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Studies for Students
DEFORMATION OF ROCKS. — IV. CLEAVAGE AND
FISSILITY ( contimied), JOINTS, FAULTS,
RELATIONS OF CLEAVAGE AND FISSILITY TO OTHER
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
STUDIES FOR STUDENTS
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
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
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.
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
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
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
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-
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-
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.
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
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-
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
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
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
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
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
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.
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-
624 STUDIES FOR STUDENTS
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
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.
- 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
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.