suggested. (1) It may be an altered olivine rock or dunite 
in the form of a dike cutting the ferruginous dolomite. (2) It may 
be derived through ser]:)entinization of forsterite developed in the 
ferruginous dolomite by contact metamorphic action. (3) The ser- 
pentine may have l:)een derived directly from the dolomite through 
the agency of hot solutions coming from adjacent intrusives without 
the previous formation of olivine minerals. 

If the rock was once a dunite no trace of olivine now remains al- 
though the arrangement of the magnetite and carbonate is suggestive 
that the}^ outline the position of original olivine grains. Objections 
to this theory are the entire absence of minerals or alteration products 
of minerals common in olivine rocks such as pyroxine, chromite, plagi- 
oclase and spinel and the absence of a sharp boundary between the 
serpentine rock and the altered ferruginous dolomite. 

The metamorphism of dolomite accompanied by the formation of 
the variety of olivine known as forsterite and the subsequent alteration 
of the forsterite to serpentine is always brought about by igneous 
intrusion. The changes that take place involve the reduction of dolo- 
mite to calcite, the magnesia combining with silica to form forsterite. 
The silica is doubtless derived from hot solutions emanating from the 
igneous intrusives. It is possible that the rock was formed in this 
way. No igneous rocks were encountered in hole 8, but this fact is of 
little importance in view of the general evidence of igneous intrusives 
in the vicinity. If forsterite was once present, subsequent alteration 
to serpentine has been complete and no trace of the mineral now 
remains. 

The third possibility, viz., the direct production of serpentine from 
the magnesian carbonate without the previous formation of an olivine 
mineral is, so far as known to the authors, a type of alteration not 
previously described. On the basis of the hmited data obtainable from 



GEOLOGY OF THE CONOVER DISTRICT. 127 

one drill hole this type of metamorphism can only be suggested as an 
interesting possibility. The magnesia could be derived from the dolo- 
mite and the silica from outside hot solutions coming from igneous 
sources. The entire absence of any trace of oHvine and the lattice 
work arrangement of the fibres of serpentine rather than their occur- 
rence in a mesh or sieve structure typical of olivine alteration makes 
this theory worthy of consideration. 

Hole 18, SEi of SE|, section 17, is about a quarter of a mile north 
of No. 8 and a short distance north of the north maximum line of 
attraction. This hole was inclined 60° to the south and the underlying 
rock was tested for a depth of 188 feet. The rock is a banded ferru- 
ginous slate containing thin seams of chert and lean iron formation, 
cut in one place by a narrow dike of greenstone. The dominant phase 
consists of alternate narrow layers of greenish gray and red slate, al- 
though in places the rock is entirely red, in others light gray or green, 
the red bands being absent. A thin section cut from one of the grayish 
green phases shows carbonate, quartz, green hornblende, and magnet- 
ite with subordinate sericite, chlorite, and epidote. The rock is very 
fine grained and decidedly schistose. Its clastic origin is plainty ap- 
parent from the rounded outlines of the larger quartz grains. 

Hole 21, NEi of. SE^, section 17, is located about 300 paces north 
of No. 18 and penetrates banded ferruginous slates essentially similar 
to the rock found in hole 18. 

Hole 23, center of section 17, T 41 N, R 10 E. is outside of the mag- 
netic belt and about one-half mile west and 350 paces north of No. 21. 
Drilling was continued for 100 feet in red and gray banded ferruginous 
slate interbedded with a black pyritic phase toward the bottom of 
the hol(\ 

Three (juarters of a mile west of the cross section just described 
the magnetic belt was tested again by two holes, in the NWj of NWj, 
section 20, T 41 N, R 10 E. No. 15 is on the point of maximum mag- 
netic attraction, while No. 11 is located 175 paces farther south. The 
rock from hole 15 is rich ferruginous slate with some ferruginous chert 
ill the ui)per 47 feet and gray talcose slate in the lower 60 feet. Analy- 
sis of the ferruginous slate near the top of the ledge gives Fe.=32% 
and P. =.028%. Hole It is in gray banded carbonate slate with occa- 
sional re<l or green layers. This slate is very similar to that found in 
Nos. 18 and 21. 

The rocks un(lci-l>ing the bch wcn^ again tested in the NWj of 
SEi, section 24, T 41 N, R 9 E, by hole 37, al)out 100 paces south of 
the maximum line of magnetic attraction. The slat(> at this locality 
is gray with i)urple bands and mottling near the surface. Small mag- 



128 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

netite octahedra and occasional minute garnets are disseminated 
throughout the core. In the lower part of the hole there is a slight 
coarsening of the grain coupled with considerable contortion in the 
bedding planes. While this rock is still slate, it seems, however, to 
be considerably metamorphosed. It is probably a gradation phase 
between the unaltered slate to the east and the highly metamorphic 
crystalline schist west of this locality. 

Hole 43, SWi of NWI, section 17, T 41 X, ROE, four miles west of 
No. 37, penetrated mica schist cut bj^ dikes of white biotite granite. 
This rock is similar to that found on the Manitowish range and in the 
Vieux Desert area and is undoubtedly the metamorphic erjuivalent 
of the slates found in this same magnetic belt a short distance east, 
the alteration being caused by granitic intrusion. This belt is con- 
tinuous, but at its western end where the underlying slates have been 
metamorphosed by the intrusion of granite the magnetic attractions 
are variable and no single maximum line can be traced. 

Hole 40, NAVi of SWJ, section 9, T 41 X, ROE, was put down in 
non-magnetic territory about one-half mile north of belt B. The test 
shows biotite schist and granite similar to that found in hole 37. 

Belt C extends from section 7, T 41 N, R 11 E, westward five miles 
through section 0, T 41 X, R 10 E. It is characterized by two and in 
places three maximum magnetic lines. This belt was tested by hole 
34, SAVi of SEi, section 11, T 41 X, R 10 E, just south of the maximum 
line of variation. The drill core is quartzose slate in the upper 15 feet 
followed by 10 feet of soft green chlorite schist. The chlorite schist 
contains seams and layers of hematite in the upper portion. In thin 
section this rock is found to be composed of a schistose aggregate of 
chlorite and green hornblende, with many clusters of titanium min- 
erals, considerable sericite or talc, a few scattered flakes of hematite, 
and an occasional grain of epidote and magnetite. It is not possible 
to determine the exact relationship of the schist to the quartz slate 
owdng to the broken character of the core near the contact. In some 
ways this contact is suggestive of a fault breccia cemented by iron 
oxide, but no positive statement is justified. 

Hole 36, SAVi of XEi section 8, T 41 X, R 10 E, is west of the hmits 
of belt C, and a short distance north of the projected strike. The 
rock is medium grained garnetiferous biotite schist with stringers and 
veins of granitic material. 

Three holes were put down at other locahties in the Conover area 
not directly connected with belts A, B or C. Hole 31, XE^ of NEi, 
section 33, T 41 X, R 10 E, just north of a small oval shaped magnetic 
area in the south part of T 41 X, R 10 E, encountered granite. Holes 



Plate XI. 



(A). (Without analizer, X 16). Kyanitic-garnetiferous-gneiss from section 
30, T. 42 N., R. 4 E., Wisconsin. Tliis plate is introduced to show the coarse 
granitoid texture of the kyanite bearing gneiss of the Manitowish range. 
Kyanite, garnet, quartz, biotite and magnetite are easily recognized. 



(B). (Without analizer, X IG). Garnetiferous-pyritic-graphitic rock from 
drill hole 5 near southeast corner of section 2, T. 42 N., R. 5 E., Wisconsin. 
This rock is identical with that found in hole 33 of the Winegar section on 
the Turtle range and is introduced here to show the appearance in thin sec- 
tion of these peculiar garnet rocks which are believed to be altered sediments. 
The mineral with the high index of refraction is garnet, the large black 
mass, pyrite, the small black specks, graphite, the light gray flaky mineral, 
biotite. The white areas are occupied by a coarsely crystalline mosaic of 
quartz. The garnets are filled with minute black dust not apparent in the 
reproduction. 



Michigan Geological aud 
Biological Survey. 



Publication 18. Geology 15, 
Plate XI. 





(.14) 



Plate XII. 



(A). (Without analizer, X 16). Serpentine-magnetite-carbonate rock 
from hole 8, NE14 of NE14 of section 20, T. 41 N., R. 10 E. The granular 
texture of the rock as outlined by the arrangement of magnetite and <'arbon- 
ate is well shown. The black areas are magnetite, the gray areas are car- 
bonate, the light areas are colorless serpentine. 



(B). (With analizer, X 16). Same as (A) showing obliteration of the 
granular structure under crossed nicols by the doubly refracting lattice work 
of serpentine. 



Michigan Geological and 
Biological Survey. 



Publication IS. Geology 15, 
Plate XII. 










(A) 







( H 



GEOLOGY OF THE COXOVER DISTRICT. 129 

Xos. 47 and 44, were put down in T 41 X, R 13 E, in non-magnetic 
territory east of the Conover belts, but in line vnth. their projected 
trend. Hole 47, SEf of NEj, section 17, is in granite, while hole 44, 
SWj of XWj, section 9, penetrates about 50 feet of ferruginous slate 
with occasional chert bands developed toward the botton of the hole. 
When the hole was abandoned about 2 feet of black slate had been 
entered'. 

17 



CHAPTER IX. 

THE PAINT SLATE AND THE WOLF LAKE GRANITE, GNEISS 

AND SCHIST. 

R. C. ALLEN AND L. P. BARRETT. 

The Paint slate and the Wolf Lake granite are described together 
in order to bring out more clearly the evidence pointing to a correla- 
tion of the mica schist series of the Turtle range with the great Paint 
slate formation of the Animikie of the Crystal Falls-Iron River dis- 
trict. The Wolf Lake granite and associated mica schist occupies the 
territory of about 75 square miles which is enclosed on the north, west, 
and south sides by the Turtle range and the AVatersmeet branch of 
the Manitowish range. This area opens out eastward into the territory 
occupied by the Paint slate. There is clear evidence that the Wolf 
Lake granite is intrusive in the Paint slate. This relation will be dis- 
cussed follo^\ang a description of these formations. 

THE PAINT SLATE FORMATION. 

The Paint slate formation comprises an unkno\vTi but undoubtedly 
great thickness of graywacke, graywacke-slate and gray slate associ- 
ated with basic intrusive and extrusive igneous rocks occupying a large 
but ill defined area north and west of the Iron River and Cr^^stal Falls 
districts, and south of the west end of the Marquette iron range. It 
constitutes a large part of what has been described as the Michigamme 
slate series. 

The Paint formation is mainly graywacke antl graywacke slate and 
contrasts strongly with the black and gray slate which are associated 
with the iron formation of the Iron River and Crystal Falls districts 
and consequently was described by Allen in 1909* under a tlistinct 
formation name. Since 1909 these rocks have been further studied by 
Allen and assistants in T's 44 and 45 N, R's 34, 35, 30 and 37 W, 
Michigan. In this area exjwsures are plentiful only in the vicinity of 
Paint River and its tributaries. A general scarcity of exposures, ren- 
<lers impossible the identification of horizons over any considiM-able 
area and makes structural mapping, wiiich is difHcult in formations of 
this type under the most favorable conditions, also impossible. In the 

♦Publication 3, MicliiKaii C.tol .<; Hiol. Survey. 



132 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

eastern part of the area of the Paint slate the rocks are, comparatively 
speaking, not highly metamorphosed, and are on the whole coarser 
grained. In many of the exposures bedding structure is preserved but 
westward and south west ward the rocks l)ecome progressively more 
highly metamorphic until, through recrystallization and the develop- 
ment of schistosity, the bedding structure is wholly obliterated. 

The composition of the Paint slate is almost uniform although there 
are variations ranging from tj^pical graywacke to what may be termed 
feldspathic quartzite and here and there fine conglomerate. The rocks 
are everywhere composed dominantly of cjuartz, orthoclase and plagi- 
oclase, evidently derived from older terranes which have broken down 
by mechanical disintegration. Most of the exposures exhibit a sec- 
ondary schistose structure which is in some places parallel to the bed- 
ding but in most localities intersects the bedding planes at low angles. 

Perhaps the most striking characteristic of the series as a whole is the 
uniform gray or grayish green color of the weathered exposures. The 
exposures in any one locality are almost exactly similar to those in nearly 
all other localities with allowance for difference in degree of meta- 
morphism. Another interesting feature is the occurrence of ellipsoidal 
cavities on the surface of many of the exposures. These cavities are 
ordinarih' 5 or 6 inches long, one or two inches vnde and an inch or 
more deep. The long dimension invariably concides with the strike 
of the secondary structure. Similar cavities occur in the upper slate 
formation of the Marenisco range. Another peculiar feature of wea- 
thering, particularly well developed in the exposures near the center 
of the NW| of section 15, T 45 N, R 36 W, is the occurrence of small 
mamillary protuberances from one-half to one inch in diameter pro- 
jecting a half inch or more above the general surface of the rock. Nearly 
all of these protuberances have a small hole in the center about a half 
inch deep. The appearance of the rock reminds one of the surface of 
boiling mush. The cause of this extraordinary phenomenon is obscure. 
The rock is typical graywacke. Thin sections cut from the mamillary 
protuberances exhibit under the microscope no dissimilarity in com- 
position or texture when compared with those cut indiscriminately 
from the body of the rock. 

PETROGRAPHIC DESCRIPTION OF PAINT SLATE. 

Microscopic study of the Paint slate affords a basis for division of 
the series into three main types of rock, viz., (1) grajnvacke and gray- 
wacke schist, (2) fine grained gray slate and (3) mica schist and mica- 
hornblende schist, the metamorphic equivalents of (1) and (2). Arkose^ 
quartzite and graj^-acke conglomerate occur here and there but are 



PAINT SLATE AND WOLF LAKE GRANITE. 133 

unimportant in comparison ^-ith the enormous mass of the graywacke 
types. 

Graywacke and graywacke schist. The important minerals in these 
rocks are quartz, orthoclase, plagioclase. chlorite, and sericite. In 
some specimens biotite and carbonate are abundant. Pyrite, epidote, 
zoisite, zircon, apatite, tourmaline and magnetite-leucoxine and rarely 
augite are the accessory minerals. 

In general the texture is medium to fine grained and clearly of clastic 
origin which is manifest in the rounded outlines of the larger grains of 
quartz and feldspar, while many of the quartz individuals exhibit 
secondary enlargement. In the schistose gra^-^vackes the flakes of 
chlorite and sericite are in parallel alignment but the grains of quartz 
and feldspar are not commonly affected by this structure although in 
some specimens they exhibit a tendency toward orientation of their 
longer axes in the plane of schistosity. With decrease in the content 
of feldspar the graj'wackes grade into arkose and quartzite, and with 
increase in size of the detrital fragments into conglomerate. 

Although quartz is the most abundant mineral, plagioclase, ortho- 
clase, chlorite and sericite are almost equally important. In exceptional 
cases biotite takes the place of chlorite. The larger grains of plagio- 
clase are fresh and exhibit the characteristic twinning lamellae. The 
orthoclase commonly shows cloudy alteration and inclusions of hem- 
atite particles. The presence of tourmahne in small crystals showing 
good development in the prism zone but \nthout terminations is prac- 
tically universal. In some instances the grains are rounded and water- 
worn and it is probable that the tourmaline is all of detrital origin 
although this question cannot be determined with certainty. This 
mineral is never abundant ])ut from one to a half dozen minute crystals 
are present in every thin section examined. Tourmaline was not re 
ported by Allen in 1909 but a re-examination of the thin sections of 
the Paint slate described by him resulted in the identification of this 
mineral in minute quantity. In some of the slides calcite occurs in 
isolated areas and less often in irregular individuals arranged in lines 
parallel to the schistosity. Pyrite and epidote occur in considerable 
abundance in some of the specimens. 

Slate. The slates are gray, grayish green and black. They are 
essentially similar in mineral composition to the graywackes. The 
important minerals are quartz, feldspar, chlorite, sericite, biotite and 
in some instances, carbonaceous material. The minor minerals are 
pyrite, carbonate, magnetite (usually coated with leucoxine), rutile, 
ai)atite, epidote, zircon, and tourmaline. The first two are frequently 
of considerable imi)ortance. The rock is .so fine grained and schistose 
that the mineral composition is only ol)servable untler high powers 



134 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

although most of the sHdes exhibit scattered rounded grains of fjuartz 
and feldspar of consideral)le size and these bear witness to its clastic 
origin. 

Mica schist, hornblende schist and altered qraywacke. For the most 
part these rocks are fine grained f(>ldspathic mica schists which are 
here and there hornblendic. They are characteristic of the Paint 
slate in the western part of the area. They are composed of cjuartz, 
feldspar, biotite or hornblende, chlorite, sericite, epidote, pyrite, tourma 
line, zircon, magnetite-leucoxine and apatite. 

There are all possible gradations between the mica schists in which 
the clastic structures and textures have been obliterated by recrystal- 
lization and the graywacke and slate. In some of the outcrops the 
schist alternates with massive or coarser grained layers in which sedi- 
mentary textures are still preserved. In other instances microscopic 
study shows that the recrystallization is onl}^ complete for the finer 
grained material in the rock, the larger fragments retaining their 
rounded outlines. Some of the schists are also marked by a banding 
which cuts the plane of schistosity at a small angle. In certain of the 
less altered varieties the biotite is oriented haphazardly but the 
schistose structure is retained through the parallel arrangement of 
chlorite and sericite. The biotite individuals enclose fragments of 
quartz and feldspar and are plainly secondary. 

SUMMARY STATEMENT. 

The Paint slate, so far as it is possible to judge from the field observa- 
tions and microscopic study, comprises a vast thickness of fragmental 
rocks laid down under subaqueous conditions. The material is ap- 
parently derived from the mechanical disintegration of crystalline 
rocks of acid or intermediate composition. In some localities, par- 
ticularly along the south border, there are associated greenstones both 
intrusive and extrusive, but throughout the greater part of the area 
underlain by the Paint formation greenstone or other igneous rocks 
are notably absent. Here and there dykes of diabase cut the slate but 
they are not of great size or of frequent occurrence. 

The rocks are characterized by a regional schistosity striking nearly 
east and west. Both northern and southern dips occun, the latter 
being characteristic of the more westerlj^ exposures. Where bedding 
is recognizable it is generally in approximate parallelism to the sec- 
ondary structures of cleavage or schistosity. It is clear that the 
rocks have been highl}^ folded by forces acting mainly in a north- 
south direction. 

The relationship of the Paint slate with adjacent formations is not 
known. It is believed to be conformably above the Middle Huronian 



PAINT SLATE AND WOLF LAKE GRANITE. 135 

(Animikie) slate-iron formation series of the Crystal Falls and Iron 
River districts. It is intruded by the Wolf Lake granite which is 
correlated with the Presqiie Isle granite of the Gogebic range. The 
age of the Paint slate is believed to be largely Middle Huronian (Ani- 
mikie) and is discussed in connection with the general correlations in 
chapter 2. Part of it may be of Upper Huronian age. 

GRANITES, GNEISSES AND SCHISTS OF THE WOLF LAKE AREA. 

The exposures in this area comprise a complex of mica schist, green 
schist, granite and gneiss. The prevailing strike of schistosity is about 
east and west with dip vertical or steeply inclined to the south, al- 
though northerly dips are not uncommon. 

The mica schists are best developed in T 46 N, R's 39-40 W, and 
granites and green schists in T 45 N, R 39 W. However, mica schists 
are locally found in the latter township and granite and green schist 
are not entirely lacking in the two northern ones. The schists are 
everywhere intruded by the granite and are therefore regarded as the 
oldest rocks of the complex. 

The Wolf Lake Granite. 

The Wolf Lake granite comprises massive and gneissose varieties of 
acid composition and subordinate more basic phases including quartz- 
diorite and syenite. The rocks are mainly white, gray, and pink, 
with typical granitic, pegmatitic and porphyritic textures. 

The primary minerals of the common normal granite are orthoclase, 
quartz, plagioclase, biotite, fine particles of red hematite, magnetite 
(often associated with leucoxine) and occasional crystals of zircon and 
apatite. Secondary minerals are microcline and other feldspars, sericite, 
chlorite and quartz. Most of the specimens exhibit in thin section a 
characteristic mortar structure. In the gneissose varieties the biotite 
and chlorite show parallel orientation. The feldspar, with the excep- 
tion of microcline, shows considerable alteration. The microcline appears 
in many cases to be an alteration product of orthoclase. It occurs in 
small scattered patches in orthoclase crystals, less commonly it forms 
a ])order around orthoclase individuals. In some specimens chlorite is 
the only ferro-inagnesian mineral but in others it is closely associated 
with biotite from which it is probably derived. Hornlileude does not 
occur in the typical normal granite. 

The syenite differs from liie normal granite ojily in the ahnost total 
absence of (luartz and merits no sjx'cial des(Tiption. 

The more basic varieties of the granite are composed of plagioclase, 
ort lioclasc, (|uart/, biotite, liornblende, magnetite, and titanite. S(H'- 



136 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

ondary minerals are feldspar, carbonate, zoisite, epidote and biotite. 
In thin section the mortar structure is typically developed, the feld- 
spar is commonly granulated, and tiie mica shows parallel orientation. 
The feldspar is predominantly plagioclase, generally highly altered to 
calcite, zoisite, quartz and biotite. Orthoclase is suliordinate, shows 
alteration to sericite and, to some extent, intergrowth with quartz. 
The rocks have a composition probably very near that of the quartz- 
diorite. 

In certain places small masses of basic material, characterized by 
the development of considerable hornblende, have separated from the 
granitic magma. 

The Wolf Lake Schists. 

The mica schist series includes biotite-quartz schist, biotite-musco" 
vite-garnet schist, hornblende-biotite schist, feldspathic biotite schist 
and chlorite schist. The greenschist may be more correctly termed 
amphibolite or hornblendite. Green hornblende is the most abundant 
and important mineral, especially in the coarse grained varieties and 
occurs largely to the practical exclusion of other minerals. Part of 
the green schist appears to be merely a differentiation product of the 
granitic magma. On the other hand, granite is found plainly intrusive 
in hornblende schist, and this is probably their prevailing relationship. 

Green schist. The green schist or amphibolite is most abundant in 
the northwest portion of T 45 N, R 39 W. It is uniformly dark green 
to black and comprises both coarse grained types and fine grained 
rocks approaching slate in appearance. Under the microscope the 
coarser grained varieties appear to be made almost entirely of large 
individuals of green hornblende with a small amount of epidote, quartz 
and magnetite, the latter closely associated with leucoxine or other 
titanium minerals. The hornblende is of the strong pleochroic greenish 
blue variety and commonly shows yellowish limonitic colored bands 
around the edges and filling the cleavage cracks. The other minerals 
occur both as inclusions within and in the interstices between the horn- 
blende individuals. Magnetite occurs in irregular grains and clusters of 
grains closely associated with leucoxine or small crystals of rutile and 
brookite. In one observation the latter mineral is present in fairly 
large sized grains some of which show in convergent light beautiful 
biaxial interference figures. 

The finer grained green schists are characterized by a greater abund- 
ance of quartz, epidote, carbonate, and secondary feldspar (limpid 
albite). There is no direct clue to the original character of the green 
schists but it is probable that they are mashed igneous rocks of basic 
composition. 

Mica schist. Throughout the Wolf Lake area there are exposures 



PAINT SLATE AND WOLF LAKE GRANITE. 137 

of acid mica schists similar in many respects to those found in the 
southern Archean (?) area described in connection ^vith the Marenisco 
range. The mica schist series is particularly well developed in sections 
29, 30, 21 and 32, T 46 N, R 39 W; in section 6, T 45 N, R 39 W; and 
in the southeastern part of T 46 N, R 40 W. The mica schists are 
characterized by an abundance of quartz veins, blebs, and stringers 
parallel to the schistosity. Banded structures at variance "w-ith the 
schistosity were not observed in the Wolf Lake area. In general ap- 
pearance, however, the mica schists are very similar to those of the 
Marenisco range and, with the exception of their greater abundance of 
garnet, mineralogical differences are revealed only under the micro- 
scope. Both rocks are generally gray, fine grained and markedly 
schistose although in both areas there are exceptional non-schistose 
and massive types which have the appearance of fine grained gra^-wacke. 
Under the microscope the Wolf Lake schists reveal the presence of 
quartz, biotite, muscovite, chlorite, secondary limpid albite, garnet, 
sericite, magnetite (usually coated with leucoxine), pyrite and hema- 
tite. Green hornblende, epidote and calcite are sparingly present. In 
general the minerals are arranged in a rather fine grained interlocking 
crystaUine mosaic. By extreme mashing the grams of quartz and 
secondary feldspar are elongated parallel to the orientation of the 
micas, but as a rule the latter only, show marked aligimient. The 
garnets in all cases were evidently developed under conditions of static 
metamorphism. They are full of inclusions of other minerals and as a 
rule have made space for themselves largely by pushing the mica 
aside, thus developing a more or less perfect augen structure. 

The predominant type of mica schist is composed mainly of quartz, 
pale brown biotite with pale green chlorite, and a lesser amount of 
colorless muscovite, limpid feldspar and small colorless garnets. Ac- 
cessory minerals are magnetite and an occasional irregular patch of 
pyrite generally associated with hematite. In thin section the min(n-als 
are arranged in a typical crystalline interlocking mosaic, and while 
many of the quartz grains show roundish outlines they fit perfectly 
into the mosaic and in no case have undoubted detrital grains been 
observetl. The very subordinate amount of secondary feldspar is in 
rather sharp contrast to its abundance in the feldspathic biotite schists 
of the Southern Archean (?) area of the Marenisco range. 

In addition to the rocks described alcove, certain restrictetl types are 
couiposed of biotite, chlorite, quartz and garnet with pyrite and raag- 
netite-leucoxine as accessories. Other types contain muscovite, l>iotite, 
and garnet in large individuals, the mica showing perfect parallel 
orientation, and in one observation the schist is composed almost 



138 



CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 



entirely of chlorite and quartz with subordinate carbonaceous (?) 
material. 

Through the assistance of Dr. C. K. Leith a composite sample of 
the mica schist of the Marenisco and Turtle ranges was analyzed both 
chemically and physically in the hope of making definite determination 
of its origin. Unfortunate!}^ neither analysis offers conclusive evidence. 
Prof. W. J. Mead of the University of Wisconsin made a solution 
separation of the crushed sample and examined the heavy residual 
minerals under the microscope. He found no evidence of rounded 
grains and to this extent the evidence, though inconclusive, is against 
sedimentary origin. The chemical analysis was made b\' Dr. W. G. 
Wilcox, and is given below: 

Analysis of Mica Schist. 

59.50% 

16.47 (does not include TiOs) 

4.08 

6.61 



Si02 

AI2O3 

Fe^Os 

FeO 

TiO, 

MnO 

CaO 

MgO 

Na20 

K2 O 

CO2 

Carbon (organic matter) 

Water below 100 deg. C 

Water above 100 deg. C 



Present but not determined. 

1.04 

1.59 

2.65 

2.40 

trace 

.35 

.12 

2.43 



Total 98.02% 



RELATIONS OF WOLF LAKE GRANITE AND MICA SCHIST TO THE PAINT 

SLATE FORMATION. 

We have shown that there is considerable evidence of sedimentary 
origin of the mica schist of the Southern Archean (?) area of the 
Marenisco range. (Chapter 4). That the Wolf Lake schists are of 
sedimentary origin is strongly supported, if not indeed conclusively 
proven, through what seems to be a direct connection with the Paint 
Slate in section 13, T 46 N, R 39 W. In the NEi of the SWi of this 
section, adjacent to the railroad, mica schist, identical in every respect 
to those described above, is inseparable from schistose graywacke. 
Both rocks occur in the same exposures and grade one into the other. 
There is no question whatever that the mica schist here is an extremely 



PAINT SLATE AND WOLF LAKE GRANITE. 139 

schistose graywacke. The same relations are exhibited about three 
quarters of a mile southeast of this locality, at and near the rapids on. 
the Middle branch of the Ontonagon river in the SEj of section 18, 
T 46 N, R 38 W. In a distance of about 250 paces the river falls north- 
ward in a series of cascades from 50 to 60 feet directly across the E-W 
strike of the schistosity of the graywacke. The dip of the plane of 
schjstosity is south about 70°, opposed to the direction of the flow of 
the stream. The rocks exhibit every gradation from dark massive 
graywacke to crystalline mica schists carrying blebs and stringers of 
quartz which are exactly similar to the Wolf Lake schists. The gray- 
wacke is identical ■^dth the most common phases of the Paint slate 
which apparently underlies the area from this locality eastward to the 
northern part of the Iron River district. The conclusion that the 
Paint slate and the mica schist of the Wolf Lake area are one and the 
same formation is almost inescapable. If this conclusion be embraced 
it follows that the Paint slate is intruded by the Wolf Lake granite. 
In chapter 2 on general correlations it is sho\vn that the Paint slate 
is probably of Middle Huronian age. The age of the Wolf Lake granite 
is probably not older than Middle Huronian (Animikie). It has been 
shown in chapters 3, 4, 5, 6 and 7 that the intrusive granite of the 
east end of the Gogebic range and the Marenisco, Turtle, Manitowish 
and Vieux Desert-Conover districts is doubtless Middle Huronian (Ani- 
mikie) and in chapter 2 these granites are correlated with those of the 
Florence and Menominee ranges which also intrude Animikie sedi- 
ments. Therefore, it is the most reasonable assumption that the Wolf 
Lake granite is merely an outlier of the great Animikie granite batholith 
of Northern Wisconsin, and is of late Middle Huronian age. For a 
further discussion of the correlation of the Paint slate and the granites 
which intrude the Animikie sediments see chapter 2. 



CHAPTER X. 

CORRELATION AND STRUCTURE OF THE PRE-CAM- 
BRIAN FORMATIONS OF THE GWINN IRON- 
BEARING DISTRICT OF MICHIGAN* 

R. C. ALLEN. 

Published information regarding the geology of the Gwinn district 
is very meager. In 1873, Major J. B. Brooks pubhshedf a brief des- 
cription of the locality now occupied by the Princeton and Stegmiller 
mines, then knowai as the S. C. Smith mine, in sections 17, 18, and 20, 
T. 45, R. 25. In speaking of the occurrence of iron ore there he says: 
"The geographical position is less remarkable than what might be 
called its geological isolation, for it appears to be in a small patch of 
Huronian rocks, in the midst of a great area of barren territory, under- 
lain by the Laurentian and Silurian systems." Brooks observed the 
black slate adjacent to the ore on the northeast in sections 17 and 18 
and in "section 20, west of the river, a talcky schist, holding grains of 
quartz," but was unable to determine the stratigraphic relation of these 
rocks to the iron formation. 

About ten years later this locality was again examined by Dr. Carl 
Rominger, Jwho writes as follows: "The Cheshire mine, formerly 
known as the S. C. Smith mine .... is working a strip of slaty and 
quartzose rock beds, knowTi to extend along the valley of the Escanaba 
River for a distance of nine miles from the northwest corner of section 
19, T. 4G, R. 26, to the center of T. 45, R. 25." Rominger describes 
the rocks showii in the mining pits in sections 18 and 20, T. 45, R. 25, 
in considerable detail. He recognizes an iron formation underlain and 
overlain by slate. Owing to his misunderstanding of the structure 
his suc(u>ssion is reversed. 

In 1011 the United States Geological Survey published a brief ac- 
count of the geology of the Gwinn (Swanzy) district l\v C. R. Van Hise 
and C. K. L('ith.§ These authors had made no detailed survey of this 
district and attempted merely a summary of the information from other 
sources available to them at the time their nioiiograi)!! was written. 
They describe the Gwiim district as a southeastwanl-iMtching syncli- 

*PuliliHtu'(l ill the Journal of Ceolopy, Vol .XXII, No. 6, St'i)leiiil)fr-October, 1914. 
tMicliiKiin (SeoloKical Survey, I. 150-51. 
tlbid, V, (IS!»4), I'lirt I. pp. 70-7S. 
§C. H. Van Hise and V. K. Leitli. Monograph r>2, V. S. Geological Survey, pp. 28.3-80. 



142 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

norium about two miles long and from one-half to two miles wide, the 
structure being unknown toward the southeast because of the deep 
overburden. They correlate the pre-Cambrian sedimentary rocks with 
the Upper Huronian series and describe them as (1) a basal "quartz 
slate and quartzite grading down into arkose or decomposed granite" 
which is overlain by (2) the Michigamme slate carrying the Bijiki 
iron-bearing formation in "lenses and layers" near its base. 

Recent studies by the writer, based on field mapping and an exami- 
nation of the records of several hundred diamond drill holes, show 
clearly that the Gwinn district contains at least two unconformable 
series of sedimentary rocks. It seems probable that the upper series, 
which will be described as the Princeton series, is equivalent to the 
Upper Huronian of the Marquette district, that the lower series, which 
will be described as the Gwinn series, is equivalent to the Middle Hu- 
ronian of the Marquette district, and that the Lower Huronian series, 
while not present in the Gwinn synclinorium, is represented by certain 
fragments of quartzite and cherty slate in the conglomerate at the 
base of the lower or Gwinn series. 

Without the information afforded by records of drill holes and other 
exploratory operations, any statement of the geology of the Gwinn 
district would probably be misleading and in any event necessarily 
fragmentary and incomplete. Outcrops are not plentiful except in 
certain restricted localities and are limited to the north two-fifths of 
the district. The records of drill holes, carefully compiled by geologists 
of the Cleveland Cliffs Iron Co. and the Oliver Iron Mining Co., are 
the main reliance for mapping the formations. Only a few of the drill 
samples were seen by the writer, but each of the formations is somewhere 
exposed either in outcrops or in excavations and was studied on the 
ground. It will be seen on the accompanying map that information is 
entirely wanting in some parts of the synclinorium and in other parts 
is insufficient for accurate mapping. Only a few of the many faults, 
which certainly occur, particularly in the north end of the district, have 
been mapped and the exact location and character of even those is not 
apparent. 

The lithology of the various formations will be considered only so 
far as essential to an understanding of the succession and the correla- 
tions, but the discussion necessarily will be more in detail than the 
account published in Monograph 52, to which reference has been made. 

Preliminary to the statement of the geology, there is given in parallel 
columns for comparison the succession and correlation of the United 
States Geological Survey and of the writer. 



GEOLOGY OF THE GWINN DISTRICT. 



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GEOLOGY OF THE GWINN DISTRICT. 145 

LOCATION AND TOPOORAPHY, ETC. 

The Gwinn synclinorium occupies an area about six miles long and 
from one to two miles wide, mainly in T 45 N, R 25 W, but extending 
a short distance into T 44 N, R 25 W. The trend of the major struc- 
ture is about N 45° W, or almost exactly parallel to the Republic trough, 
the southern end of which is 22 miles west and 6 miles north of the 
north end of the Gwinn fold. Gwinn, the principal village, is 16 miles 
south of the city of Marcjuette. 

The southeast three-fifths of the Gwinn fold is buried beneath a 
featureless and almost flat sand plain which extends north and east to 
the hills of the Marquette range.- In the opposite direction the surface 
is broken and hilly with occasional rock exposures. Granite hills en- 
circle the northwest and north sides of the synclinorium. The district 
is drained by the Escanaba River, which follows the northeast side of 
the trough to Gwinn and then turns south across the sand plains. On 
the plains the water table is within a few feet of the surface and the ore 
bodies are deeply buried under water-saturated sand and gravel, a 
condition which is a serious menace to mining operations. 

The first shipment of ore was made in 1872 from the Cheshire mine, 
now known as the Princeton No. 1 pit. About 1902 the Cleveland 
Cliffs Iron Co. purchased the Princeton (Swanzy, Cheshire) mine and 
during the time which has since elapsed has extended its holdings by 
purchase and lease until it now controls all of the known workable ore 
bodies with the exception of the Stegmiller, which is mined by the 
American (Oliver) Mining Co. Since the building of the beautiful and 
princii)al village of (iwinn by the Cleveland Cliffs Iron Co. the name 
of the district has been changed by common usage from Swanzy to 
Gwinn. There are five j^roducing mines in the district. Concrete shafts 
have been sunk to a number of additional ore bodies but it is not known 
when these will be ecjuipped for mining operations. 

NOTES ON THE STUUCTUnE OF THE GWINN SYNCLINOUHIM. 

The Gwinn synclinorium contains two unconformable series of 
sedimentary rocks, having a combined thickness of from 800 to 1,000 
feet. Outliers of flat-lying Paleozoic (Cambrian or Ordovician) sand- 
stone and limestone occur throughout this lu'ca. The pre-Canibrian 
beds are renmants of formations, originally nuich more extensive, which 
have escaped erosion by downfolding or depression in the Archean 
basement. 

The synclinoriuni is constricted to iK)t more than t hrc(>-fourths of 
a mile in width in the vicinity of the NW-, of section 29, T 45, R 25. 
North of tlic constricted poiiion, the rocks arc folded and faulted in 
lit 



146 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

a complex manner hut south of it the Ktructure is apparently somewhat 
less complicated. 

The southern three-fifths of the synclinorium is a spoon-shaped 
basin four miles long with a maximum width of about two miles. The 
deepest part of the fold is adjacent to the northeast limb where the 
Archean granite is reached in many drill holes at depths of from 1,000 
to 1,200 feet (see cross-section III-IV., Fig. 9). Drilling along the 
southwest limb indicates a number of sharp drag folds pitching north- 
west. The folds on the opposite limb are not so sharp and are apparent- 
ly simple cross-folds. The most prominent one appears in the SEj of 
section 35. The synclinorium practically terminates against a faulted 
zone on the southeast. It is not possible to determine from present 
information the full extent of this zone nor the character of the faulting. 
The rocks in the faulted area are largel}^ slate, chert, conglomerate, 
and breccia resembling lithologically the succession in the upper or 
Princeton series, but the regular succession of formations shown on 
both limbs of the fold terminates abruptly at the line indicated as a 
fault on the map. Another cross-fault trends diagonally northesat 
through section 28, producing a horizontal displacement of not less 
than 700 or 800 ft. in the NEJ of section 32 and from 150 to 200 ft. in 
the N^ of the NWj of section 28. The offset in the latter locality may 
be explained by folding, but the sharpness of the break in the former 
locality strongly suggests faulting. In any case, the extension and 
direction of the fault as indicated on the map is to a couvsiderable 
degree hypothetical. 

Knowledge of the structure of the northeast two-fifths of the Gwinn 
synclinorium pertains chiefly to the northeast liml). The most con- 
spicuous structural feature of this limb is the broad cross-anticline 
responsible for the extraordinary surface exposure of the iron forma- 
tion in the vicinity of the Austin and Stephenson mines, giving rise to 
two prominent synclines, the northern one carrying the Princeton No. 
2 ore body and the southern one the Austin-Stephenson deposit (see 
cross-section I-II Fig. 9). Northward from Princeton No. 2 mine the 
east limb is overturned and dips at an angle of about 80° to the north- 
east, about parallel to a faulted contact with black slate extending from 
somewhere north of the Old Swanzy pit in the SWj of the NEJ of sec- 
tion 18 southeast for a distance of probably more than a mile. Where 
observed in the Swanzy pit and in the Princeton No. 1 pit in the SE| 
of section 18, the dip of the fault plane is northeast about 75° or 80°. 
Both the iron formation and the adjacent slate are intensely sheared 
along the zone of faulting. The belt of slates adjacent to the fault on 
the northeast may be stratigraphically either above or below the iron 
formation so far as the writer has proof. The upper and the lower slate 



147 

Drill 
Q this 

series 
This 
elongs 
Idle of 
)ution 
ggests 



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orium 
hwest 
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holes 
vithin 



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hvinn 
)ly of 
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nown 



ie up- 

e, (3) 

•ios is 
irfaco 
ver a 
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dom- 
It is 
ubse- 
•Iving 



GEOLOGY OF THE GWINN DISTRICT. 147 

members of the Gwinn series are lithologically very similar. Drill 
holes and the mine workings show that the iron formation in this 
vicinity lies directly on the basal arkose member of the Gwinn series 
with here and there a few feet of black slate lying between them. This 
makes it very probable that the slate belt northeast of the fault l:>elongs 
to the upper slate member of the Gwinn series. North of the middle of 
section 18. details of the structure are unknowai but the distribution 
of formations indicated by the few exposures and drill holes suggests 
deformation by both folding and faulting of a complex character. 

ARCHEAX SYSTEM. 

The Archean system comprises both acid and basic plutonic rocks, 
granite greatly predominating. These rocks inclose the s^aiclinorium 
on the west, north and east sides, encircling the north and northwest 
sides in bold hills and protruding through the drift in low knobs on 
the east side from New Swanzy northward. Numerous drill holes 
reach the system after penetrating the overlying sedimentaries ^^^thin 
the borders of the synclinorium. 

ALGONKIAN SYSTEM. 

The Algonkian system is represented by two unconformable series 
of Huronian sedimentary rocks, the Princeton (upper) and the CiAnnn 
(lower) series. Both series are intruded by basic dikes, probably of 
Keweenawan age. The basal conglomerate of the G\\'inn series con- 
tains pebbles and boulders of quartzite, quartz slate, and siliceous, 
cherty, slightly dolomitic slate derived from a third sedimentary series 
unconformably below the Gwinn series but not present so far as known 
in the Gwinn synclinorium. 

MIDDLE HURONIAN. 

Gwinn Series. 

There are four members of the Gwinn series, viz., from the base up- 
ward, (1) conglomerate and arkose, (2) black slate and gray slate, (3) 
iron formation, and (4) l^lack slate, gray slate, and graywacke. 

1. Conglomerate-arkose — The basal member of the Gwinn series is 
mainly arkose and arkose conglomerate. It lies on an uneven surface 
of Archean granite and is reported to occur in isolated patches over a 
considerable area outside of the Gwinn synclinorium. Within the fold 
its thickn(>ss varies from practically nothing to al)ove ()0 ft. The dom- 
inant phase of the member is arkose or decomposed granite. It is 
evident that the arkose has its origin in the disintegration and subse- 
quent sedimentation of the disintegrated |)articles of the underlying 



148 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

granite which in many places it resembles so closely that distinction 
is difficult. There are phases of the arkose in which the feldspar crys- 
tals show little perceptible wear, much less the quartz grains. It is 
particularly difficult to separate from granite in places near the contact 
where secondary mica has developed and veins of quartz and pegmatite 
occur like those in the granite. Phases in which there has been per- 
ceptible or conspicuous rounding of the quartz and feldspar particles 
are commonest and these may be either massive or schistose. The 
schistosity in the arkose is the result of masliing of the feldspars, by 
which process the quartz grains are generally not greatly affected. 
Where the arkose is overlain by the iron formation and particularly by 
iron ore, as in the mines north and west of CJwinn, it is in many places 
highly decomposed, soft, and iron stained, the feldspars being largely 
kaolinized. 

The conglomerate is much less abundant than the arkose and ac- 
cording to drill records is not present in most locaUties. Its occurrence 
seems to be erratic and, curiously enough, where exposed in the SE^ 
of the SWJ of section 19, T. 45, R. 25, it lies some distance above the 
base of the formation. Drift bowlders of the conglomerate are rather 
plentiful but the only exposures knowTi to the writer are in the SWj 
of section 19. Here there are 12-15 ft. of it exposed in layers from 1 to 
2 ft. thick dipping about 16° E. and striking N. 15° W. At this locality 
the contact with the granite is about 150 paces west. The matrix of 
the conglomerate is chiefly arkose but in one exposure it is sihceous, 
gray slate interbedded with the arkose. The pebbles are up to several 
inches in diameter and are mainly vein quartz which is abundant in 
the underlying granite. There are also many fragments of greenschist, 
dense, vitreous, gray quartzite and siliceous, chert}^ shghtly dolomitic 
slate of grayish-green color. The composition of the conglomerate 
may also be studied to advantage on the waste dump of the Gwinn 
mine in the NEJ of the NW^ of section 28 where a bowlder bed was 
encountered in cutting the pumping station in the shaft. All of the 
bowlders are well rounded and vary up to 6 to 7 inches in diameter. The 
matrix is arkose so decomposed that many of the bowlders are lying 
free on the dump. In addition to the rocks represented in the exposures 
in section 19 there are many bowlders of granite and greenstone. 

The origin of the quartzite and slate pebbles is of great interest in 
its bearing on the correlation of the Gwinn series. Near Little Lake, 
about five miles east, in a range of hills on the north side of section 
19, T. 45, R. 24, ther are numerous outcrops of quartzite, quartz slate 
and arkose. Van Hise and Leith considered these rocks to be the 
base (Goodrich quartzite) of the Gwinn series which we have described. 
In fact, their description seems to apply mainly to these exposures and 



GEOLOGY OF THE GWINN DISTRICT. 



149 



M in the basal member of the Gwinn series as it actually exists in the 
S^nn s^^^^^^^^^^ There is an arkose and arkose conglomerate m 

Gwmn syncnnorm ^^ .^^ .^^ associated 

which Tre as much as 2 ft. in diameter. The exposures at Little Lake 
Ire n t to «^ Gwinn sj^linorium but .-ill be described m a ater cha^ 
ter The point is emphasized, however, that the presence oquart.rte 
and Che ty'^ quartz-slate pebbles in the basal member of the Gw.m> 
Is prows\hat there is at '^f «- "uconformable sene^ o sed 
iments between the Archean and the Gwmn ser.es. T^e ™te; b^ 
lieves that this series is the Lower Huroman as represented m the Mar 
quette district a short distance north. fifth • „f t lie district 

2. The Lower SlaU.-U the southeastern three-fifths of the district 
a black, graphitic, and a gray slate formation "^^^^]^^^ 
basal arkose member and the iron formation. It is less geneialK 
pr s ntfrom the Stephenson mine northward, in this area never ex- 
ce d ng a few feet in thickness, but south of the Stephenson mine it 
varts up to above 60 ft. thick. Were it not for hthologic dissimilarity 
Ih" slate would be included in the basal member, but inasmuch as it 
repre ents a distinct change in conditions of sedimentation and more- 
over s"ms to maintain a definite stratigraphic rCatioi. to the^over- 
lytag and underlying formations, it should perhaps be described as a 

distinct member of the series. . , , ,■ ;„ tK,. 

3 The Iron-Bearing Member.-Uko the other formations in the 
G^rim. series the iron-bearing member varies markedly m thickne^ 
but is nevertheless persistent, occupying a constant^ and definit.- stiat - 
gnphic positicm in the series. The description of the occmreiue o 
tlU member in "lenses and layers" in slate by Van Hise and Leith is 
mislead ng in so far as this implies that the member is discontinuous 
"in the syn<.linorium. The thickness »f 'f'" -" '""";^^»",';; *; 
narilv 50-100 ft. with a maximum of probably less than 125 ft. ..ml .i 
m mum of only a few feet as shown in some ilriU holes toward the 
"r oTthe bin west of the Princeton and StegniiUcr mines, ^olne 
Ins show a greater thickness than 125 ft. which is ^^^'^^ 
by folding. The formation is thinner and at the same lime Uamr 
toward th' w,.st si.lc of the ,syn,-linorium. .K\X of the known ore bodies 

are on the oast liml) of the fold. , . • -i 4-^ 

4Z iron formation is mainly banded, ferruginous chert similar to 
the "soft ore jasper" of the other Michigan rang.'s. !«■ original o 
lltill Phaie 'is cherty iron carbonate. North ol .he Swan.y pit 



150 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

in section 18, the base of the formation, as shown by drilling, seems to 
be mainly grunerite schist. This part of the district shows evidence 
of greater deformation by folding and faulting than areas farther south. 

The upper part of the iron bearing member is slaty in many places 
and the base of the overlying slate is here and there so ferruginous 
that it is a matter of choice as to whether it should be mapped as slate 
or iron formation. On the map these phases are included in the over- 
lying slates. 

The iron ores are both Bessemer and non-Bessemer grades, the 
latter greatly predominating, very soft and fine textured in the main 
and generally high in moisture. A purplish satin luster is a peculiar 
characteristic of the Gwinn ores. There are some pits in the upper 
part of the formation west of the Austin mine that show hard jasper 
and hard, blue hematite. Localization of the ores is largely coincident 
with synclinal troughs and faulted zones but is not limited to these 
structures. An inclined position of the iron formation between the 
overlying slate and underlying slate or arkose satisfies the structural 
requirements for ore concentration. 

4. The Upyer Slate. — The upper slate member is from 30 to 100 ft. 
thick. It is unconformably overlain by the basal conglomerate of the 
Princeton series. Its relation to the underlying iron-bearing member is 
largely gradational. It comprises an interbedded series of black slate, 
gray slate, and dark graywacke-quartzite. The black graphitic phase 
is more commonly directty above the iron formation than the gray 
slate, and the graywacke-quartzite phase seems to be in upper and 
middle horizons. 

UPPER HURONIAN. 

Princeton Series. 

The Princeton series consists of an interbedded series of slates, fer- 
ruginous slates, and cherts, quartzites, ferruginous quartzites, and gray- 
wacke with a basal conglomerate. The series is 400-500 ft. thick. 
Probably the entire thickness is not represented in the Gwinn fold. It 
is rarely seen in outcrops but it has been penetrated by numerous drill 
holes and some open pits. For the purpose of this article the interest- 
ing member is the basal conglomerate. 

The basal conglomerate varies from 30 to 50 ft. to more than 100 
ft. in thickness. Nearly all of the many drill holes which cross its 
horizon show its presence but here and there it is represented by a 
coarse graywacke. So far as known, the only exposures are in the 
SEi of the NE^ of section 18, T. 45, R. 25, where a number of expo- 
sures occur on a low brush-covered ridge. Adjacent to them on the 



GEOLOGY OF THE GWINN DISTRICT. 151 

east the upper slate member of the Gwinn series is exposed in pits. 
The strike of the conglomerate is X. 70° W. and the dip 80° N. 

The matrix of the conglomerate is coarse, dark graywacke-quartzite, 
the pebbles are chert and siliceous black slate, quartz, and arkose, 
derived from the underlying Gwinn series, and quartzite. The matrix 
carries a good deal of disseminated ferruginous material and some verj' 
small fragments of iron ore. There are also a good many small irregular 
cavities in the rock which are lined with hematite and limonite pro- 
duced by weathering-out of iron-bearing fragments of some kind. The 
largest chert fragments are two to three inches long and one-half to 
an inch wide. All of them show wear by attrition, the smaller ones 
being generally lens shaped. 

So far as can be ascertained, the Princeton and Gwinn series are 
structurally almost accordant. The strike of the conglomerate where 
exposed in section 18 indicates discordance in trend with the Gwinn 
series, but too little is knowii of the structure in that vicinity to place 
any importance on this observation. 

KEW^EENAWAN SERIES (?) 

Basic dikes have been cut in a few drill holes and may be observed in 
section 20, T. 45, R. 25, cutting the basal arkose member of the G^^^nn 
series. These dikes intrude both the Princeton and the Gwinn series. 
They are older than Paleozoic and younger than the Princeton series. 
Their age is therefore probably Keweenawan. 

PALEOZOIC. 

Isolated remnants of limestone and sandstone of Cambrian or Ortlo- 
vician age, or possibly both, occur throughout the district. Some of 
these are in excess of 50 ft. thick. No fossils or other means of deter- 
mining the exact age of these outliers is available at the present time. 

CORRELATION OF THE GWINN AND THE PRINCETON SERIES. 

It has been shown that the pre-Cambrian sedimentary rocks of the 
Gwinn synclinorium consist of two unconformable series. The imcon- 
formity between them is marked by a basal conglomerate the ])osition, 
extent, and thickness of wliich imi)ly an important erosion interval 
which intervened between the periods of deposition of the two series. 

( 'oiicerning the respective ages of these two series, it may be said 
that probably no geologist faiiiihar with the ))re-Gambrian formations 
of the Lake Superior region would correlate the (Jwinn (U)wer) series 
with the Lower Iluroiiian. It contains an important iron formation 
associated with gra])hitic slates, an assemblage of rocks not known in 



152 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

the Lower Huronian. Moreover, the l)asal coiislomerate carries frag- 
ments of quartzite and quartz slate dissimilar to any known Archean 
sediments in Michigan but exactly similar to certain Lower Huronian 
rocks in the adjacent Marquette district. This evidence considered in 
connection with the unconformity separating the Princeton and the 
Gwinn series is a sufficient basis for the correlation suggested, l:)ut an 
additional consideration tending to show that the CiAvinn series is 
Middle Huronian appears in the absence from its basal conglomerate 
of jasper fragments from the Negaunee formation so strongly developed 
in the adjacent Marquette district. 

Escape from the correlations suggested in this paper involves a 
chsregard or subordination of the importance of the unconformity 
separating the Gwinn and the Princeton series. There is no certain evi- 
dence in this synclinorium of great structural discordance between 
these two series but it may be and probably is as great as that sepa- 
rating the Upper Huronian and the Middle Huronian series of the 
Marquette district. Great structural discordance could hardly be 
expected inasmuch as the main deformation took place after the depo- 
sition of the Princeton series. Some structural discordance is implied 
in the consideration that although the upper slate member of the Gwinn 
series was probably not cut through in this district, there was sufficient 
erosion in adjacent territory to uncover the different members of the 
entire Gwinn series prior to the deposition of the basal conglomerate 
of the Princeton series. 



CHAPTER XL 

EVIDENCE OF THE AIIDDLE-UPPER HURONIAN 
UNCONFORMITY IN THE QUARTZITE HILLS 
AT LITTLE LAKE, AIICHIGAN 



\T* 



R. C. ALLEX AND L. P. BARRETT. 

A critical examination of the exposures of quartzite, quartz slate, 
and arkose in the hills near Little Lake in T 45 N, R 24 W, :Marquette 
County, Michigan, was inspired by the results of studies by the senior 
writer in the Gwinn s>Ticlinorium, which Ues between five and seven 
miles west. 

The Gwinn synclinorium contains two series of Huronian sedimentary 
rocks, separated by an unconformity which is characterized by a con- 
glomerate at the base of the upper (Princeton) series containing frag- 
ments derived from the various formations (including a productive 
iron-bearing member) of the lower (Gwinn) series and also from a 
third sedimentary series not represented in the s:\mclinorium. The 
work at Little Lake resulted in the identification of an unconformity 
which, in connection with other data to be described, establishes a 
basis for correlation of the formations at Little Lake with certain of 
those in the Gwinn s\Ticlinorium. 

So far as the writers are aware, no previous mapping and careful 
study of the rocks at Little Lake has been made. Rominger barely 
mentions the locality in 1894 in the statement that "iron-bearing rock 
beds occur in the vicinity of Little Lake."t Reference was again made 
to this locality in 1911 by Van Hise and Leitht who correlated the 
quartzite, quartz slate, and arkose in the hills at Little Lake with the 
Goodrich quartzite or basal member of the Upper Huronian as developed 
in the Marquette district and the arkose and arkose conglomerate at 
the base of the Gwinn series in the adjacent Gwimi (Swanzy) 
synclinorium. 

The .succession and correlation of the formations in the Gwinn 
sjTiclinorium and those at Little Lake are given below: 

♦Published in the Journal of Geology Vol. XXII. No. 6, September-October, 1914. 

tMichigan Geological Survey, 1894, Vol. V, Part I, p. 71. 

tC. 11. Van Hise and C. K. Leith, Monograph 52, U. S. G. S , pp. 283-86. 



154 



CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 



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MIDDLE-UPPER HURONIAN UNCONFORMITY. 155 

STRUCTURE OF THE LITTLE LAKE HILLS. 

Rising to a height of possibly 100 feet above a featureless flat sand 
plain near the station of Little Lake are two hills on which there are 
many exposures of pre-Cambrian arkose, quartzite, and quartz slate 
with associated conglomerates. These hills present today, in reference 
to the fluvio-glacial sand plains in which their bases are buried, some- 
what the same appearance that they seem to have had near the 
close of pre-Cambrian time, when they were monadnocks on a ])re- 
Cambrian peneplain, for remnants of flat lying Paleozoic (Cambrian 
or Ordovician) limestone still cHng to their sides and summits. 

The eastern and larger hill is nearly a half-mile in diameter: the 
western and smaller one is about three-eighths of a mile long in an 
E-W direction with a basal width of about one-eighth of a mile. The 
exposures are most abundant on the north half of the east hill, but on 
both hills there are a large number of pits and trenches which were 
dug many years ago by prospectors whose dihgence deserved a better 
reward than this locality seems to have offered. Aside from the red 
color of some of the quartz slate beds in the upper series, iron-stained 
shear zones in the quartzite and arkose, and an exposure at locality F 
(see Fig. 10) of about eighteen inches of hematite occupying a lens- 
shaped cavity along a zone of thrust faulting in massive quartzite, 
there appears no present evidence of the attractiveness which these 
hills seem to have presented to the early prospector for iron ore. 

The structure of the north side of the east hill is apparently an 
anticline, the crest of which has been cut away by erosion, thus ex- 
posing the arkose and associated conglomerate of the lower (Gwinn) 
series flanked on the north, east, and west sides by conglomerate, 
quartzite, and quartz slate of the upper (Princeton) series. This is 
the only complete structural feature which can be determined from the 
available data. There is evidence in the development of cleavage 
and schistose structures, shear zones and faults of both normal and 
thrust type, that general deformation has been severe. Further evi- 
dence of the intensity of deformation is afforded in the overturning of 
the formations, with consequent apparent reversal in succession, in 
exposures at locality A at the southeast extremity of the east hill. 
While evidence of minor faulting is abundant in outcrops and pits, it 
is found impossible with information available to trace the course or 
measure the throw of any of these faults. The fault at locality C-H 
is a partial excej^tion but the only thing known about this fault is its 
direction and the fact that its vertical displacement is inconsiderable. 
In reference to the .structure of the west hill perlKips no inferences are 
warranted. So far as known, the arkose of the lower .series is not 
exposed but the (listril)ution of the lower and liigher members of the 



156 



CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 





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MIDDLE-UPPER HURONIAN UNCONFORMITY. 157 

upper series together with the topographic expression faintly suggests 
a shallow syncline trending across the hill in a XE-SW direction carry- 
ing the quartz slate member in the trough and exposing the underlying 
quartzite on its opposite flanks. But the structure is probably not so 
simple as this for there is evidence of faulting in some of the pits. 

THE LOWER (gWINN) SERIES. 

Arkose and conglomerate. — The major portion of the arkose forma- 
tion is in reality now an abundantly sericitic quartzite, the sericite 
being a metamorphie derivative of the original feldspar. The abund- 
ance of sericite affords on cleavage surfaces, a characteristic pearly 
luster. From the dominant phase there are gradations through inter- 
mediate phases to tj^pical arkose with feldspar practically unaltered. 
Of subordinate importance are interstratified lenses of conglomerate 
varying from a foot or two up to eight feet in thickness. The pebbles 
are mainly vein quartz well rounded and of various sizes under four 
inches in diameter. Other pebbles of dense, vitreous, gray quartzite, 
black chert, and siliceous dolomitic slate are much less abundant. 
The matrix of the conglomerate beds has the composition of quartzite 
rather than arkose and is usually dark, dense, vitreous, and slightly 
sericitic. 

Bedding structure is not observable in any of the various phases of 
the formation, except as it may be represented by an occasional thin 
layer of gray chert. The deposition of these cherty layers probal^ly 
heralded the approach of a change in conditions of sedimentation 
represented by an iron-bearing member in the adjacent Gwinn syncli- 
norium which lies in part directly on a similar arkose-conglomerate 
formation. At Little Lake the iron-bearing member appears to have 
been removed by erosion prior to the deposition of the overlying con- 
glomerate and quartzites. The similarity of the arkose-conglomerate 
of Little Lake to that at the base of the Gwinn series extends to the 
pebble content. Rounded fragments of dolomitic siliceous slate, and 
gray quartzite are common to both localities, but the bowlders of 
granite and green schist which occur in the conglomerate of the Gwinn 
district were not observed in the exposures at Little Lake. 

THE UPPER (PRINCETON) SERIES. 

The upper series, so far as represented at Little Lnk(\ coini)rises a 
higher horizon of red-aud-gray-banded (juartz slate and slaty quartzite 
grading down througii banded (juartzite and massive non-beddeil 
quartzite into a basal conglomerate. 

CotKjlonicrate. — The contact of the u|>])er aii<l the lower series is ex- 



158 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

posed at localities B and C (see Fig. 10). At locality B this contact is 
distinguishable only on careful examination. The base of the upper 
series on weathered exposures is not conspicuously dissimilar to the 
underlying arkose except on freshly fractured surfaces which reveal, 
in contradistinction to the underlying sericitic, quartz-feldspar rock, a 
dense, hard matrix of quartzite holding pebbles of vein (juartz of sizes 
less than an inch in diameter. At locality C, however, all tloubt of the 
unconformable relations of the arkose-conglomerate and the overlying 
series is dispelled. The change from arkose to dense, black, vitreous 
quartzite is abrupt at a wav}' contact of knife-like sharpness. In addi- 
tion to the quartz pebbles observed at locality C there are pebbles of 
chert and large bowlders of the underlying arkose above one foot in 
diameter. The arkose bowlders are much softer than the embedding 
matrix of quartzite and weather out to form characteristic pit-like 
depressions. The full thickness of the basal conglomerate is not ex- 
posed at locality C, but at locality B it is apparently only six feet. At 
C only about four feet are observable. 

Quartzite and quartz slate. — There are three distinct main phases of 
this series, viz., (!)• a massive phase associated with the basal con- 
glomerate, grading upward into (2) a banded phase which in turn is 
over-Iain rather sharpl}^ by (3) beds of gray- and red-banded quartz 
slate. Although these three phases correspond to definite stratigraphic 
horizons, considerable difficulty is experienced in correlating the vari- 
ous exposures of the different members of this series. The chief difl&- 
culties refer to the relation of the quartzite on the west hill to that 
exposed on the east hill and to the determination of the stratigraphic 
position of the two outcrops of quartzite north of the slate at the base 
of the east hill. The outcrops of gray quartz slate and red-banded 
quartz slate on the north slope of the west hill are apparently strati- 
graphically above the exposures of quartzite in outcrops and pits on 
its northwest and northeast sides. Whether the quartzite at the base 
of the north slope of the east hill is stratigraphically above the quartz 
slate or represents the underlying massive quartzite brought up by 
faulting cannot be determined. 

Extended description of the different phases of the quartz rocks in 
the upper series has little interest for present purposes. The dissimi- 
larities of the different members refer mainly to texture and bedding 
structures rather than to composition. The red color of certain layers 
in the quartz slates is caused by the presence of small particles of finely 
disseminated hematite. 

NOTES ON THE CORRELATION. 

In chapter 10 the senior writer discussed the importance of the un- 
conformity separating the Princeton, (upper) and Gwinn (lower) series 



MIDDLE-UPPER HURONIAX UNCONFORMITY. 159 

in the G"\\inn synclinorium and adduced evidence in support of the cor- 
relation of these two series with the Upper and the ]\Iiddle Huronian. 
The hthologic similarity of the arkose-conglomerate formation at Little 
Lake to the basal member of the Gwinn series, only a few miles distant, 
considered in connection \nth the unconformity separating it from the 
overlying quartzites and quartz slates is a sufficient basis for extend- 
ing the arguments for the correlations in the Gwinn district to cover 
the two unconformable series at Little Lake. The geology of each area 
accounts for three unconformable series of sedimentary rocks corre- 
sponding to the Lower, ^Middle, and L'pper Huronian of the adjacent 
Marquette district. The upper two series are present while the lower 
one is represented in both areas by fragments of some of its formations 
in the base of the middle series. 

The absence in the lower series at Little Lake of the slate and iron 
formation members developed in the G^inn s^mclinorium strengthens 
the evidence of the importance of the erosion interval which inter- 
vened between the deposition of the Princeton and G^nnn series. 
Incidentally it has a practical bearing on the possibilities for success 
attendant on drilling for iron ore in the immediate vicinit}' of the Little 
Lake hills, t^ome drilling of which the writers have no records, has 
already been done and we understand that additional drilling has been 
contemplated by parties who are likewise ignorant of the results of the 
former explorations. 



CHAPTER XII. 

RELATIVE TO AN EXTENSION OF THE MENOMINEE 

IRON RANGE EASTWARD FRO^I WAUCEDAH 

TO ESCANABA, MICHIGAN.* 

R. C. ALLEN. 

The Menominee Iron Range of Michigan, so far as knoA\Ti, includes 
a folded series of Huronian rocks trending a little south of east from 
the Menominee River in T. 40, R. 31 to the longitude of the village 
of Waucedah, Dickinson county, a chstance of about 19 miles. The 
Lower Huronian is represented by a series of thick formations of quartz- 
ite rSturgeon) and dolomite (Randville), the Middle Huronian (?) by 
cherty quartzite not exceeding 70 feet in thickness, and the Upper Hu- 
ronian by a vast thickness of slate (Hanbury) overlain by volcanic 
green schists (Quiimesec) and carrying two productive iron bearing 
formations in basal horizons. The Huronian series lies unconformably 
on the Archean Complex, f 

The areal distribution of the iron formation is shown on the accom- 
panying figure 11. 

It is reported that drilling a short distance west of the Menominee 
river in Florence county, Wisconsin, failed to locate the iron forma- 
tion of the southernmost belt in the territory where it may 
have been expected to occur judging from its structure and strike 
between the city of Iron Mountain and the river. In the opposite 
direction the iron formations are overlapped by Cambrain sandstone 
in the vicinity of Waucedah. East of Waucedah the thickness of the 
Paleozoic rocks increases, presumably rather regularly, to about 800 
feet in the vicinity of Escanaba. 

It has been obvious for many years that the Menominee range 
Huronian series, including the iron formations, extends an unknown 
distance east of Waucedah beneath the flat lying Paleozoic formations. 
. The magnetism of certain of the Huronian rocks, particularly (but 
not exclusively) the iron formation, furnishes the only means short of 
actual drilling or underground exploration by which they may be traced 
into the Paleozoic territory. Magnetic surveys of a part of the area 
east of Waucedah have been made by certain mining companies, and 

♦Published in Kcononiic Geology, Vol. IX. No. 3, April. 1914 
tFor description of peology of the Meiioniinee iron range .set- Monograph r>2. V. S. G. S. 
page 328-46. 

21 



162 



CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 




EASTERN EXTENSION OF MENOMINEE RANGE. 163 

the results of some of their work were placed at the disposal of the 
writer. The economic importance and scientific interest of the problem 
furnished the incentive to complete the magnetic survey' of the range 
eastward to Lake Michigan. 

The survey Avas executed in the field bj- L. P. Barrett, assisted for 
a portion of the time by H. J. Allen. Where information was available 
efforts were extended only to checking the accuracy of former surveys. 
The results are shown in a general way on the accompanying plate.* 

CONCLUSION. 

It will l)e seen that the magnetic belts which are coincident with the 
two separate belts of iron formation near Waucedah extend eastward 
without break for about six miles. The assumption that these belts 
have the same relation to iron formation beneath the Paleozoic rocks 
that they have to knowii occurrences at Waucedah and westward 
seems warranted. It is, however, unsafe to conclude that the magnetic 
belts further east to Little Bay de Noc are all or even partially under- 
lain by iron formations of the ore bearing type of the Menominee 
range. From their general and linear-s\Tnmetrical characters it may 
be inferred that the magnetic rocks are folded sedimentaries and that 
they contain iron, at least partly, in the form of magnetite. But the 
magnetic beds may or may not be the stratigraphic eciuivalents of the 
iron formations of the ^lenominee range. In this connection the re- 
ported extension without break of a magnetic belt from the known 
iron formation at the opposite end of the range westward into magnetic 
slates, as shown by drilling, is instructive. 

A recent drill hole in the general projection of the northernmost 
magnetic belt on Stonington Peninsula in the SW^ of the NW| of 
section 8, T 39 N. R 21 W encountered dense, gray, vitreous quartzite 
similar to the Sturgeon (Lower Huronian) quartzite of the Menominee 
range after penetrating 780 feet of Paleozoic l)eds. It is reported that 
iron formation was penetrated at a depth of 732 feet at the Escanaba 
l>rewery, granite at 810 feet at the Richter brewery, and granite at 931 
feet at the plant of the Escanaba Manufacturing C'omjiany, all in the 
city of Escanaba. There is little doubt that the above figures represent 
in each instance the approximate thickness of the Paleozoic rocks, but 
inasmuch as no records of the drillings were kept and no samples of 
the cuttings or cores preserved of the holes in Escanaba little credence 
should attach to the hearsay reports of the characttM- of the under- 
lying i)re-Caml)rian rocks. 

From the evidence available it may In- fairly assumed (1) that the 

*Lar>fe srale plats showiiiK the iniiKiietic otiscrvatioiis iiiiiv Ix' obtairu'il on application to the 
state (;eoloKist. LanstiK, MichiKan. 



164 CONTRIBUTIONS TO PRE-CAMBRIAN GEOLOGY. 

magnetic belts mark the course of pre-Cambrian sedimentary rocks 
from Waucedah to Escanaba, (2) that these rocks contain magnetite, 
(3) that the magnetic beds are probably associated with other sedi- 
mentary formations which have little or no magnetism, (4) that it is 
not improbable that the succession and in a general way the structure 
of the pre-Cambrian beds may be closely related to those of the Menom- 
inee range west of Waucedah, and finally (5) that the occurrence of 
productive iron formation may be related in some degree and in some 
place or places between Waucedah and Escanaba to the magnetic 
belts. In advance of actual drilling operations no further assumptions 
are warranted. 



THE GEOLOGY OF LIMESTONE MOUNTAIN AND SHER- 
MAN HILL IN HOUGHTON COUNTY, MICHIGAN. 



E. C. CASE AND W. I. ROBINSON. 



THE GEOLOGY OF LIMESTONE MOUNTAIN AND SHERMAN 
HILL IN HOUGHTON COUNTY. MICHIGAN. 

E. C. CASE AND W. I. ROBINSON. 

In southeastern Houghton county, there are three outUers of Paleo- 
zoic dolomite called respectively, Big Limestone Mountain, Little 
Limestone Mountain, and Sherman Hill. Big and Little Limestone 
Mountains lie half a mile east of the little station of Hazel on the Mass 
City branch of the Mineral Range Railroad, and directly north of the 
track. Big Limestone is a ridge a mile long and a half a mile wide 
running nearly north and south, and terminating in steep walls at its 
southern end. The western face rises abruptly 300 feet from the sur- 
face of the swampy land which surrounds the outliers but the eastern 
face slopes gently to the swamp level and is covered by a heavy growth 
of hardwood timber. Little Limestone lies south of Big Limestone 
and is separated from it by a deep irregular gully partly occupied by 
swamps and partly filled by glacial debris and talus. This hill is much 
smaller than Big Limestone, and runs NE-SW in contrast to the north 
and south trend of Big Limestone. It is also lower, rising but L50 
feet from the swamp. The surface has been robbed of its timber by 
forest fires and the top is now under cultivation. The third and smallest 
outlier, Sherman Hill is one and a half miles northeast of Big Limestone. 
It rises 150 feet above the swamp and is covered by a thick second 
growth of young hardwood. The dolomite appears as high bare cliffs 
on the northern and eastern part. All three masses of dolomite lie 
upon the .lacobsville Sandstone which forms the surface rock of the 
immediately surrounding country, and can be detected through the 
masses of talus at intervals along the bases of the hills. 

The drift is locally thin around the hills, being mostly sand and 
gravel with a few scattered Ijoulders. Long tapering slopes of drift 
extend from the southwestern, lee, sides of Little Limestone and Sher- 
man Hill. The toi)s of the hills show little drift and no glacial markings 
were foinid in the few places where the dolomite appears through the 
soil and thick vegetation. Northwest of the hills a very heavy drift 
occurs in the vnllry of the Lit1l(> Otter which has cut down through 
several lnnulnMl iVd of red clay to the .lacobsville sandstone. The 
.lacnbsvilU' is here darker in color and composed of liner grains th;in 
at th(> outcrops beneath the dolomite of the hills, and -is it is considerably 
lower topographically it is very possibly a lower member. 



168 



LIMESTONE MOUNTAIN AND SHERMAN HILL. 




Figure 1. Limestone Mountain and vicinity modified from a map by W. L. Honnold and A. C. Lane. 

Contour interval 50 feet. 



LIMESTONE MOUNTAIN AND SHERMAN HILL. 169 

These outliers have been visited by several geologists who have 
reported variously upon their age and structure. C. T. Jackson*, 
in 1849, published a report upon the region in which he assigned the 
sandstone to the Triassic and considered it equivalent to the "New 
Red" of the Connecticut valley. He regarded it as overlapping the 
dolomite. He collected Peniamerus oblongus and so gave the age of 
the dolomite as Niagaran. His analysis of the dolomite is as follows: 

Calcium Carbonate 44 . 49 

Magnesium carbonate 44 . 65 

Peroxide of Iron 1 . 98 

Silica 8.91 



100.03 



Foster and Whitneyf in their "Report on the Geology and Top- 
ography of the Lake Superior Land District," established the correct 
position of the dolomite above the sandstone by obser\ang the cal- 
careous nature of the upper layers of the sandstone and the abundant 
sand grains and silica in the lower layers of the dolomite. They made 
a considerable collection of fossils which were submitted to James 
Hall for identification. His conclusion was that "The evidence from 
the whole together goes to prove that the rocks from which they were 
obtained belonged to the older Silurian period." The rocks were 
assigned to the Potsdam and Calciferous sandstones, the Chazy, Birds- 
eye, and Black River limestones, and perhaps Trenton and even Hudson 
River groups. Hall lists the following fossils, 

Maclurea. 

Murchisonia or Loxonema. 

Arnhonychia (near orhiculata and amygdalina) . 

Modiolopsis (near truncatus). 

Edmondia (near subtruncata and subangidata) . 

Edmondia (near ventricosa). 

Leptaena sericea. 

Or this sp. 

Glyptocrinus (stems) . 

Orthoceratiiea. 

Dr. Carl Romingert visited the locality and included a description 
of it in his report on the Upper Peninsula. He noted the very great 
disturbance, the complex faulting, and the varying dip of the layers 

•3l8t Cong. Ex. Doc. No. 1, pp. .199 ami 4.12. 1K49 
tSlat. Cong. Ex. Doc. No. 09. pp. 117-119. lS.->t) 
JMlch. Geological Survey, vol. 1, part W. pp 69-71, 1873. 



170 LIMESTONE MOUNTAIN AND SHERMAN HILL. 



of which ho said, " it appears to me more probable that there 

was an underwashing and sinking of the strata during the drift period 
rather than an actual upheaval of later date." He was essential in 
agreement with Hall as to the age of the beds and gave the following 
report on the fossils. 

"In the lower ledges, casts of Bivalves and of Gasteropods are numer- 
ous, but not well enough preserved for determination; the same is the 
case with fragments of Orthoceras and C'yrtoceras. I have identified, 

Orihis occidentalis. 

Orthis testudinaria. 

Orthis similar to pectinella. 

Orthis lynx. 

RhynchoneUa increbescens. 

Leptaena alternata. 

Lingula quadrata. 

Leytaena sericea. 

Pleurotomaria lenticularis. 

SubuUtes similar to elongatus. 

Murch iso?} is m a jo r . 

Bucania. 

Ambonychia orbicidaris. 

Cyrtodonta subtruncata . 

Nucula levataf larger than Hall's specimens. 

Streptelasma cornicidum. . 

The valve of a Brachiopod similar to the dor- 
sal valve of Orthis occidentalis, but with the 
hinge hne extended ear like, and exhibiting an 
internal septum like the ventral valve of a Pen- 
tamerus. A specimen of this kind may possibly 
have induced Jackson to mistake it for Penta- 
merus oblong us.''' 

In 1891 W. L. Honnold, then serving as geologist on the Michigan 
Geological Survey, spent some time in the vicinity and in connection 
with his studies made excavations at the base of the hills to determine 
the nature of the contact of the dolomite with the sandstone. He 
reported that the dolomite lay in apparently conformable contact -with 
the sandstone which in his opinion forms a gentle syncline. He, also, 
noted that the upper layers of the sandstone are calcareous and the 
lower layers of the dolomite siliceous with no transition beds between 
the two. Unfortunately, Honnold's work has never been published 
but short abstracts have appeared.* 

*Am. Jour. Sci., vol. 42, 3rd. Series, pp. 170-71, 417-19, 1891. Trans. Am. Inst. Min. Eng. 
vol. 27, pp. 684-8.5, 1897. Mich. Geol. Surv., Ann. Rep., p. 178, 190.3. 



LIMESTONE MOUNTAIN AND SHERMAN HILL. 171 

In 1909 Lanet gave a further account of Honnold's work and an 
account of a visit by Hubbard, Seaman and Lane. This report in- 
cluded a map prepared by Honnold and the following list of fossils, 
which were identified by W. F. Cooper: 

Orthoceras vertebrale. 
Orthoceras undulostriatum. 
Troch onema heloitensis. 
Pleurotomaria subconica. 
Orthoceras. 

Orthoceras {vertebrale) darus. 
Cypricardites veniricosusf 
Cypricardites ventricosus. 
Cypricardites obtusus. 
Buthiatrephis. 
Cypr ica rdites m ega mbonus . 
Cypricardites niota. 
Modiolopsis lata. 
Cypricardites latusf 
Cypricardites glabella. 
Cyrtodonta billingsi. 
Cypricardites amygdalinus. 
Cuneamya subtnmcata. 
Pentamerus. 
Rafinesquina alternata. 
Orthis testudinaria. 

In the summer of 1913, the authors spent six weeks working upon 
these outliers in an attempt to finally determine the age and structural 
relations of the beds. The work has not resulted in as d(>finite conclu- 
sions as were ho])ed for, because the outcrops are largely obscured by 
heavy talus and a thick growth of vegetation which covers the hills, 
but it is believed that the information gained is as full as can be ob- 
tained in the present condition of the country. The dolomite layers 
have been nuich disturbed as is shown by th(> widely varying dips 
determined at various iioints of the outcrojjs. How this disturbance 
was caused is still i)r()blematical and certain tentative exi)lanati()ns are 
offered at tlie close of this ))aper. 

STKATKiHAI'irv AND CORKELATION OF THK HKDS. 

From our sections and the fossils obtained, the following general 
section has been made out: {Fig. 2). The fossils were determined by 

tMicli. fifol. Siir\ . |)\il. t;, (;i-(.|. Series 4, vol 2, iip. ,")1;:M>I, UKMI. 



GENERALIZED SECTION 


J. 






2 






o 


Middle 


Thickness undetermined 


> 






Ul 






a 








NIagann 




as 

3 


Lockport 


Thickness undetermined 


to 






■ 


Middle to Upper Richmond 


Thickness undetermined 


Upper Part of Lower Richmond 


Thickness undetermined 




Arnhelm 




Lower Richmond 


lOfeet-J-. 




Upper Galena 


eOfcet. 




Stewartville 


Upper layers fossiljferout 


m 






tf 






«ii 






1 

fit 
o 


Decorah 


20 feat 


Upper Blue 






Upp«- Black RIvw 


5 felt at the top foislllferaus 




Upper Blufr 


75 feet below barren 


a: 






< 






oe 


Jacobivllle 




X 




.100. feet^exposed 


u 







Figure 2. The succession of the beds. 



LIMESTONE MOUNTAIN AND SHERMAN HILL. 173 

the junior author and finally submitted to Dr. E. 0. Ulrich of the U. S. 
Geological Survey for revision and the determination of the exact 
horizons. For this, and for many helpful suggestions, we desire at this 
point to express our thanks to Dr. Ulrich. 

IX. Mid-Devonian. — All that is kno-«-n of this horizon is a single 
angular mass of chert, found in the talus on the southeastern slope of 
Big Limestone. It yielded four fossils: 

Chonetes coronatus var. 

Produdella cf. navicella and spinulicosta. 

Spirifer aff. Pennatus. 

Cystodiciya cf. hamiltonensis. 

VIII. Niagaran. (Lockport). — A bed of very siHceous material 
was found on the south slope of Big Limestone (marked N on the map, 
see Fig. 1). It could not be traced a great distance up the slope and 
it was so badly broken that no satisfactory dip reading could be taken. 
No trace of .such a layer was observed on the tops of any of the hills. 
The fossils collected are: 

Streptelasma spongaxis. 
Zaphrentis stokesi. 
Duncanellaf spf 
Clorinda cf. ventricosa. 
Pentamerus sp. 
Conckidium decussatum f 
Dalmanella cf. elegantula. 
Leperditia aff. cylindrica. 
Loxonema sp. 

VII. Middle to Upper Richmond. — Three fossils were found with 
the Niagaran material, the beds not being distinguishable: 

Favosites asper. 
Columnaria alveolata. 
Plectorthis whitfieldi. 

VI. Upper part of the Lower Richmond. (Arnheim). — At the locality 
marked R on the map, a single thin layer of dolomite was found. The 
rock was almost i;)eriien(li('ular l)ut as it was in the zone of ])roken talus 
we cannot be certain that it was in place. The following fossils were 
found : 

Crinoid cohimnals. 

Coeloclema o we n i . 

Mitoclema minxlum. 



174 LIMESTONE MOUNTAIN AND SHERMAN HILL. 

Mesotrypa patella. 

Bythopora striata. 

Rhynchotrema perlamellosa. 

Rhynchotrema capax. 

Conularia formosa. 

Primitia cincinnatiensis. 

Tedradella persulcata var. 

Ceratopsis robusta. 

Calymene a new species allied to C. fayettensis 

and C mamillatus, Hall. 
Conodont. 

V. Lower Richmond. — A })ed about ten feet thick, containing fossils 
of this age occurs at the top of Little Limestone and on the southeastern 
talus slope of Big Limestone. The matrix is a siliceous dolomite and 
the fossils are as a rule, sihcified. The following fossils were collected: 

Streptelasma rusticum? 

Haly sites gracilis. 

Inocrinus aff. I. crassus. 

Orhicidoidea? (Schizotreta) new species. 

Rafinesquina new species. 

Leptaena unicostata, two new varieties. 

Plectambonites sp. 

Plectorthis ivhitfieldi 

Plectorthis kankakensis . 

Dalmanella aff. rogota. 

Dinorthis suhquadrata . 

Hebertella new species aff. H. insculpta, H. fansfa. 

Platystrophia sp. 

Rhynchotrema capax. 

IV. Galena. (Stewartville or Upper Galena). There is a heavy 
bedded cream colored dolomite which underlies bed V on Little Lime- 
stone, and occurs at the top of Sherman Hill. The fossils were found 
near the top of the layer which has a thickness of sixty feet. A list 
of the fossils collected follows: 

Cyrtolites cf. retrorsus. 
Liospira cf. angusiata. 
Hormotomaf major. 
Lophospira minnesotensis. 
Maclurea crassa. 
Maclurina manitobensis. 
Maclurina cuneata. 



LIMESTONE MOUNTAIN AND SHERMAN HILL. 175 

Trochoyiema umhilicatum. 

Fusispira suhhrevis. 

S-piroceras sp. 

Salpingostoma cf. expansa Hall, and huelli Whitfield. 

III. Decorah. (Upper Blue). Below bed IV on the eastern face of 
Little Limestone there is an old quarry in a thin bedded dolomite. The 
top layers are gray, the lower layers cream colored with irregular spots 
of dark red iron stain. The fossils show that the two layers belong at 
the same horizon. A similar mottled layer carrying Decorah fossils, 
occurs at Sherman Hill, but here it shades up into the heavy dolomite 
without the thin bedded laj-er intervening. The following fossils 
were found: 

Crinoid columnals. 

Ceramophylla frondosa. 
• Trematopora f prim igen ia . 

Halloporina crenulata. 

Arthrostylus sp. 

Arthoclema sp. 

Rhin idictya mntabilis. 

Rhinididya fidelis? 

Arthropora simplex. 

Eschar opora subrecta . 

Escharopora confluens. 

Streptelasma profundiim . 

Or this tricenaria. 

Strophomena incurvata. 

Strophomena septata. 

Dalmanella rogota? 

Hormoioma salteri canadensis. 

Aporchites sp. 

II. Upper Black River. (Upper Bluff). At the extreme southwestern 
part of Big Limestone and eighty feet al)ove the sandstone, there 
occurs a heavy Ix'dded cream colored fossiliferous dolomite. The beds 
above and below are coinj^letely l)arr(>n. The following fossils are 
found. 

Utcnodonta nasula. 

Ctenodonta gibberula . 

Endodesma? new species. 

Vyrtodonta billinqsi. 

Cyrtodonta cf. billi?igsi new species. 

Cyrtodonta cf. hiirnnensis and snbcarinata. 



176 LIMESTONE MOUNTAIN AND SHERMAN HILL. 

Cyriodonta tenella. 

Cyrtodonta new species. 

Vanuxemia aff. niota Hall and subrotunda Vlrich. 

Vanuxemia sp. 

I. Potsdam. (Jacobsville). The lowest members of all the outliers 
and the only stratum which was observed at all three localities is a 
dull brown, coarse, poorly cemented sandstone with occasional streaks 
of a very fine conglomerate or very coarse sandstone. On Big Lime- 
stone and Little Limestone the pebbles of the conglomerate layer are 
of quartz, well rounded, and no larger than a pea. South of Sherman 
Hill, the conglomerate carries larger pebbles of greenstone and chert. 
This sandstone has been referred by Lane* to the Jacobsville Sandstone 
of probably Potsdam age. 

STRUCTURE. 

The dips in the uppermost of all the exposures show a remarkable 
diversity although there seems to be quite generally a dip toward the 
center of the outlier in each case. There is no suggestion of folding. 
Figure 2 shows the aspect of the southern slope of Big Limestone as 
seen from Little Limestone. The rocks forming the western face of 
Big Limestone dip quite regularly eastward. At the south end of the 
west face a dip of 30° E. 4° N. was obtained. Where the line between 
sections 14 and 23 crosses the cHff, a dip of 32°, E. 10° X. was found 
and between these two locations dips easterly were obtained varying 
from 14° to 20°. From figure 2 it will be seen there is great variation 
in the dip along the southern face of Big Limestone. The western 
block dips 20°, N. 60° E., east of this a block dips 14°, N. 51° E., and 
east of this still another block dips 50°, E. 5° S. Talus covers much 




bS.E; 

" On«-ftlBhthmll« 



Figure .3. Cross section of southern slope of Big Limestone along B. B. (See map). The 
white areas indicate talus covered slopes. Vertical scale X 2. 

of the remainder of the south cliff but readings were obtained which 
show that steeply inclined strata occupy the remainder of the section 
except at the eastern end where a block occurs which dips westerly 
at various angles. Readings of 21°, 22°, 28°, and 41° were obtained 
here. 



*Jour. Geol., vol. 15, p. 680, 1907. 



LIMESTONE MOUNTAIN AND SHERMAN HILL. 



177 



On Little Limestone a similar irregularity is noticeable. On the 
northwest cliff a large section of layer IV probably carrying layer V 
has slumped away from the main chff and lies with a dip of 31°, n! 
64° E. South of this block on the west face there is a dip of 30°, E. 
14° S. Other dips which were noticeable follow: 
- East face 35°, W. 18° N. 

Northeast face 32°, W. 23° N. 

Southeast face (Decorah beds) 34°, N. 30° E. 

.u'^^lo^^'^o ^^^^*>'^^g the Decorah and about 20 paces northeast of 
them 52°, S. 30° W. 

These dips, and the dip of the layers of Sherman Hill, are plotted 
on the map {Fig. 1). That the faults which separate these blocks 
ot varying dip are very minor, is proved by the slight displacement of 
layer IV which can be traced from the east side to the north side of 
LiUle Limestone m which distance there are two abrupt changes of dip 

there is evidence however, of a fault of larger importance between 
iJig and Little Limestone. This evidence is mainly in a displacement 
01 the Jacobsville sandstone and is presented in figure 3. From the 




Ofi«-hait»nlt> 



Figure 4. A Reneraliml section of Bis aiul Little Limestone. The vertical scale X 4. 

swamp to the crest of Little Limestone is 150 feet. The dolomite has 
a combined thickness of 90 feet. If those layers are uniform and pr.^s' 
ent under the whole mountain, there is 60 feet of sandstone above the 
swamp. In a similar way, Big Limestone rises 300 feet above the 
swamp. The dolomite is 140 feet thick leaving 100 feet of sandstone 
above the swamp. Allowing 20 feet for change in swamp level there 
IS at least 80 feet difference in level bc^tween tln^ sandstone of the two 
hills. Since these sandstones are lithoiogiealiy identical, and there is 
a decided topographic break between them a fauh is assumed to be 
present in the valley. 

The succession of Ik.Is in Hig and Little Limestone mountains in 
the few cases in which the beds may be said to be approximately in 
place, IS very different. In Big Limestone laytT I, the Jacobsville is 
followed by II, the Black Hiver, while at Little Limestone I is followed 
by 111, the Deeorah, with not a trac(> „f Hln,k River lH>twe(>n The 
fossils of the Black River (layer II) wen. found near ih. fop of Bi^ 
Limestone, the highest point from which fossils were collec-ted- below 



178 LIMESTONE MOUNTAIN AND SHERMAN HILL. 

them topographically layers V, IV, and III were found apparently in 
place on Little Limestone (see Fig. 4). 

From the foregoing account of the stratigraphy' it is evident that 
several new points have been added to the geological history of Northern 
Michigan. Not only was the region covered by an Ordovician sea but 
by seas of Silurian and Devonian time as well. Ordovician fossils 
were discovered by Allen* near Iron River, Michigan and Silurian has 
been noted by earlier observers at Limestone Mountain, but this is 
the first time that Devonian fossils have been found in place or near 
their original position, so far north in the state. Our paleogeographic 
maps must so far be revised as to extend the Silurian and Devonian 
seas well into, if not over, the Northern Peninsula. The similarity 
of the Ordovician fossils with those of Minnesota and Wisconsin shows 
that the same sea reached from Michigan into these areas. Dr. Ulrich 
in a letter to the authors cites the peculiarity of the pentameroid forms 
and their similarity with forms found throughout the extreme western 
part of North America, indicating a wide connection of the Silurian 
sea in that direction. 

The Devonian material is small in amount but so characteristic that 
there can be no doubt of the presence of marine waters in Mid-Devonian 
time. How far the Devonian sea and deposits extended over the 
Northern Peninsula is impossible to state. Our fossils were obtained 
from a large fragment on the southeast slope of Big Limestone, involved 
in the great talus from the lee of the hill, but its size, position, and 
angular condition, lead us to doubt that it has been transported any 
great distance, though it may easily have come from some region to 
the northeast of the dolomite hill. The heavy drift northwest of the 
mountains also may easily conceal remnants of Paleozoic deposits 
beyond any hope of detection. We consider it very doubtful that the 
Devonian deposits were originally connected with the nearest rocks 
of that age in Canada, in the vicinity of Lake Winnepeg and Hudson 
Bay. 

HISTORY. 

We are unable to propose any hypothesis for the preservation of 
these outliers, so far removed from the deposits with which they were 
originally connected. That the seas did not endure for any great length 
of time, is apparent from the relatively thin deposits and it may be 
that the northern peninsula was a region of limited sedimentation, 
toward the limits of the invading waters. The upper layers show no 
peculiar hardness nor consistency which would have enabled them to 
resist the degrading forces, and, as is shown below, the faulting does 
not account for the preservation. 

*Mich. Geol. and Biol. Surv., Pub. 3, Geol. series 2, pp. 113-16, 1910. 



LIMESTONE MOUNTAIN AND SHERMAN HILL. 179 , 

While the hills discussed in this paper are the most remote outcrops 
of Paleozoic sediments later than the Cambrian kno^Nai m Miclugan, 
we cannot but believe that more of the same material is buried by the 
heavy drift to the northwest. . , , . . t + 

\11 obtainable evidence shows that the erosional history began, at 
the earliest, later than Mid-Devonian; how much later camiot be made 
out but considering the amount of material removed, and the com- 
pleteness of the removal, we are inclined to the opinion that the region 
was exposed from sometime late in the Paleozoic. 

As the preceding discussion shows, the layers are disturbed by nu- 
merous faults in an intricate manner. The fault between Big and 
Little Limestone is the largest that was detected, with a throw ot at 
least 80 feet, .vdth Big Limestone upthro^Ti. Whether the steep clih 
faces of Big and Little Limestone are fault scarps is less certain but 
this may be possible. The remaining faults are of minor character and 
importance, and may be accounted for by processes involving only 

verv local conditions. , . r u 

The fault hehoeen Big and Little Limestone. -This is the largest fault 
observed and throws more light than the others on the history of the 
hills As has been shown before, layer II occurs near the top oi Big 
Limestone. It is the lowest stratigraphically and the highest topo- 
graphically of any fossiliferous bed. On the opposite side of the fault 
laver III occurs directly upon the sandstone. It is evident that the 
whole thickness, or nearly so, between the sandstones and layer 111 
is missing on Little Limestone. This may be explained in various 
ways but because of the lack of evidence, the explanations which are 
offered are very tentative. 

There is no doubt in our minds that the sandstone m the two hills 
is the same; texture, color, material, pecuHarities of cross bedding and 
included layers of coarser sand grains leave no doubt on this subject 
This being true, we must suppose a lack of deposition of layer II on 
Little Limestone or account for its disappearance by erosion or solu- 
tion. Under the first hypothesis, that of a lack of deposition on the 
site of Little Limestone, we would postulate an erosional irregularity 
of surface which permitted the deposition of a considerable thickness 
of Ordovician in the position of Big Limestone while the site of Little 
Limestone was occupied by an elevation not covered until much later 
by the invading sea. This idea is strengthened by the occurrence of 
a layer of dolomite, mottled by irregular spots of red, just above the 
sandstone wherever the sandstone and dolomite were together, irre- 
spective of locality or stratigraphic i^sition. This is seen on Big and 
Little Limestone, and Sherman Hill. TUr faulting took place along the 
edge of the elevation (see Fig. o). 



180 



LIMESTONE MOUNTAIN AND SHERMAN HILL. 




(5 — 1 ^ 










1 1 \ 




1 l\ 




1 1 \ 




1. 1 \ 




1 1 \ 








4 ...1 .' --jA, 


/^i 


1 ' i ' ;i 




/ 1 


1 1 


/ 1 


r 


/ 1 1 


1 1 ' 


/ , 1 , 

l^^^^g^vA. .J u,. .1. .. 


1 1 1 

1 ' 1 ' ! J= 


1 


' 


■^ :!-ir^--- ■■: :■■.■ 


;/.:v;;^\;v/'-/;: 


1 1 




.^■. .•:•■.■.-• :■•..■.• 


1 


1 


->r' ■-••.:.■ ..-•..••• 


1 1 




yr- :■:■•■■■■.■■ ■■: ■ 


1 


\~ 








b?^' 



Figure 5. 



The upper figure shows a hypothetical progressive overlap. The lower figure shows 
the present condition due to faulting and erosion. • 



As alternative hypotheses, we may suggest the following: (1) the 
sandstone maj^ be different in the two hills, that below layer II on 
Little Limestone being higher stratigraphicall}- than that at the base 
of Big Limestone. This we regard as an impossibility for reasons 
given above. (2) The outcrops of layer III on Little Limestone may be 
large blocks fallen from a higher position because of undercutting and 
slumping of the layers. That such undercutting and slumping has 
occured at places on both hills in pre-glacial and glacial times is certain, 
but to assume it for laj^er II, ivolves the further assumption that the 
core of Little Limestone is formed by laj^er I, at least 80 feet thick, 
and since the whole hill does not rise over 90 feet from the sandstone, 
there is not room for such a core. It is very peculiar that no fossils of 
layer I were found on Little Limestone if any remnant of such a core 
exists. There is some possibility that Little Limestone has been split 
by a fault equal in throw, to the fault between Big and Little Limestone 
and parallel to it, and that the full series is represented on the east end 
of Little Limestone. One or two points support this suggestion. The 
east face of the north end of Little Limestone is very steep, descending 
abruptly to the swamp level. On the south end of the same face the 
slopes are less steep and there is a slight but well marked terrace indi- 
cating the position of the sandstone which outcrops here some distance 
above swamp level. This assumption of structure while possible is not 
less complicated than the one of an irregular surface of sandstone upon 



LIMESTONE MOUNTAIN AND SHERMAN HILL. 181 

which the dolomite was deposited, and on the whole seems less satis- 
factory to us. 

Layers VI, VII, VIII and IX are so irregular in position, and so 
evidently involved in the debris, that we are inclined to believe that 
they are not in position but attained their present attitude as land- 
slides or slumps due to undercutting by surface or underground waters 
in comparatively recent times. 

In our opinion these outliers have been broken both by major faults, 
which involve the sandstone below, and by minor faults or breaks, the 
result of erosional forces. Unfortunately our work does not throw 
much light upon the age of the great Keweenawan fault, the outliers 
do not approach near enough to the fault line to afford definite evi- 
dence. All that we can say safely is that there were considerable move- 
ments later than Mid-Devonian time, involving the Cambrian rocks. 

In the preparation of this report the authors have had the advantage 
of a study of a manuscript "Report on the fossils of Limestone Moun- 
tain" prepared by Professor A. C. Lane while he was State Geologist 
of Michigan. 

In conclusion the senior author wishes to state that most of the field 
work was done by the junior author and that to him is largely due 
the credit for the discovery of the wide extension of the Paleozoic seas 
over the Northern Peninsula of Michigan. 



INDEX. 



INDEX. 



A. 

Page 

Algonkian. relations of intrusives to 83 

Algonkian system, presence of in Gwinn district 147 

Allen, H. J., acknowledgment to 19 

Allen, R. C. cited 22, 131 

Analysis of drill core from Mercer section 98 

Animikie. composition of 38 

Animlkie extrusives. occurrence of 47 

Animikie series, correlation with Middle Iluronian 31 

description of in Turtle range 90 

equivalence of 22 

structure of 61 

Animike, succession of 69 

Archean, relations of intrusives to 83 

Archean rocks, composition of 37 

Archean system, occurrence of in Gwinn district 147 

Ascham, R. E., acknowledgment to 19 

B. 

Bad River limestone, composition of 38 

Banner location, result of diamond drilling on 103 

Barrett, L. P.. acknowledgment to 19 

Basal quartzite, occurrence of in Ford-Lucas section 92 

occurrence of in Whiteside exploration 93 

Basic intrusives, occurrence of 48 

Batholith (granite), occurrence of in northern Wisconsin and Gogehic range.. 17 

Bayley, W S.. cited 22. 24, 20. 30. 98 

Big Limestone Mountain, dip of rocks forming 176 

situation of 167 

Brewer, Luther, acknowledgment to 33 

Brooks, J. B., cited 141 

Brooks, T. B., cited 30 

Broomhandle exploration, location and character of material 96 

C. 

Carpenter, F. L. Syndicati-. acknowledgment to 19 

formation of 15 

results of drilling on Manitowish range Ill 

results of recent diamond drilling 88 

(Mark, K. W., acknowIiMlgineiit (o 19 

Clements, .1. M., cited 27, 107 

«'oiiglomerate-arkose of tlie (iwinn series descril)ed 147 

Conover district, location and extent of 123 

Conover slates, i)et rograplilc description of 124 

Cooper, W. F., Idt-nllllcation of fossils l)y 171 

('opps formation, lltliology and location 54 

ri'inlive to adjacent formations 57 

slru<ture of 60 

thickness of 57 

Corless, (!e<>. B.. ix'knowledgment 15» 

CoiTflal ion III' I'riiii'cloii and <i\vliiii series, notes on LIS 



186 INDEX. 

Page 

Correlation tables, Gogebic iron range 36 

Gwinn syncliuorium and Little Lake hills 154 

Marquette and Gwinn districts 143-144 

Correlations, table of 31 

I). 

Decorah, fossils from 175 

Diabase, occurrence of 49 

occurrence of on Marenisco range 83 

Dip of rocks of Limestone Mountain 176 

Sherman Ilill 177 

Dolomite, analysis of 169 

occurrence of on Turtle range 90 

E. 

Extrusives, occurrence of on Marenisco range 77-80 

occurrence of on Turtle range 105 

F. 

Fault between Big and Little Limestone Mountain 179 

Ferruginous slate, Conover district, analysis of 127 

Ford-Lucas section, character and composition of material 91 

Foster and Whitney, cited 168 

G. 

Galena, fossils from 174 

Gogebic range east of Wakefield, structure of 59 

Goldsberry, J. P., acknowledgment to 19 

Granite, exposures of on Manitowish range Ill 

Granite intrusive into Lower and Middle Huronian .' . . . 17 

Granite, occurrence of on Marenisco range 82 

Granitic intrusion, exomorphic effect of '. 18 

Graywacke-quartzite, occurrence of in Middle Huronian 70 

Green schist, occurrence of in Wolf Lake area 136 

Greenstone (ellipsoidal), occurrence of near Michigan mine 96 

occurrence of on Turtle range 105 

Greenstone, occurrence of 49 

occurrence of as earliest intrusives 81 

occurrence of in Broomhandle exploration 96 

Greenstones (porphyritic), character of on Turtle range 107 

Groveland formation, correlation of 28 

Gwinn district, correlation and structure of formations 141 

location and topography 145 

table of correlations 143-144 

Gwinn series, composition of 147 

correlation of 151 

notes on correlation 158 

occurrence of arkose and conglomerate 157 

Gwinn syncliuorium. correlation tables 154 

notes on structure 145, 146 

H. 

Hall, James, cited 169 

Hemlock mine, result of recent development in 26 

Honnold, W. L., cited ITO 

Hornblende schist, occurrence of T9 

Hore, R. E., acknowledgment to 19 

Hotchkiss, W. O., cited 31 

Huronian group, definition of position 21 

former correlations 21 



INDEX. 187 

Page 

Huronian group in Gogebic and Marquette ranges, correlation of 23 

Huronian group, tripartite division of 22, 24 

Huronian, relation of mica schist to 69 

Huronian rocks north of Marquette, examination of 16 

Huronian rocks, relation of effusives to 79 

Huronian series, groups in 34 



Igneous rocks, occurrence of on Turtle range 104 

Intrusives, occurrence of on Marenisco range 80-85 

occurrence of on Turtle range 104 

Iron bearing member of the Gwinn series 149 

Iron formation, depth at Banner location 104 

Iron formation, occurrence of in Ford-Lucas section 92 

occurrence of in Michigan mine 96 

occurrence of in Whiteside exploration 93 

Iron formation of Marenisco range, igneous intrusives in 74 

Iron formation of Marenisco range, lithology 71 

ore bearing possibilities of 74 

Ironwood formation, lithology 44-46 

distribution and exposures 43 

Ironwood series, correlation with Xegaunee series 23 

correlation with Vulcan series 29 

Irving, R. D., cited 22, 24 



Jackson, C. T., cited 169 

Jacobsville sandstone, occurrence of 167 



Keweenawan gabbro, occurrence of 58 

Keweenawan, relation of mica schist to 69 

Keweenawan series, occurrence of in Gwinn district , 151 

relation to adjacent formations 58 

structure of 60 

King, F. H., cited 88, 89 

L. 

Lane, A. C, cited 171, 176. 181 

Leith, C. K.. acknowledgment to 33. 138 

cited 17, 28, 29, 30, 33, 38, 39, 98, 141, 149, 153 

Little Lake area, occurrence of exposures in 153 

Little Lake hills, correlation tables. 154 

Little Lake hills, structure of 155 

Little Limestone Mountain, situation of 167 

Longyear, E. J., Company, result of diamond drilling on Banner location 103 

results of recent diamond drilling S8 

Longyear. .1. M., Jr., cited 55 

Lower Huronian series, composition cif 37 

Lower liichniond, fossils from 173, 174 

M. 

McKennn, I'. <;.. acknowledgment to 19 

Magnetic belts, appearance of on early geological maps 1C> 

occurrence of In Merrer section i»7 

occurrence of on Turtle range ,S7 

Manitowlsh rang(>. geiicral summary ^ 117 

location an<i extent of Ill 

results of iliiiiiinnd drilling on 114-117 



188 INDEX. 

I'ago 

Miirenisco range, character and extent of 6.5 

succession on (JO 

Marquette district, table of correlations 14.S-144 

Mead, W. J., acknowledsnient to 138 

Menominee iron range, extent of 161 

Mercer section, analysis of drill core from 98 

result of diamond drilling in 97 

Mica schist, chemical analysis of 138 

occurrence of 67 

origin of 68 

occurrence of in Wolf Lake area 1.36 

relation of adjacent rocks 69 

relation to Paint slate formation 138 

Michigan mine, location and cliaracter of material 95 

result of recent development in 26 

Mid-Devonian, fossils from 173 

Middle Huronian, composition of 38 

correlation of Animlkie series with 31 

Middle Huronian extrusives, occurrence of 47 

Middle Huronian series, description of in Turtle range 90 

structure of 61 

succession of 69 

Middle or Upper Richmond, fossils from 173 

N. 

Negaunee series, correlation of with Vulcan series 25, 27 

correlation with Ironwood series 23 

equivalence of 22 

Niagaran, fossils from 173 

P. 

Paint slate, characteristics of 134 

Paint slate formation, composition and location of 131 

relation to Wolf Lake granite and mica schist 138 

Paint slate, petrographic description of 132 

Paleozoic, occurrence of in Gwinn district 151 

Palms formation, composition of , 39 

distribution, exposure and lithology 40 

metamorpbic phases 41 

Porphyrites (altered) , occurrence of 77 

Potsdam, composition of 176 

Presque Isle granite, lithology of 51 

occurrence of 50 

relation to Copps formation 54 

relation to Middle Huronian 51 

Presque Isle Mining Company, acknowledgment to 33 

Princeton series, composition of 150 

correlation of 151 

notes on correlation 158 

occurrence of conglomerate 157 

occurrence of quartzite and quartz slate 158 

Q. 

Quartzite, occurrence of on Turtle range 90 

R. 

Robinson, W. I., acknowledgment to 19 

Rominger, Carl, cited 141, 169 

Rose, Robert Selden, acknowledgment to 33 

Rupp, Geo., acknowledgment to 33 



INDEX. 189 



S. Page 

Seaman, A. E., cited 22 

reference to notes by 16 

Sherman Hill, dip of rocks 177 

situation of 167 

Slate formation of Marenisco range, lithology of 76 

occurrence of 75 

relations to adjacent formations 77 

Slate, occurrence of in Whiteside exploration 93 

Slate (lower) of the Gwinu series 149 

Slate (upper) of the Gwinn series 150 

Smyth, II. L.. cited 22, 24. 25. 28. 98 

Smith, R. A., acknowledgment to 19 

Sunday quartzite, composition of 37 

T. 

Turtle range, extent of 87 

petrographic description of rocks on 107-109 

succession of 89 

r. 

Ulrich, E. O.. acknowledgment to 173 

Upper Black River, fossils from 175 

V. 

Van Hise, C. R., acknowledgment to 33 

cited 17, 22, 24. 26-28. 30. 33, 38. 39. 50. 55. 98. 113. 141, 149 153 

Vieux Desert district, character of rocks in 121 

location and extent of 119 

results of diamond drilling in 119-121 

Vulcan series, correlation with Ironwood formation 29 

correlation with Negaunee series 25, 27 

W. 

Whiteside exploration, character and composition of material 93 

Whiteside, R. B., results of recent diamond drilling 88 

Winegar section, result of diamond drilling on 99 

Wolf Lake area, exposures in 135 

Wolf Lake granite, age of 139 

composition of 135 

relation to I'aint slate formation 138 

Wolf liake schists, composition of 136 

petrographic description of 137 

Wriglit. Oha!^. E., cited 30 

Y. 

Youngs. L. .(.. a<kii(iwlcdgniciit t<i 1<) 



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