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Studies for Students 



DEFORMATION OF ROCKS. — IV. CLEAVAGE AND 

FISSILITY ( contimied), JOINTS, FAULTS, 

AUTOCLASTIC ROCKS. 



RELATIONS OF CLEAVAGE AND FISSILITY TO OTHER 
STRUCTURES. 

RELATIONS OF CLEAVAGE AND FISSILITY TO BEDDING. 

Where a rock series is composed of layers of different litho- 
logical character, and is in the zone of combined fracture and 
flowage, the deformation includes the development both of 
cleavage by normal plastic flow and of fissility in the planes of 
shearing, in both homogeneous and heterogeneous rocks, and 
perhaps of all gradations between the two. The beds in the 
heterogeneous rock may be each approximately homogeneous. 
There is necessary rearrangement within the beds as well as 
readjustment between them. Therefore the rearrangement 
within the beds, in so far as it is not affected by the readjust- 
ment between the beds, will tend to produce cross cleavage and 
cross fissility, while the readjustment between the beds will 
mainly be by parallel slipping, and will tend to produce parallel 
cleavage and parallel fissility (Fig. lo, p. 478). 

In passing from the limbs of the folds toward the crests or 
troughs, parallel readjustment becomes less and less important, 
and normal plastic flow or fracture along the shearing planes 
becomes more and more important. At the arches and troughs 
the thrust for a given bed are approximately equal, in opposite 
directions, and when deeply enough buried its entire thickness 
is under compression. The direction of least resistance is ver- 
tical. Therefore the conditions which here prevail are those of 

593 



594 



STUDIES FOR STUDENTS 





1 



the formation of cross cleavage and cross fissility. It follows 
that in heterogeneous rock strata, parallel structures may pre- 
vail on the limbs of the fold, and cross structures on the crests 
and in the troughs. At intervening places may be found all 
the complex effects of the interaction of 
the two (Fig. 7). In many cases where 
there is almost perfect accordance of pri- 
mary and secondary structures on the limbs, 
and the rocks are so crystalline that the 
two cannot readily be discriminated, at the 
crests and troughs both structures may 
readily be seen intersecting each other. 

Formations are but divisions of rock 
masses greater than beds which are roughly 
homogeneous. In each formation, consid- 
ered as a whole, cross secondary structures 
; will usually be produced, while at the con- 

tacts between the formations, where major 
readjustment is sure to occur, nearly par- 
FiG. 7(reprmtedfrom ^jj^j structures may be found. In the dis- 
p. 470). — Parallel fissility •^ 

on the limbs of the folds cussion of each separately it has been seen 
and cross fissility on the that in the cases of extreme folding the 
anticlines, and gradations relations between the different secondary 

between the two. After 1 1 , i- 11 

jj^.^ structures and bedding are nearly the same, 

The deformation is and therefore that cleavage and fissility 

mainly by folding, but on developed under each of the laws will merge 

the anticlines, where the . ^^ 1 1 ^1 i - . \ 

, . , . ,, T -, to2"ether, and both be approximately par- 

material is partly relieved o ' rr j r 

from stress, the deforma- allel to the reduplicated beds. They are 
tion is partly by the mul- all brought into nearly parallel positions, 
tipie minor slips of fissility. -^^^ ^^ ^^^ pebbles in the folding process. 

In order that this should be done, it is plain that there must 
be such extreme rearrangement of the rock material that it 
could not inaptly be compared with kneading. 

In rock masses in which the alternating layers of different 
strength are not beds, the principles of the development of 
cleavage and fissility are the same as in the heterogeneous 



DEFORM A TION OF ROCKS 595 

bedded Focks. The different layers may be due to secondary 
structures. They may be due to the flowage of igneous material 
along primary or secondary planes of weakness, or to secondary 
water-deposited impregnations along such planes. They may be 
due to heterogeneous injections. In all of these cases, as in any 
other, the stronger beds will to a certain extent control 
the movements. The accommodations will occur along the 
layers when folded, and there will be a tendency for cleavage 
and fissility to develop parallel to them. 

The more unequal the layers in thickness and strength, the 
more likely is the major accommodation to take place parallel 
to them, and thus produce cleavage and fissility which nearly 
accord with them. In proportion as the rocks approach massive 
ones, the law of cross structures prevails, but in a minor way 
readjustments along the laminae may occur, and these laminae 
still retain their integrity. In rocks as they occur in the field 
both tendencies are always present in all the parts, from the 
minute laminae to the largest masses. Sometimes the first is 
predominant, sometimes the second. If the lamination is not 
strongly marked the first tendency will control, although the 
lesser layers or laminae of unequal strength may in a minor way 
control the movements. 

Ordinary shale is a representative of rocks having minute 
layers of slightly different strength. Usually the average of the 
cleavage or fissility distinctly cuts the beds. The material yields 
to thrust by flowage or fault slips combined with minute pucker- 
ings. If the process be continued far enough the individual 
layers are folded upon themselves in a large number of parallel 
folds, nearly at right angles to, or inclined to, their original 
positions. Therefore, while cleavage does nearly correspond 
with the original beds, the particles have been so much deformed 
and rearranged and the layers have been so far readjusted as to 
make the term bedding scarcely applicable. 

As has been seen (p. 474) , beds are particularly likely to 
largely control the direction of cleavage or fissility if the folds 
are monoclinal and overturned. In such folds the major dif- 



59^ STUDIES FOR STUDENTS 

ferential movements are along the longer limbs. Therefore the 
cleavage develops in a corresponding direction, being nearly 
parallel to the bedding on one limb of each fold and cutting 
across the bedding on the other, steeply inclined or overturned 
limb. As the area of outcrop of the steeper limbs is much less 
than that of the more gently inclined ones, the fact that the 
cleavage cuts the bedding on one side of each fold is very likely 
to be overlooked. As a result of the greater mashing thus 
developing cleavage or fissility the longer limbs of the folds are 
thinned more than the shorter limbs. 

If for any cause cleavage be parallel to the bedding planes, 
as these are apt to be sh .aring planes in the zone of fracture, 
the predominant fissility would be likely to be parallel to the 
bedding. The other direction of fissility would be transverse to 
the bedding, and might have a wider spacing. The first might 
be called fissility and the second joints. 

In another case, after a cross cleavage has developed and the 
rocks have passed into the zone of fracture, the stresses may 
result in the development of fissility along two sets of shearing 
planes, one of them being controlled in direction by the cleav- 
age, the other by the bedding. The more regular parting, par- 
allel to the cleavage, might be called fissility, and the less regular 
parting, parallel to the bedding, might be called either fissility 
or joints, depending upon its closeness. Whether the intersect- 
ing planes of fissility are at right angles to each other would 
depend upon the inclination of the cleavage and the bedding. 

In regions of complex folding it is difficult to make accurate 
general statements of the relations of cleavage and fissility to 
bedding. However, as a result of the action of the various 
forces, a bed has a definite strike and dip, and the cleavage and 
fissility have definite relations to these. As there are rapid 
variations in strike and dip in regions of complex folding, it is 
to be expected that there will be variations in the directions 
and character of cleavage and fissility. Certain of the specific 
relations of bedding and secondary structures in regions of com- 
plex folding have already been considered (pp. 347-349). 



DEFORM A TION OF ROCKS S97 

RELATIONS OF CLEAVAGE AND FISSILITY TO THRUST FAULTS. 

Between cleavage and fissility developed along the longer 
limbs of folds and thrust faults, which accord in dip with the 
beds on the longer limbs, there is only a difference in the 
magnitude and frequency of the movements. 

When fissility or cleavage develops there are many slightly 
or infinitesimally separated movements of small degree. When 
a thrust fault develops there is a single major movement. It is 
believed that the relief is more likely to be by faulting at little 
depth, and at greater depth is more likely to be by the 
development of cleavage, and often, secondary to it, fissility, 
or more rarely by the development of fissility directly. The 
passage of cleavage by gradation into minute overthrust faults 
is beautifully illustrated a short distance northwest of Blowing 
Rock, N. C. Where fissility is not developed throughout the 
rock mass it may occur adjacent to thrust faults, due to the 
shearing adjacent to the thrust planes. Rock masses deformed 
by thrust faults and showing fissility adjacent to the faults will 
have zones of fissility which alternate with others in which this 
structure is absent. Where fissility varies in perfection of devel- 
opment in alternating zones, but is present throughout the 
rock mass, it implies that the relative movements were con- 
centrated to a certain extent along definite zones, but that move- 
ment everywhere occurred. It is plain that shearing developing 
cleavage and fault slips along planes of fissility accomplish the same 
mass deformation as do thrust faults^ only it is averaged through- 
out the rocks instead of being largely concentrated at certain 
planes. 

Differential movements similar to those described in the 
above paragraph may occur along a lamellar structure in any 
kind of a rock, sedimentary or igneous. 

By differential movements, such as are above described, 
enormous masses of material may move forward long distances 
The top of the mass, having the advantage of all the differential 
movements below, will travel the farthest (Figs. 5 and 6, p. 468). 
The base will move the least. From this mass at some later time 



598 STUDIES FOR STUDENTS 

mountains may be carved. As each stratum grinds over the one 
below it the former presses against the latter with all the weight 
of the superincumbent material. Under such circumstances it is 
no wonder that a coarse-grained, massive granite may be trans- 
formed into an evenly laminated schist. In the zone of fracture 
the schist, developed in the zone of flow, may become fissile, and 
the slickensided, wavy folia may be thinner than paper. It is 
by folding combined with differential movement that the abnor- 
mal folds described on a previous page are produced. 

The cleavage or fissility so frequently present upon the flanks 
of anticlinal core-rocks in great mountain ranges may be 
explained by similar movements. In the section on the analysis 
of folds it has been shown that much readjustment must occur 
upon the limbs of folds. The flanks of an anticlmal mountain 
core are such limbs, and hence the development of cleavage or fis- 
sility parallel to the central massif. In passing toward the center 
of the core we approach nearer the crown of the anticline, and 
penetrate to a greater depth ; hence less readjustment is necessary, 
and therefore the secondary structures are less prominent. 

RELATIONS OF CLEAVAGE AND FISSILITY TO THICKNESS OF STRATA. 

Without reference to the origin of secondary structures, or 
any evidence upon this point, bedding and secondary structures 
are often spoken of as corresponding. Even reputable text- 
books make such statements. This confusion is most unfortu- 
nate for two reasons. (i) It often leads to great overestimates 
of the thickness of strata, the real thickness of the beds being 
supposed to be the apparent thickness as observed across the 
secondary structure, where, as shown by the foregoing analysis, 
the same bed may be repeated many times. (2) The mistake 
is likely to give erroneous ideas of structure. If the primary and 
secondary structures are thought to correspond, the whole 
breadth of a slate or schist may be regarded as a bed of enor- 
mous thickness, and this will lead to the preparation of sections 
in which the mass is represented as extending to a great depth, 
where it may be comparatively superficial. (Fig. 12.) 



DEFORM A TION OF ROCKS 599 

The assumption that bedding and secondary structures cor- 
respond is still less justifiable when no remaining evidence of 
bedding is found. If only cleavage or fissility be found and the 
relations of the beds with other beds are not such as to give the 
direction of stratification, no inference in reference to this point 
should be drawn. 




Fig. 12. — Closely plicated shale underlain by bed of limestone. 

It is apparent that attempts to estimate the real thickness of 
cleaved or fissile beds must take into account two difficulties : 
(i) The same bed may be folded on itself many times, and 
these folds must be followed, or at least some estimate must be 
made of the thickness of the beds which would be present if the 
minute plications could be straightened. (2) In the complex 
folding of the beds there is readjustment, mashing and conse- 
quent lengthening of the layers upon the limbs, and they are, 
therefore, on the average, thinner than originally. So far as 
such thinning occurs, it compensates for the reduplication of the 
beds, but it is believed that this compensation is far short of full 
correction. To fully overcome these difficulties is often impossi- 
ble, and estimates of the thickness of closely folded, cleaved, 
and fissile beds, even when all the difficulties are wholly under- 
stood and allowances made, are usually only approximate. 

DEVELOPMENT OF CLEAVAGE BY OTHER CAUSES THAN THRUST. 

Thus far I have considered cleavage developed in connection 
with and dependent upon orogenic movements. It is probable 
that this structure develops in other ways. It may be that 
deeply buried beds may become cleavable with the structure 
parallel to bedding, where superincumbent pressure, cementa- 
tion, and metasomatic changes are the predominant forces. 



600 STUDIES FOR STUDENTS 

Such deep-seated rocks, if below the level of no lateral stress, 
are in the zone of great vertical compressive stress and circum- 
ferential tension. They would, therefore, be shortened verti- 
cally. If under the stress of gravity movement goes far enough, 
this would develop a cleavage parallel to the surface. Such 
cleavage in sedimentary rocks would be parallel cleavage and 
would emphasize the bedded structure originally formed. Just 
below the level of no lateral stress it is probable that the circum- 
ferential dilation would be slight, but would increase with 
depth. Whatever its amount, it is a real cause so far as it goes. 

It is not asserted that rocks in which cleavage may thus 
develop reach the surface by subsequent denudation, but per- 
fectly crystalline schistose rocks in which the cleavage corre- 
sponds exactly with the bedding, and which are but gently folded, 
suggests that such may have been the conditions under which 
the structure formed, and if the estimates given for the depth of 
the level of no lateral stress, from two to eight miles,'' are correct, it 
is certain that rocks which have been below this level in some 
regions have subsequently reached the surface by denudation. 
It is generally believed that the Laurentian and Adirondack 
areas are regions of profound erosion, and here are found excellent 
illustrations of gently folded cleavable schists, the structures of 
which apparently correspond with original bedding. The above 
explanations may be applicable to these regions. This method 
of the development of cleavage parallel to the surface of the 
earth below the level of no lateral stress is also applicable to 
igneous rocks. 

Laccolitic or batholitic intrusives might promote this process 
by giving great pressure parallel to the bedding and by heating 
percolating waters, thus rendering them more active. In the 
Adirondacks the cleavage of the schists and the periphery of the 
batholite of gabbro have in some places a parallel arrangement, 
and the intrusion of the igneous rock has probably been one of 
the causes of the metamorphism and the parallel relations 

'Origin of Mountain Ranges, by Joseph Le Conte, Jour, of Geol., Vol. I, 1893, 
pp. 566-568. James D. Dana, Manual of Geology, 4th ed., 1895, PP- 3^4, 385. 



DEFORM A TION OF ROCKS 6o I 

obtaining between schistosity and bedding. Readjustment 
between the beds, as explained on a previous page, may also 
have assisted in the process. 

In a third case, a great boss of intrusive igneous rock may 
cause the secondary structure to be everywhere parallel to it, 
without reference to the direction of previous structures. This 
case probably differs but little from that of direct thrust, but the 
direction of thrust gradually varies through 360° in circumscrib- 
ing the mass, being at all times radial. The material pushed 
aside obeys at each point the law of normal plastic flow, just as 
in ordinary orographic movements. The new minerals develop, 
with their shortest diameters in the direction of thrust. Old 
minerals are mashed into similar forms in parallel positions. 
The heat of the igneous rocks furnishes hot solutions which help 
to transform the old minerals. As the direction of the thrust 
varies gradually around the intrusive, the secondary structures 
follow, and therefore form a zone around the intrusive, the 
layers of which may be compared to those of an onion (see pp. 
454-456). 

The process may be complete, and old structures, such as 
bedding, previous cleavage, or previous fissility, may be wholly 
destroyed. In other cases traces of these structures may still be 
found. Where the earlier structures are wholly obliterated near 
the intrusive, they may appear gradually in passing away from 
it. Thus in the same rock mass several structures may occur. 
The mica-schists about the intrusive granite core of the Black 
Hills are an excellent illustration of this case. 

MODIFICATIONS OF SECONDARY STRUCTURES. 

The partings of fissility may be concentrated here and sparse 
there. In the cracks between the laminae a new mineral or 
minerals may be deposited from water solutions, and the second- 
ary zones of greatest fissility would then have a composition 
different from that of those which are less fissile. Moreover, 
the parted laminae and the minute layers of infiltrated material 
may be of different composition. This would give a minute 



602 STUDIES FOR STUDENTS 

alternation of layers of different characters. Such a major and 
minor alternation of different materials may simulate the 
appearance of bedding to a remarkable degree. If such struc- 
tures are taken for bedding, mistakes in structural work will 
follow. 

Where fissility is developed in an igneous rock, secondary 
impregnations may occur between the laminae, just as above 
described. Thus there would be formed a rock with alternating 
layers of different mineral character, no part of which is sedi- 
mentary, and yet which closely simulates a sedimentary struc- 
ture. If either the sedimentary or the igneous rock which has 
become fissile be intruded by igneous materials, these might 
follow the cracks in a minute way, and thus again produce a 
structure which is very similar to bedding. 

In the above cases both the process of water impregnations 
and that of igneous injections tend to cement the rock. If the 
process be complete the crevices of the rock may be entirely 
healed. The once fissile rock will then have lost its fissility. It 
may, however, have the property of cleavage parallel to the 
banding. Such a cleavable rock may give no evidence that it 
v/as once fissile. From the foregoing I conclude that banded 
rocks ?nay owe their structure to fissility and secondary impregnations 
or injections, or both, and the bands may or m.ay not accord with an 
original structure. 

After a first secondary structure has developed, later move- 
ments may produce a new cleavage or fissility, which cuts this 
earlier structure at right or oblique angles, or the new force may 
be so intense as to produce a structure which wholly destroys 
the earlier structure. Usually, in order that a new structure may 
be produced, it is necessary that the new force shall vary con- 
siderably in direction from the first, so that it cannot be decom- 
posed into two components, one parallel to the old structure and 
one at right angles to it. To this fact is doubtless due the 
comparative frequency of cleavage and fissility in several direc- 
tions in the same rock mass ; but while the secondary structure 
is ordinarily in a single direction, an exposure or even a hand 



DEFORMATION OF ROCKS 603 

specimen may show the original bedded structure and secondary 
structures in two or more different directions. In fact, as has 
been shown, a single simple orogenic movement may produce 
both cross and parallel secondary structures, and the cross fissility 
may be in two directions. 

While it is rare to find more than two or three structures in 
a rock, theoretically there is no limit to the numbers which might 
be produced ; but practically, as has been seen, new movements 
usually emphasize old structures, or else produce new structures, 
which tend to obliterate the old structures. However, in a thor- 
oughly crystalline rock, if there be two or three structures, it is not safe 
to assume that the oldest and most intensely plicated one is bedding. 
This has been done frequently in the case of the crystalline 
schists and gneisses by those who would not regard cleavage or 
foliation, if but a single structure existed, as evidence of bedding. 
The older the structure the greater is the probability that it is 
really bedding, but the fact that it is the earliest structure which 
now exists in the rock cannot be regarded as conclusive, for it 
may have been produced by an early orogenic movement which 
simultaneously obliterated bedding. 

After a secondary structure has developed in a forma- 
tion it may be folded into anticlines and synclines. In order to 
be thus folded it is usually necessary that the secondary struc- 
tures be not steeply inclined. As indicated on a previous page, 
where such a structure develops in a horizontal position it may 
correspond with bedding, but also it had been seen that cleavage 
or fissility may form with slightly inclined planes of movement 
which cut diagonally across the bedding. Such a cleavage or 
fissility may be emphasized by secondary impregnations and 
injections, in which case it simulates bedding to a remarkable 
degree. In either of the above cases a careless observer would 
be almost certain to regard the structures as bedding. 

The number and severity of orogenic movements may in 
many places have been so great along old ranges that it is not 
strange that it is impossible to differentiate or separate the 
various formations upon a structural basis. The beds have 



604 STUDIES FOR STUDENTS 

been kneaded again and again by the orogenic forces ; cleavage, 
fissility, and band banding may have developed in different 
directions ; earlier structures may have been destroyed by later 
transformations ; until it is no longer possible to determine the 
position of original bedding. 

APPLICATION TO CERTAIN REGIONS. 

In many mountainous regions in which there has been pro- 
found erosion, illustrations of nearly all of the foregoing princi- 
ples may be found. Attention may be directed to one or two of 
the more important. 

It has already been pointed out (pp. 337-338) that in the 
Appalachian and New England crystalline areas the main direc- 
tion of active stress was probably from the southeast toward the 
northwest. At any rate, the couple composed of force and 
resistance was such as to make the higher strata move toward 
northwest as compared with the lower strata, or to make the 
lower strata move to the southeast as compared with the upper 
strata. As a consequence of this, the folds of that area have 
axial planes which have a very general tendency to dip to the 
southeast. If the force be supposed to have been directed 
toward the northwest, and to have been equal for different depths 
throughout the thickness of the rocks now exposed at the sur- 
face, the cleavage which developed in the normal planes would 
dip to the southeast. This would have been due to two causes : 
First, the direction of normal pressure, compounded of thrust 
and of gravity, would have been northwest and downward, which 
would, therefore, have given a southeasterly dip to the cleavage. 
Also, because of increasing resistance and probably lessening 
thrust with increasing depth, the force would have caused the 
higher strata to have moved differentially over the lower ones. 
There would, therefore, have been a shearing motion, the higher 
strata moving upward and northwestward as compared with the 
lower. As a result of this shearing and of shortening, the old 
and new mineral particles would lie with their longer diameters 
in southeasterly-dipping planes and give a cleavage in that direc- 



DEFORM A TION OF ROCKS 605 

tion. The conjunction of these two forces would have given a 
flatter dip than would follow from either one of them alone. 

While the above statement is true on the average, the case is 
complicated because of the differential movements in individual 
folds. The cleavage for a given section is in a single direction 
to the southeast only when the folds have a decided monoclinal 
attitude, and this is especially marked where the folds are all 
overturned. Even here, however, the cleavage tends to be flatter 
upon the limbs in normal positions than on the overturned limbs. 
In the areas in which the folds approach a symmetrical character, 
cleavage with northwest dips is found on the northwest limbs of 
the folds. The explanation of these phenomena is given on 
pages 461-475- 

While there is a general tendency in this region for a south- 
easterly-dipping cleavage, there are great variations in the steep- 
ness of the dip in different beds in the same locality, and varia- 
tions in the average steepness in different localities. The varia- 
tion in steepness in the same area is explained by the fact that 
the differential movement between the strata was largely concen- 
trated in the weaker beds, so that the cleavage in them is flatter 
than in the more resistant beds. The general variation in the 
dip of cleavage in passing from area to area may be explained 
by a difference in the character of the rocks, by a difference in 
the intensity of the forces at varying depths, or by a difference 
in the depth of burying. The particular average inclination for 
a given area depends upon a combination of all these variables. 

With given forces, if the rocks are more resistant in one area 
than in another, there is less shearing motion, and therefore 
steeper cleavage in the former than in the latter. Other things 
being equal, if the forces are more intense near the surface than 
at a greater depth, the shearing motion is greater at higher 
horizons, and hence the cleavage is flatter in passing toward the 
surface. A given force would produce less and less shearing 
motion with increasing depth, because of the increased friction, 
and hence the cleavage may be flatter in passing toward the sur- 
face, just as in the foregoing case. 



6o6 STUDIES FOR STUDENTS 

For any given area, after cleavage developed, as denudation 
progressed the zone of flowage passed upward into the zone of 
fracture. It is clear that the cleavage planes already developed 
were then probably shearing planes, and this was true even if the 
horizontal thrust was the same and in the same direction in the 
zone of fracture that it was in the zone of flowage, for the direc- 
tion of greatest normal pressure is composed of thrust and 
gravity, and therefore at a great depth is steeply inclined to the 
horizon, whereas in the zone of fracture, gravity being less impor- 
tant, the direction of greatest normal pressure is less steeply 
inclined, and therefore normal planes in the zone of cleavage 
become shearing planes in the zone of fracture. In the develop- 
ment of fissility along the cleavage planes there were slight 
differential movements between the laminae, and hence was formed 
the very extensive fault-slip cleavage so well known in the 
Appalachians. It is believed that the more regular and wide- 
spread fissility is thus secondary to cleavage, but it is recognized 
that fissility or joints formed in other directions, and that in the 
outer zone, which was never in the zone of flowage, original 
fissility or jointing only was developed. 

As pointed out by Willis, in the western area of little altered 
rocks in the southern Appalachians the deformation was mainly 
by faulting ; in the corresponding area in the northern Appala- 
chians the deformation was mainly by folding. At intermediate 
areas the deformation was by faulting and folding. Parallel to 
the fault-planes fissility developed to some extent in the area of 
fracture, and dipping in the same direction as the monoclinal 
folds cleavage developed in the area of folding. In an inter- 
mediate area the deformation was by major faulting, by minor 
fault-slips along fissility, by the pure shortening and shearing 
motion producing cleavage, and by monoclinal folds, all com- 
bined. The interactions of these are more fully described in 
other places (see pp. 595-598,620-622). 

Returning to the crystalline area, in the cracks and crevices 
between the fissile laminae mineral impregnation from water 
solution occurred at many places, and thus gave the rocks a 



DEFORM A TION OF ROCKS 607 

parallel banding, as described by Hobbs in the Searls quarry. 
In other districts parallel injections of igneous rocks occurred, 
and here the metamorphosed schists are wedded together along 
the fissile planes by igneous material. Such are the conditions 
in southeastern New York, and especially on northern Manhattan 
Island in the vicinity of New Rochelle. 

As shown in other places (pp. 600-603), there may be all 
gradations between aqueous impregnations, through aqueo- 
igneous deposits, to true igneous injections. Usually the origi- 
nal rock differs in chemical and mineral composition from the 
impregnations or injections. The process gives a distinctly 
banded structure, which has often been mistaken for metamor- 
phosed original sedimentary layers. Also the chemical com- 
position may be so changed, if the amount of secondary mate- 
rial is large, as to make it impossible to tell from its chemical 
analysis whether the original rock is aqueous or igneous. 

If the parallel mineral impregnation, or the parallel igneous 
injection, or the two together, had been general throughout the 
Appalachian semicrystalline and crystalline areas, we should 
have a vast series of crystalline schists with parallel banding, the 
bands generally dipping to the southeast, and these layers might 
be mistaken for beds, and thus lead to estimates of enormous 
thickness for the series, when in reality the original sedimentary 
beds were of no unusual thickness. This error has not been 
made for much of the Appalachian region because in most areas 
the metamorphism has not been so extreme but that the cleav- 
age is detected intersecting the bedding at the sharp turns of 
the layers on the crests of anticlines and in the troughs of syn- 
clines. Had the metamorphism gone as far throughout the 
region as it has in certain places, the bedding could not be dis- 
covered, and there woufd be now no means of tracing out the 
different steps of the process of modification. But in the 
Appalachian and New England regions the steps of the process 
of the development of cleavage and fissility, the stages of the 
parallel impregnation and injection, and all degrees of metamor- 
phism may be observed, so that in some of the regions of most 



6o8 STUDIES FOR STUDENTS 

extreme change the genesis af the rocks and their structures 
are determined with reasonable certainty. 

In the eastern region the areas in which the sedimentary rocks 
have gone through the process above described are more exten- 
sive than those of igneous rocks, but all steps of the process 
have also affected extensive areas of igneous rocks. Examples 
of the latter are the pre-Cambrian granitoid gneiss of the Green 
Mountains, and, more extensive than this, the great areas of 
ancient plutonic and volcanic rocks of the Blue Ridge and Pied- 
mont Plateau. 

It is believed that the regularly banded and laminated Lau- 
rentian gneisses which have an isoclinal dip over great areas in 
Canada, along the Madison Canyon in southwestern Montana, 
and in other regions, may be explained by the same processes 
completely carried out as applicable to the Appalachians. It is 
not asserted whether the original rocks in these regions were 
igneous or aqueous. The general drift of opinion in recent years 
is in favor of the former origin. 

RELATIONS OF CLEAVAGE AND FISSILITY TO STRATIGRAPHY. 

Cleavage or fissility may be developed in one set of beds, 
and not in another set of beds in the same set of formations. 
Secondary structures develop readily in a shale, less readily 
in a fine grit, still less readily in a limestone, and perhaps 
with the least readiness in a quartzose sandstone, quartzite, 
or conglomerate. As a consequence of this, a shale between 
beds of limestone may take on a thoroughly cleaved or fissile 
character, the cleavage stopping abruptly at the beds of the 
limestone. The same is true of layers of shale between beds of 
grit, or sandstone, or quartzite, or conglomerate. In general, in 
the strata constituting a formation, cleavage may develop in the less 
rigid beds and be absent or imperfect in the more rigid beds. In 
such cases it has sometimes been assumed that the lower bed was 
deposited and cleavage or fissility developed in it before the 
superior bed was formed, the fact that a secondary structure 
was not so ready to develop in the upper bed being ignored. 



DEFORM A TION OF ROCKS 609 

In determining unconformities the use of cleavage and fissil- 
ity follows the same principles as given on pages 351-353 in 
reference to folding. To safely infer that there is a structural 
break between series it is necessary to show that cleavage and 
fissility would be as likely to develop in the superior formations 
as in the inferior ones. It is further necessary that the two 
series be in actual superposition, not in adjacent lateral posi- 
tions. 

It has been seen that in proportion as cleavage and fissility 
develop, the original structures of the rock are obliterated. 
Where there is apparently complete destruction of the bedding 
for parts of folds, at areas of less movement, as, for instance, 
the crests of anticlines and the troughs of synclines, the beds 
may still be recognized, and thus the relations between the 
primary and secondary structures be determined. 

JOINTS. 

ORIGIN OF JOINTS. 

Various causes have been assigned for joints, of which the 
more important are tension, torsion, earthquake shocks, and 
shearing. It is believed that joints may be classified into ten- 
sion joints and compression joints. The first are ordinarily in the 
normal planes, the second are in the shearing planes. 

Tension joints, — Tension is often due to the contraction caused 
by cooling or by desiccation. It is well known that the peculiar 
columnar jointing of igneous rocks is due to the contraction and 
consequent tension caused by cooling, and the mud cracks of 
sedimentary rocks are due to the contraction and consequent 
tension caused by desiccation. However, it is probable that 
neither cooling nor desiccation is important in the production 
of systematic sets of joints in the sedimentary rocks. 

It has already been seen that when rocks are simply folded 
and not too deeply buried the convex halves of the anticlines 
and synclines are subjected to simple tension (pp. 205-208, Fig. 
i). If the tension goes beyond the limit of elasticity, radial 
cracks will be formed which strike parallel with the rocks. 



6 10 STUDIES FOR STUDENTS 

Joints of this class are at right angles to the tensile force. 
This class of joints is beautifully illustrated in the sharp folds 
of the graywackes of the Hiwassee river, in the Ocoee series. 
If the folded rock has planes of weakness of any kind, due 
either to a primary or secondary structure, the fracture due to 
the tensile stress may be controlled by these, and thus deviate 
from the normal planes.^ 

Joints produced by tensile stress may have smooth or rough 
surfaces, depending upon the character and strength of the rock. 
If it is a weakly cemented sandstone, the fracture, as pointed 
out by Becker, is around the grains. If, however, it is a strong, 
tolerably homogeneous graywacke, quartzite, or limestone, or 
similar rock, the fractures may be clear-cut and sharp. After 
joints due to tensile stress have formed, subsequent movements 
may press the surfaces together, or may fault the strata in a 
minor or major way, and thus produce slickensided surfaces. 

It has been seen in the discussion of folds that, instead of 
being simple, and, therefore, in a horizontal attitude, they usually 
have a pitch ; or, in other words, the rocks are folded in a com- 
plex manner. In such regions there may be tensile stresses in 
two directions at right angles to each other, thus producing two 
intersecting sets of joints. One of these sets, that roughly 
parallel to the more conspicuous folds, would be called strike 
joints, while the other set of joints, parallel to the transverse 
folding, would be called dip joints. Both sets would intersect 
the bedding nearly at right angles. The fact that two sets of 
joints in these positions so frequently accord in direction with 
the strike and dip is strong evidence that many joints are pro- 
duced by the tensile stress of folding on the stretched half of 

^ Becker states that " Tension will not produce joints or cleavages. The theory of 
the distribution of tension cracks is the explanation of columnar structure " (this Jour- 
nal, Vol. IV, footnote, p. 444). Where equal contraction occurs in all directions as 
the result of cooling or desiccation, this is well known to produce columnar forms, 
but where there is tension in only a single direction, this may produce one set of approxi- 
mately parallel joints. Tension at a later time in a direction at a large angle to this 
may produce another set of joints. Or, finally, unequal tension at the same time in 
two directions nearly at right angles to each other may produce two regular sets of 
intersecting joints. 



DEFORM A TION OF ROCKS 6 1 1 

the mass folded. If the folds are nearly horizontal — that is, 
if the force was mainly in a single direction — the strike joints 
may be strongly developed and few dip joints produced. If, on 
the other hand, the folds are important in both directions, the 
strike and dip joints will both be important. 




Fig. 13. — Radial cracks due to tension in sharply flexed stratum. 

Daubree has shown that if a brittle plate fractures when it is 
subjected to torsion beyond the limit of elasticity, a double set 
of parallel fractures nearly at right angles to each other are pro- 
duced.^ The forces which produce complex folding deform the 
strata, where not too deeply buried, in two rectangular directions 
by tensile stress ; or, in other words, they are subject to torsion. 
It therefore appears that Daubree's explanation of joints by tor- 
sion is but another statement of the production of joints by 
complex folding normal to the two principal directions of ten- 
sile stress. 

A question for investigation is the extent of the area over 
which joints and faults run in rectangular directions. In the 
case of a large and strongly pitching fold, the force of torsion 
may produce rupture in different directions upon different parts 
of the folds. Upon the flank of the pitching fold, at a proper 
point, strike joints and dip joints may be formed, striking half- 
way between the ordinary strike joints and faults, and dip joints 
and faults at the crests and trouo;hs, and between the two there 
would be all gradations. 

^Geologic experimentale, par A. Daubree, pp. 306-314, Paris, 1879. 



6l2 STUDIES FOR STUDENTS 

Crosby has recently suggested' that when torsion has nearly, 
but not quite, reached the. limit of elasticity in rocks, an earth- 
quake shock may act as the trigger which sets the process in 
motion, and he thus combines this cause with torsion in explain- 
ing joints. Crosby also emphasized the fact that a plate when 
subjected to torsion does not crack along a single plane of frac- 
ture, the weakest plane, but that many parallel fractures are pro- 
duced. He applies this fact to rock beds, and suggests that the 
fracture must first occur at some one plane, that of the greatest 
stress or least strength in the distorted belt. The shock of the 
fracture, added to the force of torsion, goes beyond the ultimate 
strength of the beds at the next weakest plane, thus fracturing 
them and producing another joint, and so on, until the compli- 
cated fracturing actually obtained in the glass plate is paralleled 
by the rocks. He thus makes the rupture of the first joint itself 
serve the purpose of a secondary shock, and this rupture a third 
shock, and so on, producing a set of joints in rapid succession 
over an extended area. Finally a place is reached where the 
rock has not been sufficiently distorted for the shock of the last 
fracture to carry the stress beyond the breaking strength. This 
theory is as applicable to simple tension as to torsion. In the 
above it appears that Crosby has overlooked the fact that he has 
not explained the first earthquake shock. The statement might 
be amended as follows : The first fracture occurs because the 
steadily acting orogenic forces finally go beyond the ultimate 
strength of the deformed beds. When rupture takes place, this 
gives the first shock. This initial shock carries the stress beyond 
the ultimate strength of the next weakest planes, and so on. 

Compression joi?its. — Daubree^ and Becker 3 show that joints 
may be produced by compression. In this case there will be 
jomting in two planes when the rocks are simply folded, and, 

^ The origin of parallel and intersecting joints, by W. O. Crosby, Am. Geol., 
Vol. XII, 1893, pp. 368-375. 

^Geologie experimentale, par A. Daubree, pp. 315-322, Paris, 1879. 

3 George F. Becker : Finite homogeneous strain, flow, and rupture of rocks. 
Bull. Geol. Soc. Am., Vol. IV, pp. 41-75, 1893. The torsional theory of joints. 
Trans. Am. Inst. Min. Engineers, ol. XXIV, pp. 130-138, 1894. 



DEFORM A TION OF ROCKS 6 1 3 

according to Becker, there may be jointing in three or four 
planes when they are complexly folded, one of these being 
normal to tensile stress and the others in shearing planes. How- 
ever, where there are more than two sets of joints at right angles 
to each other, it is probable that in many cases these have been 
caused by successive orogenic movements, the second being in 
a different direction from the first. Becker has explained that 
minor faulting is a common phenomenon of compression joints. 

When the folding is simple, both sets of joints developing in 
the shearing planes, although dipping in different directions, 
would accord in their outcrop with strike, and might therefore 
be regarded as strike joints. When the folding is complex it 
may be that different sets of shearing planes would correspond 
to strike joints and dip joints, but upon this point further obser- 
vation is needed. 

In the Knox dolomite of east Tennessee the formation of 
joints along both tensile and shearing planes is beautifully illus- 
trated at numerous localities. Commonly the joints produced 
by tensile forces are nearly perpendicular to the bedding. Two 
sets of joints, equally inclined to the bedding and making obtuse 
angles with each other, are clearly in the shearing planes. 

The attitudes of joints produced by shearing and their rela- 
tions to bedding would be identical with fissility, as described on 
pages 593-597. Whether the structure be called fissility or 
joints would depend upon their number. If numerous and close 
together the structure would be called fissility ; if fewer and 
farther apart, jointing. The same compression might produce 
fissility along one set of shearing planes, and joints along another. 
If the above be true, it is clear that there are all gradations 
between joints and fissility. It has been suggested that the term 
''fissility" might perhaps be wisely restricted to the cases where 
the structure is secondary to a previous one, such as cleavage or 
bedding, and that the term ''joint" should be used to cover 
fractures along independent shearing planes. 

In thus explaining many joints as the result of the same 
forces which produce folds, it is not meant to imply that there 



6l4 STUDIES FOR STUDENTS 

are not joints of other origins, but merely that the master joints, 
which run in different directions approximately parallel to the 
strike and dip, may be thus explained, and these are the joints 
which are the most useful in determining the structural relations 
of different series. 

ZONE AFFECTED BY JOINTS. 

Joints, implying as they do openings in the rocks, are neces- 
sarily confined to the outer zone of fracture and the middle zone 
of fracture and flowage. In the first zone they are of more 
importance and probably more regular than in the second. In 
the deeper zone of rock-welding no joints can develop. In 
rocks once buried to this depth, which subsequently reached the 
surface by erosion, joints may be formed ; for in approaching 
the surface they passed from the zone of flowage, through the 
zone of fracture and flowage, into the zone of fracture. 

RELATIONS OF JOINTS TO STRATIGRAPHY. 

If there is a greater number of sets of joints in an inferior 
formation or formations than in a superior formation or forma- 
tions, the two divisions being of such lithological character as to 
be equally likely to take on jointing, this argues that there may 
be discordance between the two sets, for it is probable that the 
lower set of formations, which has the more complicated joint- 
ing, was subjected to orogenic movements which produced a part 
of these fractures, before the upper series was deposited. Com- 
stock has applied this as the determining criterion in separating 
three supposed series of rocks in the Llano district of Texas. 
The lower series of formations is said to have three sets of pro- 
nounced joints running in definite directions, while the middle 
series has only two sets of joints running in definite directions, 
these two being common with two of the three in the lower series, 
and the upper series has a single set of joints running in a defi- 
nite direction, this system being common to both the inferior 
series. Comstock's inference is that the system of joints in the 
lowest series not found in the upper two series was produced 



DEFORM A riON OF ROCKS 6 1 5 

before the upper two series were deposited, and that the two sets 
of joints found in the lower series, one of them also affecting the 
middle series, and both absent in the upper series, were present 
before the latter series was deposited. That is, the lower series 
was subjected to three orogenic movements, the middle series to 
two, and the upper series to one. Considering that two or more 
sets of joints may be developed by a single orogenic movement, 
it would seem that such a conclusion should be supported by 
other criteria. 

FAULTS. 

ORIGIN OF FAULTS. 

Faults differ from other rock fractures, in that there is import- 
tant dislocation along the fractures and often also they are far 
more extensive. Like joints, faults may be classified into tension 
faults and compression faults ^ the first forming in the normal planes 
and the second in the shearing planes. Faults are, however, usu- 
ally defined as normal and reverse. A normal fault is one in 
which the overhanging side descends in reference to the other, 
while in the reverse fault the overhanging side ascends in refer- 
ence to the other. Another term applied to reverse faults is 
thrust faults^ implying that tangential thurst is the controlling 
factor. As equivalent to normal fault may be placed the gravity 
fault, implying that gravity is the predominant force. 

In the case of the normal fault the overhanging side has a 
smaller base than the other. Consequently by force of gravity it 
descends, as compared with the other side. In all cases both of 
normal and reverse faults, gravity is a never-ceasing force. At 
first explained by Le Conte,^ the principle of the inclined plane 
thus applies to these two forces, the hade of the fault giving the 
inclination of the plane. Where the hade is greater than 45° 
if the forces of gravity and tangential thrust are equal the fault 
is normal, because gravity controls the movement (Fig. 14). If, 
on the other hand, the hade is less than 45°, tangential thrust is 

^ On the origin of normal faults, and of the structure of the Basin region. Joseph 
Le Conte. Am. Jour. Sci. (3), Vol. XXXVIII, 1889, pp. 257-263. 



6l6 STUDIES FOR STUDENTS 

the predominant force, and the fault is a reverse one (Fig. 15). 
As the hade becomes steep, gravity has greater and greater rela- 
tive power, and if the hade is very steep, gravity may be able to 
overcome the tangential thrust, even if the latter is several times 
as great as the former. So, also, if the hade is flat, tangential 
thrust even much weaker than gravity may overcome it and pro- 




FiG. 14. — Normal or gravity fault. 

duce a reverse fault. This is one reason why, as a rule, normal 
or gravity faults have steep hades, while reverse or tangential 
faults have flat hades. 

There is, however, another reason. Since tension faults form 
in the normal planes, they are usually steeply inclined or nearly 
vertical. But the very idea of tension faults implies that there 
is no thrust. Hence, gravity has its full effect, and the overhang- 
ing side goes down with reference to the other side. Tt does not 
follow, therefore, that all gravity faults are tension faults, 
although this may be the case. Compression faults form in the 
shearing planes, and they are therefore likely to be much inclined 
to the vertical. In order that rupture shall occur the thrust must 
be great, and hence compression faults are usually, and perhaps 
always, thrust faults. 

Perhaps it would be well to classify faults as gravity faults 
and thrust faults rather than normal and reverse faults, and thus 
give them names which refer them to the predominant causes. 
For the present this classification is preferable to the classifica- 



DEFORM A TION OF ROCKS 



617 



tion into tension faults and compression faults ; for it is possible^ 
though hardly probable, that an inclined fracture may result from 
compression, and after a time thrust lessen in amount, so that 
gravity controls the final differential movement. 




Fig. 15. — Reverse or thrust fault. 
RELATIONS OF FAULTS TO EXPANSION AND CONTRACTION. 

Gravity faults result in the dilation of the part of the crust 
affected by them (Fig. 14). Thrust faults result in the contraction 
of the part of the crust affected by them (Fig. 15). In a region 
in which many parallel faults occur, all of the same character^ 
the dilation, or contraction may be a considerable percentage of 
the breadth of the area disturbed. Since the amount of dilation 
or contraction with a given vertical movement increases as the 
hades become flat, and since thrust faults have flatter hades on 
the average than gravity faults, the shortening of the crust in a 
region of thrust faults is usually greater than the elongation in 
another region in which gravity faults are about equally abun- 
dant and in which the vertical displacements are the same. 



RELATIONS OF FAULTS TO STRIKE AND DIP. 

While there is great variability in the direction of faults, due 
to exceptional causes, faults are more parallel to the strikes and 
to the dips, other things being equal, than in other directions, so 
that faults are sometimes spoken of as strike faults and dip faults. 



6l8 STUDIES FOR STUDENTS 

Since a fault may be no more than a displaced joint, this relation 
is easily explained in the same manner as in the case of joints. 
(See pp. 610-612). 

RELATIONS OF FOLDS TO THRUST FAULTS. 

It has been long recognized that thrust faults are often related 
to overfolds. If the strata are in the zone of combined fracture 
and flowage, the overfolds may be broken along the reversed 
limbs and the arch limbs be thrust over the trough limbs. In a 
region of overfolds and thrust faults, if it could be determined 
whether the differential movements are such as to carry the 
material moved toward the surface or away from the surface, it 
could be decided whether such folds and faults should be called 
overthrusts or underthrusts.' But the differential movements, 
the forms of inclined and overturned folds, and the character of 
the thrusts are identical, whether a given bed above be consid- 
ered as moving forward and upward as compared with the layer 
below, or be considered as moving forward and downward as 
compared with the layer above. In Fig. 16, if the force be con- 
sidered as applied at A, it would be called an overthrust fault ; 
if the force be considered as applied at B, it would be called an 
underthrust fault ; and yet the phenomena are identical. The 
movements must be such that the material goes in the direction 
of relief, and it is probable that this is more often toward the 
surface of the earth (see pp. 338, 339) rather than deeper within 
the earth. It is probable that in certain cases thrust has been 
transmitted by a strong formation or series and pushed under 
other strata. This is particularly likely to occur where the lower 
strata are weaker or where the material in advance of the active 
strata transmitting the force has been already raised into folds, 
and thus partly escapes the pressure. (See pp. 316-319, and 
Fig. 6.) 

As explained by Willis, in regions which are but lightly 
loaded the forces producing thrust faults may result in clean-cut 

^The term underthrust is taken from Professor E. A. Smith. Am. Jour. Sci., 
3d series, Vol. XLV, 1893, pp. 305, 306 ; see also this Journal, Vol. IV, p. 339. 



DEFORMATION OF ROCKS 



619 



fractures, with scarcely any bowing of the layers of the rocks 
along the shear planes (see Figs. 4 and 6 on p. 468, and pp. 596- 
598). In passing to the greater depths the load is greater, and 
the layers, instead of all having the full movements of the clean- 
cut thrust faults, adjacent to the fault planes may be found to 
be in sharp overfolds in opposite directions upon opposite sides 




Fig. 16. — Fold passing into fault. 

of the faults (Fig. 16). Where the load is still greater these 
folds are of increased importance. Under still greater load the 
rocks may be first bent into an overfold, with little faulting, and 
finally at a greater depth the deformation may occur altogether 
by overfolding. It is therefore clear that in the same mountain 
mass there may be all gradations between clean-cut thrust 
faults and overfolds without faults. The transition may be 
longitudinal, as in the case of the Appalachians, where thrust 
faults which occur in the extreme southeast are gradually 
replaced by overfolds to the northwest. Also the transition 
may be transverse. In the latter case, if erosion cuts the strata 
to different depths after such a region was deformed, the over- 
folds may be found in the central parts of the mountain mass, 
the transition phases upon the immediate areas, and the 
thrust faults without overfolds upon the outer flanks of the 
mountains. 

Rocks at a certain depth, and therefore under a definite load. 



620 STUDIES FOR STUDENTS 

may be deformed first by folding and afterwards by faulting. 
Suppose a rock is deeply buried, but not so deeply as to cause 
the superincumbent load to equal its ultimate strength. Sup- 
pose the differential stress for a given stratum under these con- 
ditions to surpass the elastic limit, but not to reach the ulti- 
mate strength of the bed. It will then be deformed by folding, 
but during the process shearing occurs on the limbs, and as a 
result the bed is thinned, and finally the stress may surpass the 
ultimate strength of the rock, which will then be fractured 
and perhaps faulted. The same result may be reached if 
before a stratum or formation is thinned the differential stress 
increases so as to go beyond the ultimate strength of the rock. 
It follows from this that deformation by folding followed by 
faulting is normal for a considerable zone, for when the moun- 
tain-making forces for a given region first begin their work it is 
to be supposed that the differential stress is moderate. As the 
stress increases in amount so as to exceed the elastic limit the 
layers would begin to be folded, and fracture would occur as 
soon as the differential stress reached the ultimate strength 
of a given layer, provided the rock was not in the deep-seated 
zone of flowage. 

It has been seen (p. 597) that accompanying thrust-faulting 
fissility may develop parallel to the faults, and accompanying 
overfolding cleavage may develop which dips in the same direc- 
tion as the axial planes of the folds. In an area intermediate 
between the zone of fracture and the zone of flowage, this being 
at successive times under the conditions of flowage and of frac- 
ture, there may be overfolding and cleavage combined with 
thrust faults and fissility. In passing from a faulted to a folded 
area, as has been noted, first there may be fissility along the 
thrust faults, then the strata may be slightly overfolded and 
tucked under along the faults, this undertucking becoming 
more and more prominent and fissility at the same time being 
replaced by cleavage, and finally we may have overfolds with 
cleavage, with or without faulting. Each of the different phases 
of the steps of change may occur on a large or small scale. In 



DEFORM A TION OF RO CKS 6 2 1 

some places in an intermediate area a dozen little overfolds 
with fault slips may be seen upon a single hand specimen. 
Hence I conclude that the average deformation of a region may 
be the same whether it be by a few great faults with little or no 
jissility, by more freque?it lesser faults with or without fissility, 
by faults and overfolds with or without both cleavage ajid fissility, 
or by folding with or without faults and cleavage ; also that there 
is every gradation betwee?i faulting and fissility, and probably 
every gradation between faulting a?id cleavage. 

ZONE AFFECTED BY FAULTS. 

A fault may vary in magnitude from a fraction of an inch to 
many thousands of feet. A fault, like a joint, is limited in hori- 
zontal as well as vertical extent. It cannot be assumed to extend 
very far beyond where observed. A fault of an inch may die 
out within a few inches, both laterally and vertically ; a fault of 
a hundred feet within a few hundred feet ; a fault of five thousand 
feet within a few miles. In following a fault longitudinally the 
throw may be found to become less and less until it is zero, 
just as a bunch of paper may be torn for a part of its length 
and the different parts of the torn ends be differently displaced. 
But while faults may thus die out within short distances, they 
may have remarkable persistence, both in direction and in 
length. This does not necessarily imply that they have great 
persistence in depth, for just as a fold has a tendency to die 
out, as explained (pp. 210, 211), a fault may also die out below, 
and sometimes also above. In the latter case the fault usually 
occurs in a stratum or a formation which is brittle as compared 
with the overlying rocks. Because of the more plastic charac- 
ter of the higher strata the deformation there occurs by folding. 
Probably most faults at sufficient depth pass into flexures, and 
deeper dow?i these flexures may die out. As already explained, 
when a bed is deformed under little weight the strain neces- 
sarily causes fracture, and the readjustment is largely by fault- 
ing along the fractures. When all the conditions are the same, 
except that there is such load that as a whole the rocks are in 



622 STUDIES FOR STUDENTS 

the zone of flowage, the necessary deformation is accomplished 
by folding. In regions of close folds it is probable that 
before the superincumbent beds were removed by erosion many 
of the latter were faulted instead of folded, for they were in the 
zone of fracture and flowage, and in the zone of fracture 
rather than the zone of flowage. 

Possibly the depth at which important faults disappear is 
in many cases not more than 5000 meters, although the dis- 
cussion of the depth of earthquake shocks due to faults leads 
to the conclusion that some faults extend to the depth of a 
number of miles. 

If there are inclined planes of weakness in the deep-seated 
zone of flowage, the deformation may largely occur by faulting 
along these planes.^ Such inclined planes of weakness may be 
in sedimentary rocks or in igneous rocks. Since masses of 
intrusive rocks, either in the form of dikes or of bosses, usually 
have vertical or steeply inclined exteriors, faulting is particu- 
larly likely to occur at the contacts of igneous rocks with one 
another, or at the contacts of igneous with sedimentary rocks.. 
It has already been explained that such deep-seated faults 
would differ in no way from the differential movements result- 
ing in cleavage or fissility, except that the movements are 
mainly confined to narrow zones. This results in great dis- 
placement at the fault zones and little displacement in the areas 
between the faults. In such supposed deep-seated faulting it 
is to be remembered that the displacement takes place without 
crevice or joint. At any given movement the rock is to be 
regarded as welded together. The different parts simply shear 
over one another along the plane of greatest weakness. 

RELATIONS OF FAULTS TO STRATIGRAPHY. 

Faults may be used to discriminate between series in pre- 
cisely the same way as joints, and the criterion has far 
greater weight. If an inferior set of formations has a more 

^ The Mechanics of Appalachian Structure, by Bailey Willis. Thirteenth Ann 
Kept., U. S. Geol. Surv., pp. 217-274, 1893. Especially Pis. XCV and XCVL 



DEFORM A TION OF ROCKS 623 

complicated faulting than an upper series which lithologically 
is equally likely to be faulted, this is strong evidence that 
the lower set of formations was faulted before the upper set of 
formations was deposited. Faults are frequently not easy to 
demonstrate or to trace out. Hence this criterion for discrim- 
inating between series is not so valuable upon the whole as are 
the more conspicuous and readily discovered phenomena of 
folds, cleavage, fissility, and joints, but if the conditions are 
favorable for tracing out the faults of a region, the informa- 
tion thus furnished may give positive evidence as to structural 
breaks between series. 

GENERAL. 

In the foregoing pages folds, cleavage, fissility, joints, and 
faults are regarded as the conjoint products of thrust and 
gravity. Similar forces acting upon heterogeneous rocks under 
various conditions produce diverse phenom.ena. Thus several 
classes of phenomena which are often treated as independent 
and unconnected are genetically connected. A fault may 
accord in inclination with any of these structures. Between 
faults and joints, fissility, or cleavage there are all gradations. 
When there is a marked displacement along a break it is called 
a fault. Whether a given minor displacement is thus named 
often depends upon the point of view. Wherever there is fissil- 
ity there is slipping or faulting, using this term in its exact 
sense. Usually minor displacements are not called faults unless 
they occur across the beds or other structures. A displace- 
ment across a prior structure of such magnitude as to be called 
a fault might be ignored if it occurred along the structure. 
Folding and cleavage belong normally to the zone of flow- 
ing ; fissility, joints, and faults belong normally in the zone of 
fracture. In the zone of combined flowage and fracture all 
the structures occur together in a complex manner, the par- 
ticular combination of phenomena depending upon the relative 
thickness, strength, and brittleness of the rock beds con- 
cerned. 



624 STUDIES FOR STUDENTS 

AUTOCLASTIC ROCKS. 
ORIGIN OF AUTOCLASTIC ROCKS. 

When rocks are folded by strong erogenic forces, and they 
are not so heavily loaded as to render them plastic, they are 
frequently broken into fragments, and '' auto clastic'''' rocks are 
produced. The autoclastic rocks which readily show their 
origin may be called dynamic breccias, and those which resemble 
ordinary conglomerates may be called pseudo-conglomerates. 
Brittle rocks are the most likely to become autoclastic ; hence 
it is that cherts, quartzites, cherty limestones, graywackes, and 
rather siliceous slates are some of the kinds which most fre- 
quently present the phenomena described. The movements of 
the broken fragments over one another in many cases so thor- 
oughly round them that they have the appearance of being 
waterworn, and the matrix between the larger fragments may 
consist almost wholly of well-rounded fragments of a similar 
character. For instance, in a semi-indurated quartzite the larger 
complex fragments may be well rounded by their mutual fric- 
tion while the matrix may consist of the simple original water- 
worn grains which are rent apart. In another case the original 
rock may have consisted of beds of mud interlaminated with 
thin beds of grit. By consolidation and cementation these beds 
may have been transformed to alternating shale and graywacke. 
The shale is plastic under slight load ; under the same load the 
graywacke is brittle. When such a set of beds is deformed 
the shale yields largely by flow and the graywacke by fracture. 
The beds of graywacke are broken into fragments of varying 
sizes, which are ground over one another, and thus are rounded. 
At the same time the shale flows and fills the spaces between 
the fragments. Also slaty cleavage may be developed. As a 
result, a pseudo-slate-conglomerate is produced, having a slate 
matrix and pebbles of graywacke, which, so far as its own char- 
acters are concerned, could not be discriminated by anyone 

^Structural geology of Steep Rock Lake, Ontario, H. L. Smyth, Am. Jour. Sci., 
3d ser., VoL XLII, p. 331. 



DEFORM A TION OF ROCKS 625 

from a true conglomerate. Fortunately, in most cases it is 
possible to find transition phases between such a rock and one 
in which the process has not gone so far, and thus one is 
enabled to determine that the rock is autoclastic. 

ZONE OF AUTOCLASTIC ROCKS. 

The zone in which autoclastic rocks may be produced is con- 
fined to the outer 10,000 meters of the earth's crust, and the 
formation of widespread autoclastic rock is probably limited to 
the outer 5000 meters. At a depth greater than the larger 
number the pressure in all directions exceeds the crushing 
strength of any rock, and therefore if it were possible for 
crevices to form such as are necessary to produce brecciation 
they would be almost immediately closed by flowage. Conse- 
quently, at great depths it is to be supposed that no crevices 
form in the rocks as the result of dynamic movements, and 
therefore that no breccias are produced. 

From the foregoing it follows that autoclastic rocks may 
develop whether the formations concerned are homogeneous 
or heterogeneous. Also that they may develop whether the 
beds are all within the zone of fracture for them or whether 
they are in the zone of fracture for a part of them, and in the 
zone of flowage for the other part. In the first case dynamic 
breccias are likely to form. In the second case only the stronger 
rocks are broken, the fragments being buried in the members 
which flowed, and pseudo-conglomerates are frequently formed. 

RELATIONS OF AUTOCLASTIC ROCKS TO BASAL CONGLOMERATES. 

Since it is possible that pseudo-conglomerates may be mis- 
taken for true basal conglomerates, the criteria which discrim- 
inate the two are of great importance. 

(i) An autoclastic rock must derive its material mainly 
from the adjacent formations. If, for instance, it is produced 
from interstratified layers of limestone and quartzite, it will con- 
tain only limestone and quartzite detritus, and the fragments 
will be mainly from the more brittle formation. Further, an 



626 STUDIES FOR STUDENTS 

autoclastic rock may have a part of its material from the superior 
formation as well as from the inferior. However, in some cases 
the brecciated layer may itself have been conglomeratic^ 
although not a basal conglomerate, and thus some material from 
extraneous sources will be found. But in most instances the 
material is of local origin. In true basal conglomerates, on the 
other hand, while the material is very frequently derived in 
large measure from the immediately subjacent formations, they 
also usually contain a small proportion of material from various 
foreign sources, and do not contain any material from the over- 
lying formations, as may the autoclastic rocks. 

(2) In an autoclastic rock, if the pebbles are closely 
examined they will in many cases be found to be less rounded 
than in a true basal conglomerate. If the belts of brecciation 
be followed for some distance a considerable variation will fre- 
quently be found in this respect, fragments being here well 
rounded and there very imperfectly rounded. The well-rounded 
fragments are concentrated, as are also the angular fragments. 
A basal conglomerate, on the other hand, has a considerable 
uniformity in the degree of the rounding of its pebbles in pass- 
ing along the same horizon, but at the same place the large frag- 
ments may be angular and the small ones well rounded. In a 
basal conglomerate very near to the underlying formation many 
of the contained fragments may be angular, but in an extreme 
case the fragments of a basal conglomerate are upon the aver- 
age usually not so angular as those of an autoclastic rock. 

(3) In many cases the interstices of an autoclastic rock are 
filled with material of a vein-like character, whereas in a basal 
conglomerate the filling material is largely finer detritus. But 
sometimes, as in the case mentioned of a semi-indurated quart- 
zite, the filling material of an autoclastic rock may be water- 
worn grains of sand, which have been separated by dynamic 
action, and are therefore indistinguishable from the ordinary 
matrix of a true conglomerate. 

(4) In most instances a bed of autoclastic rock, if followed,, 
may be traced into an ordinary brecciated or partly brecciated 



DEFORM A TION OF ROCKS 



627 



form. A basal conglomerate, on the other hand, if followed 
along the strike and dip, may change its character, but it will 
be a gradual change into the ordinary mechanical sediments, 
whereas an autoclastic rock is likely to have very sudden varia- 
tions in character. 




Fig. 17,- 
of limestone. 



- Chert breccia, an autoclastic rock resting upon truncated minor folds 



Using all of the above criteria, it is difficult in some cases to 
discriminate between an autoclastic rock and a true conglomer- 
ate. Usually, however, if an area be studied sufficiently long, 
and if the relations be examined closely a true judgment may 
be reached. 

Autoclastic rocks are not likely to develop from shales and 
limestones, but if near enough to the surface even these rocks 
may become brecciated. As a consequence of orogenic move- 
ments, in the zone of combined fracture and flowage, where the 
alternate layers are thick, the shales and limestones may flow 
and the cherts and quartzites become brecciated. The brecciated 
and nonbrecciated layers, under these circumstances, may not 
become mingled to any considerable degree. Thus we may 
have a set of autoclastic rocks interstratified with layers which 
show no sign of brecciation. It may be that the plastic layers, 
as a result of the stress, may be minutely corrugated. The 
movement of the broken particles in the rigid layers against the 
crests and troughs of the folds may have truncated them. We 
might then have pseudo-conglomerates resting upon folded 
truncated layers (Fig. 17), and it might be concluded that there 



628 STUDIES FOR STUDENTS 

is a structural break between the two, the inference being that 
one formation was folded and truncated before the overlying 
clastic formation was deposited upon it. This occurs in the 
Marquette district of Michigan. 

Another case is as follows : A lower shale or grit may be 
overlain conformably by a sandstone. By cementation these 
formations may become indurated ; the grit into graywacke, and 
the sandstone into quartzite. After this an orogenic movement 
may develop cross cleavage or cross fissility in the softer, lower 
formation, the secondary structure abutting against and sharply 
terminating at the overlying quartzite. The same movement 
may develop a pseudo-conglomerate in the overlying formation. 
In the later stages of the process the differential movement may 
tear off fragments of the lower slate or schist and include them 
with the broken, harder formation. Such a pseudo-conglomerate 
simulates to a remarkable degree a basal conglomerate resting 
unconformably upon an earlier series, in which it might be sup- 
posed that the secondary structure was produced before the 
overlying formation was deposited. Exactly these relations 
obtain within the Ajibik quartzite formation of the Lower Mar- 
quette series, northeast of Palmer. At first it was supposed that 
the pseudo-conglomerate was basal and marked an unconformity, 
and it was only after the locality was repeatedly visited and 
studied in the utmost detail that the true relations between the 
two formations were discovered. 

Similar phenomena may occur between formations different 
from those above described, in which the inferior formation is 
weaker than the superior formation. 

As another illustration of the great difficulty in sometimes 
distinguishing between the two, a case in the Adirondacks may 
be cited in which a thick formation of gneiss is overlain by a bed 
of crystalline limestone containing interlaminated smaller beds 
of gneiss. The whole series has been closely folded. The 
gneiss, as a result of the folding, is closely corrugated, and to 
a certain extent its upper folds are truncated by the shearing 
action. The limestone has acted like a fluidal substance, accom- 



DEFORM A TION OF ROCKS 629 

modating itself easily to its new position, and by recrystallization 
has taken on a massive character. The thin belts of gneiss 
within the limestone have been broken to fragments. The frag- 
ments in the limestone matrix have ground against one another 
until they became well rounded. They are disseminated through 
the limestone. As the layers of gneiss are thicker and more 
numerous near the base of the limestone, this part of the forma- 
tion appears as a limestone containing numerous bowlders and 
smaller fragments of gneiss resting upon a gneiss formation. 
Thus an unconformable contact was inferred when the area was 
first examined, but an extended and close examination of the 
region showed all stages of transition, from the phase of the 
rock which appeared to be a true conglomerate to that in which 
the thin layers of gneiss are interstratified with limestone. Sim- 
ilar phenomena have been observed in the Original Laurentian 
area and in the Marquette district of Michigan. 

C. R. Van Hise.