In lithology we investigate the nature of the materials composing the
earth’s crust—the various minerals and aggregates of minerals, or rocks;
while in petrology we consider the forms and modes of arrangement of the
rock-masses,—in other words, the architecture of the earth.
Petrology is the complement of lithology, and in many respects it is the
most fascinating division of geology, since in no other direction in
this science are we brought constantly into such intimate relations with
the beautiful and sublime in nature. The structures of rocks are the
basis of nearly all natural scenery; for what we call scenery is usually
merely the external expression, as developed by the powerful but
delicate sculpture of the agents of erosion—rain and frost, rivers and
glaciers, etc.—of the geological structure of the country. And to the
practised eye of the geologist, a fine landscape is not simply a
pleasantly or grandly diversified _surface_, but it has _depth_; for he
reads in the superficial lineaments the structure of the rocks out of
which they are carved.
But, while the magnitude of the phenomena adds greatly to the charm of
the study, it also increases the difficulties and taxes the ingenuity of
the teacher whose work must be done indoors. According to our ideal
method, natural science ought to be taught with natural specimens; and
yet here our main reliance must be upon pictures and diagrams.
Nature, however, has not been wholly unmindful of our needs; for she has
worked often upon a very small as well as a very large scale; many of
the grandest phenomena being repeated in miniature. Thus we observe
rock-folds or arches miles in breadth and forming mountain masses, and
of all sizes from that down to the minutest wrinkle. So with veins,
faults, etc. And the wonderful thing is that these small examples, which
may be brought into the class-room, are usually, except in size, exactly
like the large. Now the aim of every teacher in this department should
be to secure a collection of these natural models. It is not an easy
thing to do, except one has plenty of time; for they can rarely be
purchased of dealers, but must usually come as the choicest fruit of
repeated excursions to the natural ledges and quarries, the seashore and
the mountains. But for the difficulty of getting the specimens there is
some compensation, since it may be truly said that for the _collector_
specimens obtained in this way have an interest, a value, and a power of
instruction beyond what they would otherwise possess.
_Classification of Structures._
The structures of rock divide, at the outset, into two classes:—(1) the
_original structures_, or those produced at the same time and by the
same forces as the rocks themselves, and which are, therefore, peculiar
to the class of rocks in which they occur (_e.g._ stratification,
ripple-marks, fossils, etc.); and (2) the _subsequent structures_, or
those developed in rocks subsequently to their formation, and by forces
that act more or less uniformly upon all classes of rocks, and which
are, therefore, in a large degree, common to all kinds of rocks (_e.g._
folds, faults, joints, etc.).
The original structures are conveniently and naturally classified in
accordance with the three great classes of rocks: (1) stratified rocks,
(2) eruptive rocks, and (3) vein rocks; while the subsequent structures,
not being peculiar to particular classes of rocks, are properly divided
into those produced by (1) the subterranean or igneous agencies, and (2)
the superficial or aqueous agencies.
[Illustration: Fig. 1.—Section through sediment deposited by
rain in a roadside pool: _a._ surface of roadway; _b._
layer of small pebbles and coarse sand; _c._ fine sand
passing into _d_; _d._ the finest sand and mud.]
Original Structures of Stratified Rocks.
STRATIFICATION.—All rocks formed by strewing materials in water, and
their deposition in successive, parallel, horizontal layers, are
_stratified_; and this structure is their _stratification_. It is the
most important of all rock structures; and there is no kind of structure
the origin of which is more fully or certainly known. The deposition of
sediment in carefully assorted horizontal layers is readily brought
within the comprehension of children by simple experiments with sand and
clay in water; and still better by the examination of the deposits
formed in roadside pools during heavy rains (Fig. 1), and by digging
into beaches and sandbars, which every child will recognize as formed of
materials arranged by water. Great stress should be laid upon the fact
that a lake like Erie or Champlain is simply a large pool with several
more or less turbid streams flowing into it, while the single stream
flowing out is clear, the sediment having evidently been deposited in
the lake; and that every lake is, like the roadside pool, being
gradually filled up with sedimentary or stratified rocks. But the ocean
is a still larger pool, receiving mud and sand from many streams; and
since we know that nothing escapes from the ocean but invisible vapor,
it is plain that the mud and sand and all other kinds of sediment
carried into the ocean must be deposited on its floor, and chiefly, as
we have seen, on that part nearest the land. The consolidation of
beaches, bars, and mud-flats is all that is necessary to convert them
into stratified formations of conglomerate sandstone and slate.
Let us notice now, more particularly, the causes of visible
stratification. As we can easily prove by an experiment with clay in a
bottle of water, if the same kind of material is deposited continuously
there will be no visible stratification in the deposit. It will be as
truly stratified as any formation, but not visibly so; because there is
nothing in the nature of the material or the way in which it is laid
down to bring out distinct lines of stratification. Continuous and
uniform deposition obtains very frequently in nature, but rarely
continues long enough to permit the formation of thick beds or strata.
Hence, while the stratification is almost always visible on the large
surfaces of sandstone, slate, etc., exposed in quarries and railway
cuttings, and may usually be seen in the quarried blocks, it is often
not apparent in hand specimens, which may represent a single homogeneous
layer. There is one important exception, and that is where the
particles, although of the same kind, are flat or elongated. Pebbles of
these forms are common on many beaches; and since they are necessarily
arranged horizontally by the action of the water, they will, by their
parallelism, make the stratification of the pudding-stone visible. The
same result is accomplished still more distinctly by the mica scales,
etc., in sandstone and slates, the leaves and flattened stems of
vegetation in bituminous coal, and the flat shells in limestone.
In all other cases, visible stratification implies some change in the
conditions; either the deposition was interrupted, or different kinds of
material were deposited at different times. The first cause produces
planes of easy splitting, or fissility, especially in fine-grained
rocks, like shale. This shaly structure or lamination-cleavage may be
due, in some cases, to pressure, but it is commonly understood to mean
that each thin layer of clay became partially consolidated before the
next one was deposited upon it, so that the two could not perfectly
cohere. Parallel planes of easy splitting are, however, by themselves,
of little value as indications of stratification, since the
lamination-cleavage is not easily distinguished from slaty-cleavage
(roofing slate) and parallel jointing, structures developed subsequently
to the deposition of sediments and quite independent of the
stratification. The second cause, or variations in the kind of sediment,
gives alternating layers differing in color, texture, or composition, as
is seen frequently in sandstone, slate, gneiss, etc.; and of all the
indications of stratification these are the most important and reliable.
[Illustration: Fig. 2.—Section showing strata and laminæ:
_a._ conglomerate; _b._ sandstone; _c._ shale; _d._
limestone.]
A layer composed throughout of essentially the same kind of rock, as
conglomerate or sandstone, and showing no marked planes of division, is
usually regarded as one _bed_ or _stratum_, although it may vary
considerably in texture or color; while the thinner portions composing
the stratum and differing slightly in color, texture, and composition,
and the thin sheets into which shaly rocks split, are the _laminæ_ or
_leaves_. In Fig. 2 the strata are designated by letters, and the fine
lines and rows of dots show the constituent laminæ, while the whole
section may be regarded as a small part of a great geological formation.
The geological record is written chiefly in the sedimentary rocks; and
the formations, strata, and laminæ may be regarded as the volumes,
chapters, and pages in the history of the earth. Now every feature of a
rock, lithological or petrological, finds its highest interest in the
light which it throws upon the history of the rock, _i.e._, upon the
conditions of its formation. Observe what the section in Fig. 2 teaches
concerning the geological history of that locality; premising that any
chapter of geological history written in the stratified rocks should be
read from the bottom upwards, since the lowest strata must have been
formed first and the highest last. The lowest stratum exposed is
conglomerate, indicating a shingle beach swept by strong currents which
carried away the finer material. Upwards, the conglomerate becomes finer
and shades off into sandstone, and finally into shale, showing that the
water has become gradually deeper and more tranquil, the shore having,
in consequence of the subsidence, advanced toward the land. The next two
strata show that this movement is probably reversed; at any rate, the
currents become stronger again, and the shale passes gradually into
sandstone and conglomerate. The beach condition prevails now for a long
time, and thick beds of sand and gravel are formed. The sea then deepens
again, and we observe a third passage from coarse to fine sediment. This
locality is now remote from the shore, the gentle currents bringing only
the finest mud, which slowly builds up the thick bed of shale, in the
upper part of which shells are abundant, indicating that the deposition
of mechanical sediment has almost ceased, and that the shale is changing
to limestone. The purity of the limestone, and the crinoids and other
marine organisms which it contains, prove that this has now become the
deep, clear sea; and this condition is maintained for a long period, for
the limestone is very thick, and this rock is formed with extreme
slowness.
The most important point to be gained here is that every line of
stratification and every change in the character of the sediments is due
to some change of corresponding magnitude in the conditions under which
the rock was formed. The slight and local changes in the conditions
occur frequently and mark off the individual laminæ and strata, while
the more important and wide-spread changes determine the boundaries of
the groups of strata and the formations.
Strata are subject to constant lateral changes in texture and
composition, _i.e._, a bed or formation rarely holds the same
lithological characteristics over an extended area. There are some
striking exceptions, especially among the finer-grained rocks, like
slate, limestone, and coal, which have been deposited under uniform
conditions over wide areas. It is the general rule, however,
particularly with the coarse-grained rocks, which have been deposited in
shallow water near the land, that the same continuous stratum undergoes
great changes in thickness and lithological character when followed
horizontally. A stratum of conglomerate becomes finer grained and
gradually changes into sandstone, which shades off imperceptibly into
slate, and slate into limestone, etc. Where the stratum is conglomerate,
its thickness will usually be much greater and more variable than where
it is composed of the finer sediments. The rapidity of these changes in
certain cases is well shown by the parallel sections in Fig. 3. These
represent precisely the same beds, as the connecting lines indicate, at
points only twenty feet apart.
[Illustration: Fig. 3.—Parallel sections showing rapid
lateral changes in strata: _c._ clay; _s._ sand; _ss._
sandstone; _l._ lignite; _f._ fireclay.]
When we glance at the conditions under which stratified rocks are now
being formed, it is plain that all strata must terminate at the margin
of the sea in which they were deposited, and in the marginal portions of
that sea, especially, must exhibit frequent and rapid changes in
composition, etc. The sediments forming the surface of the sea-bottom at
the present time may be regarded as belonging to one continuous stratum;
and it is instructive to examine a chart of any part of our coast, such
as Massachusetts Bay, on which the nature of the bottom is indicated for
each sounding, and observe the distribution of the different kinds of
sediment. On an irregular coast like this, especially, the gravel, sand,
and mud of different colors and textures, and the different kinds of
shelly bottom, form a patchwork, the patches being, for the most part,
of limited extent and shading off gradually into each other.
On a more regular coast, like that of New Jersey, the sediments are
distributed with corresponding uniformity, the changes are less frequent
and more gradual, and we have here a better chance to observe the normal
arrangement of the sediments along a line from the shore
seawards—gravel, sand, mud, and shells. On the beach we find the shingle
and coarse pebbles, shading off rapidly into fine pebbles and sand. The
zone or belt of sandy bottom may vary in width from a mile or two to
twenty miles or more, becoming gradually finer and changing into clay or
mud, which covers, usually, a much broader zone, sometimes extending
into the deeper parts of the sea, but gradually giving way to calcareous
sediments. Hence we may say that the finer the sediment the greater the
area over which it is spread; but, on the other hand, the coarser the
sediment the more rapidly it increases in thickness. In other words, the
horizontal extent of a formation deposited in any given period of time
is inversely, and the vertical extent or thickness is directly,
proportional to the size of the particles.
Observations made in deep wells and mines, and where, by upturning and
erosion, the edges of the strata are exposed on the surface, show that
the vertical order of the different kinds of sedimentary rocks in the
earth’s crust is extremely variable. But when we take a general view of
a great formation, it is often apparent that it consists chiefly of
coarse-grained rocks in the lower part and fine-grained rocks in the
upper part. This is, in general, a necessary consequence of the fact
that a great thickness of sediments can only be formed on a subsiding
sea-floor. Such a formation must consist chiefly of shore deposits, and
be deposited near the shore where the sea is shallow. Hence, 10,000 feet
of sediments implies nearly that amount of subsidence. In consequence,
the shore line and the several zones of sediment advance towards the
land; and sand is deposited where gravel was at first, and as the
subsidence continues, both clay and limestone are finally deposited over
the original beach. When the sea-floor rises, the order of the sediments
is reversed; and it will be observed that in consequence of the advance
and retreat of the shore-line, the formations grow edgewise to a
considerable extent.
[Illustration: Fig. 4.—Overlap and unconformability.]
OVERLAP AND INTERPOSITION OF STRATA.—Another consequence of the constant
oscillation of the shoreline is that successive deposits in the same sea
will often cover different and unequal areas. When, in consequence of
subsidence, one formation extends beyond and covers the edge of another,
as shown in Fig. 4, we have the phenomenon described as overlap.
Interposition is similar, being the case where a formation (Fig. 5,
_c._) does not, in certain directions, cover so wide an area as the
strata (_b. d._) above and below it, which are thus sometimes found in
contact, although normally separated by the entire thickness of the
intermediate and, seemingly, interposed stratum.
[Illustration: Fig. 5.—Interposition of strata.]
UNCONFORMABILITY.—We have already seen that the rocks on the land are
being constantly worn away by the agents of erosion; and it is also a
matter of common observation that the strata thus exposed are often not
horizontal, but highly inclined, having been greatly disturbed and
crumpled during their elevation. Now, when such a land-surface subsides
to form the sea-bottom, and new strata are spread horizontally over it,
they will lie across the upturned and eroded edges of the older rocks,
and fill the hollows worn out of the latter, as shown in Fig. 6; and the
new formation is then said to rest unconformably upon the older. Two
strata or formations are unconformable when the older has suffered
erosion (Fig. 6), or both disturbance and erosion (Fig. 4) before the
deposition of the newer.
[Illustration: Fig. 6.—Unconformability.]
When strata are conformable, the deposition may be presumed to have been
nearly or quite continuous; but unconformability clearly proves a
prolonged interruption of the deposition during which the elevation,
erosion, and subsidence of the sea-bottom took place. The section in
Fig. 7 shows a second unconformability, proving that the sea-bottom has
here been lifted three times to form dry land. An unconformability may
sometimes be clearly established when the actual contact of the two
formations cannot be seen, as where the new formation is a conglomerate
containing fragments of the older.
IRREGULARITIES OF STRATIFICATION.—These are especially noticeable in
sandstone and conglomerate, which have been deposited chiefly by strong,
local, and variable currents; the kind and quantity of sediment, of
course, varying with the strength and direction of the current. Two
kinds of irregularity only may be specially noticed here: (1)
contemporaneous erosion and deposit, where, in consequence of a change
in the currents, fine material recently deposited is partially swept
away and its place taken by coarser sediments; and (2) oblique
lamination, or current-bedding, where the strata are horizontal as
usual, but the component laminæ are inclined at various angles. This
structure is characteristic of sediments swept along by strong currents,
especially when deposited in shallow basins or depressions.
[Illustration: Fig. 7.—Double Unconformability: _q._
quartzite; _s._ sandstone; _d._ drift.]
RIPPLE-MARKS.—All who have been on a beach or sand-bar must have noticed
the lines of wavy ridges and hollows, or ripples, on the surface of the
sand. These are sand-waves, produced by water moving over the sand, or
by air moving over dry sand, as ordinary waves are formed by air moving
over water. Each tide usually effaces the ripple-marks made by its
predecessor and leaves a new series, to be obliterated by the next tide.
But where sediment is constantly accumulating, a rippled surface may be
gently overspread by a new layer, and thus preserved. Other series of
ripples may, in like manner, be formed and preserved in overlying
layers; and when the beach becomes a firm sandstone, a section of it
will show the rippled surfaces almost as distinctly as when they were
first formed (Fig. 8). Ripple-marks are most perfect in fine sand. They
are not formed in gravel, because it is too coarse; nor in clay, because
it is too tenacious. They are usually limited to shallow water; and are
always regarded as proving that the rocks in which they occur are
shallow-water or beach deposits. They are normally at right angles to
the current that produces them, and where this changes with the
direction of the wind, cross-ripples and other irregularities are
introduced. Ripple-marks are also usually parallel with the beach, and
when they are found in the rocks they give us the direction, as well as
the position, of the ancient shore-line.
Again, the friction of the water pushes the sand-grains along, rolling
them up on one side of the ripple and letting them fall down on the
other. Hence ripples, formed by a current are always moving and are
unsymmetrical on the cross-section, presenting a long, gentle slope
toward the current, and a short, steep slope away from it, the arrow in
the figure indicating the direction of the current, or of the sea in the
case of a beach. And we may thus learn from the fossil ripples, in some
cases, not only the position and direction of the ancient shore, but
also on which side the land lay, and on which side the sea. When the
water is in a state of oscillation, without any distinct current, more
symmetrical ripples are produced.
[Illustration: Fig. 8.—Ripple-marks in sandstone.]
RILL-MARKS, RAIN-PRINTS, AND SUN-CRACKS.—“One of the most fascinating
parts of the work of a field-geologist consists in tracing the shores of
former seas and lakes, and thus reconstructing the geography of
successive geological periods.” His conclusions, as we have already
seen, are based largely upon the nature of the sediments; but still more
convincing is the evidence afforded by those superficial features of the
strata, which, like ripple-marks, seem, by themselves, quite
insignificant. And among these he lays special emphasis upon those which
show that during their deposition strata have at intervals been laid
bare to sun and air.
During ebb tide water which has been left at the upper edge of the beach
runs down across the beach in small rills, which excavate miniature
channels; and when these are preserved in the hard rocks, they prove
that the latter are beach deposits, and, like the ripple-marks, show the
direction of the old shore.
If a heavy shower of rain falls on a muddy beach or flat, the sediment
deposited by the returning tide may cover, without obliterating, the
small but characteristic impressions of the individual drops; and these
markings are frequently found well preserved in the hardest slates and
sandstones, testifying unequivocally to the conditions under which the
rocks were formed. In some cases the rain-prints are found to be ridged
up on one side only, in such a manner as to indicate that the drops as
they fell were driven aslant by the wind. The prominent side of the
marking, therefore, indicates the side towards which the wind blew.
Muddy sediments, especially in lakes and rivers, are often exposed to
the air and sun during periods of drouth, and as they gradually dry up,
polygonal cracks are formed. The sediment of the next layer will fill
these sun-cracks; and when, as often happens, it is slightly different
from the dessicated layer, they may still be traced. Sun-cracks
preserved in this way are very characteristic of argillaceous rocks,
and, of course, prove that in early times, as at the present day,
sediments of this class were exposed by the temporary retreat of the
water. The foot-prints or trails of land-animals are often, as in the
sandstones and shales of the Connecticut Valley, associated with, and of
course strongly corroborate, all these other evidences of shore
deposits. From the foot-prints preserved in the rocks we pass naturally
to the consideration of the fossil remains of plants and animals found
entombed in the strata.
FOSSILS.—Although fossils find their highest interest in the light which
they throw upon the succession of life on the globe, they may also be
properly regarded as structural features of stratified rocks; and any
one who has seen the dead shells, crabs, fishes, etc., on the beach will
readily understand how fossils get into the rocks. It is not our
province here to study the structure of the fossils themselves, for that
would involve us in a course in paleontology, a task belonging to the
biologist rather than the geologist; but we will merely observe the
three principal degrees in the preservation of fossils:—
1. _Original composition not completely changed._—Extinct elephants have
been found frozen in the river-bluffs of Siberia so perfectly preserved
that dogs and wolves ate their flesh. The bodies of animals are also
found well preserved in peat-bogs. All coal is simply fossil vegetation
retaining in a large degree the original composition; and the same is
true of ferns, etc., preserved as black impressions in the rocks. All
bones and shells consist of mineral matter which makes them hard, and
animal matter which makes them tough and strong. In very many cases,
especially in the newer formations, the animal matter is still
partially, and the mineral matter almost wholly, intact.
2. _Original composition completely changed, but form and structure
preserved._—All kinds of fossils are commonly called petrifactions, but
only those preserved in this second way are truly petrified, _i.e._,
turned to stone. “Petrified wood is the best illustration, and in a good
specimen not only the external form of the wood, not only its general
structure—bark, wood, radiating silver-grain, and concentric rings of
growth—are discernible, but even the microscopic cellular structure of
the wood, and the exquisite sculpturing of the cell-walls, are perfectly
preserved, so that the kind of wood may often be determined by the
microscope with the utmost certainty. Yet not one particle of the
organic matter of the wood remains. It has been entirely replaced by
mineral matter; usually by some form of silica. The same is true of the
shells and bones of animals.”—LE CONTE.
3. _Original composition and structure both obliterated, and form alone
preserved._—This occurs most commonly with shells, although fossil trees
are also often good illustrations. The general result is accomplished in
several ways: (_a_) The shell after being buried in the sediment may be
removed by solution, leaving a _mould_ of its external form, (_b_) This
mould may subsequently be filled by the infiltration of finer sediment,
forming a _cast_ of the exterior of the shell. (_c_) The shell, before
its solution, may have been filled with mud; and if the shell itself is
then dissolved, we have a cast of its interior in a mould of its
exterior.
TIME REQUIRED FOR THE FORMATION OF STRATIFIED ROCKS.—Many attempts have
been made to determine the time required for the deposition of any given
thickness of stratified rocks. Of course, only roughly approximate
results can be hoped for in most cases; but these are at least
sufficient to make it certain that geological time is very long. The
average relative rate of growth of different kinds of sediment is,
however, less open to doubt, for we have already seen that coarse
sediments like gravel and sand accumulate much more rapidly than finer
sediments like clay and limestone; and we are sometimes able to compare
these two classes of rocks on a very large scale.
Thus, during what is known as the Paleozoic era, a sea extended from the
Blue Ridge to the Rocky Mountains. Along the eastern margin of this sea,
where the Alleghany Mountains now stand, sediments—chiefly conglomerate
and sandstone, with some slate and less limestone—accumulated to a
thickness of nearly 40,000 feet. Toward the west, away from the old
shore-line, the coarse sediments gradually die out, and the formations
become finer and thinner. In western Ohio and Indiana, slate and
limestone predominate; while in the central part of the ancient sea, in
Illinois and Missouri, the paleozoic sediments are almost wholly
limestones, and have a thickness of only 4000 to 5000 feet. In other
words, while one foot of limestone was forming in the Mississippi
Valley, eight to ten feet of coarser sediments were deposited in
Pennsylvania.
The best estimates show that coral-reefs rise—_i.e._, limestones are
formed on them—at the rate of about one foot in two hundred years. But
coral limestones grow much more rapidly than limestones in general.
Sandstones sometimes accumulate so rapidly that trees are buried before
they have time to decay and fall (Fig. 9). Such a buried forest, like a
coal-bed, represents a land surface, and proves a subsidence of the
land; and in some cases, as indicated by the section, repeated
oscillations of the crust may be proved in this way.
The mud deposited by the annual overflow of the Nile is forty feet thick
near the ancient city of Memphis; and the pedestal of the statue of
Rameses II., believed to have been erected B.C. 1361, is buried to a
depth of nine feet, four inches, indicating that 13,500 years have
elapsed since the Nile began to spread its mud over the sands of the
desert.
[Illustration: Fig. 9.—Erect fossil trees.]
But the greatest difficulty in estimating the time required for the
formation of any series of strata arises from the fact that we cannot
usually even guess at the length of the periods when the deposition has
been partially or wholly interrupted. Now and then, however, we find
evidence that these periods may be very long. A layer of fossil shells
in sandstone or slate proves an interruption of mechanical deposition.
Beds of coal, fossil forests, and other indications of land surfaces are
still more conclusive. The interposition of strata (Fig. 5) proves a
prolonged interruption of deposition over the area not covered by the
interposed bed. But the most important of all evidence is that afforded
by unconformability; and the length of the lost interval between the two
formations is measured approximately by the erosion of the older.
Original Structures of Eruptive Rocks.
The structures of this class are divisible into those pertaining to the
volcanic rocks and those pertaining to the fissure or dike rocks. But
since volcanoes are rare in this part of the world, while dikes are well
developed in many sections of our country, it seems best to give our
attention chiefly to the latter.
[Illustration: Fig. 10.—Typical dikes.]
[Illustration: Fig. 11.—Section of a granite mass.]
The term _dike_ is a general name for all masses of eruptive rocks that
have cooled and solidified in fissures or cavities in the earth’s crust.
But the name is commonly restricted to the more regular, wall-like
masses (Fig. 10), those having extremely irregular outlines, like most
masses of granite (Fig. 11), being known simply as _eruptive masses_.
The propriety of this distinction is apparent when we consider the
origin of _dike_ as a geological term. It was first used in this sense
in southern Scotland, where almost any kind of a wall or barrier is
called a dike. The dikes traverse the different stratified formations
like gigantic walls, which are often encountered by the coal-miners, and
on the surface are frequently left in relief by the erosion of the
softer enclosing rock, so that in the west of Scotland, especially, they
are actually made use of for enclosures. In other cases the dike has
decayed faster than the enclosing rock, and its position is marked by a
ditch-like depression. The narrow, straight, and perpendicular clefts or
chasms observed on many coasts are usually due to the removal of the
wall-like dikes by the action of the waves. Dikes are sometimes mere
plates of rock, traceable for a few yards only; and they range in size
from that up to those a hundred feet or more in width, and traceable for
scores of miles across the country, their outcrops forming prominent
ridges. The sides of dikes are often as parallel and straight of those
of built walls, the resemblance to human workmanship being heightened by
the numerous joints which, intersecting each other along the face of a
dike, remind us of well-fitted masonry.
FORMS OF DIKES.—A dike is essentially a casting. Melted rock is forced
up from the heated interior into a cavity or crack in the earth’s crust,
cools and solidifies there, and, like a metallic casting, assumes the
form of the fissure or mould. In other words, the form of the dike is
exactly that of the fissure into which the lava was injected. Now the
forms of fissures depend partly upon the nature of the force that
produces them, but very largely upon the structure—and especially the
joint-structure—of the enclosing rocks. Nearly all rocks are traversed
by planes of division or cracks called joints, which usually run in
several directions, dividing the rock into blocks. And it is probable
that dike-fissures are most commonly produced, not by breaking the rocks
anew, but by widening or opening the pre-existing joint-cracks. Hence
the straight and regular jointing of slate, limestone and most
sedimentary rocks is accompanied by wall-like dikes—the typical dikes
(Fig. 10); while the more irregular jointing of granite and other
massive rocks gives rise to sinuous, branching, variable dikes. The
general dependence of dikes upon the joint-structure of the rocks is
proved by the facts that dikes, like joints, are normally vertical or
highly inclined, and that they are usually parallel with the principal
systems of joints in the same district. The wall-like dikes also give
off branches, but usually in a regular manner, as shown in Fig. 12.
[Illustration: Fig. 12.—Dike with regular branches.]
STRUCTURE OF DIKES.—The rock traversed by a dike is called the _country_
or _wall_ rock. Fragments of this are often torn off by the igneous
material, and become enclosed in the latter. Such enclosed fragments may
sometimes form the main part of the dike, which then, since they are
necessarily angular, often assumes the aspect of a breccia. This is the
only important exception to the rule that dikes are homogeneous in
composition; _i.e._, in the same dike we can usually find—from end to
end, from side to side, and probably from top to bottom—no essential
difference in composition. But there is often a marked contrast in
_texture_ between different parts of a dike, and especially between the
sides and central portion. The liquid rock loses heat most rapidly where
it is in contact with the cold walls of the fissure, and solidifies
before it has time to crystallize, remaining compact and sometimes even
glassy; while in the middle of the dike, unless it is very narrow, it
cools so slowly as to develop a distinctly crystalline texture. There is
no abrupt change in texture, but a gradual passage from the compact
border to the coarsely crystalline or porphyritic middle portion. It is
obvious that a similar gradation in texture must exist between the top
and bottom of a dike.
CONTACT PHENOMENA.—Under this head are grouped the interesting and
important phenomena observable along the contact between the dike and
wall-rock. These throw light upon the conditions of formation of dikes,
and are often depended upon to show whether a rock mass is a dike or
not. The student will observe here:—
1. The detailed form of the contact. It may be straight and simple, or
exceedingly irregular, the dike penetrating the wall, and enclosing
fragments of it, as in Fig. 11, which is a typically igneous contact.
2. The alteration of the wall-rock by heat. This may consist in: (_a_)
_coloration_, shales and sandstones being reddened in the same way as
when clay is burnt for bricks; (_b_) _baking and induration_, sandstone
being converted into quartzite and even jasper; clay, slate, etc., being
not only baked to a flinty hardness, but actually vitrified, as in
porcelainite; and bituminous coal being converted into natural coke or
anthracite; and (_c_) _crystallization_, chalk, and other limestones
being changed to marble, and crystals of pyrite, calcite, quartz, etc.,
being developed in slate, sandstone, and other rocks.
3. The alteration of the dike-rock by (_a_) more rapid cooling, and
(_b_) the access of thermal waters.
The alteration of the wall-rock may extend only a few inches or many
yards from the dike, gradually diminishing with the distance; and the
cases are surprisingly numerous where there is no perceptible
alteration; and, again, the alteration is usually mutual, the dike-rock
being altered in texture, color, and composition.
[Illustration: Fig. 13.—Ideal cross-section of a laccolite.]
[Illustration: Fig. 14.—Ideal cross-section of a volcano.]
INTRUSIVE BEDS.—We commonly think of dikes as cutting across the strata,
but they often lie in planes parallel with them; and the same dike may
run across the beds in some parts of its course and between them in
others (Fig. 12), or the conformable dike maybe simply a lateral branch
of a main vertical dike, as shown in the same figure. All dikes or
portions of dikes lying conformably between the strata are called
_intrusive beds_ or _sheets_.
When a dike fails to reach the surface, but spreads out horizontally
between the strata, forming a thick dome or oven-shaped intrusive bed,
the latter is called a _laccolite_ (Fig. 13). Laccolites are sometimes
of immense volume, containing several cubic miles of rock. Fig. 14
enables us to compare the laccolite with the volcano.
In the one case a large mound of eruptive material accumulates between
the strata, the overlying beds being lifted into a dome; while in the
other case the fissure or vent reaches the surface, and the mound of
lava is built up on top of the ground.
COTEMPORANEOUS BEDS.—When the lava emitted by a crater is sufficiently
liquid, it spreads out horizontally, forming a volcanic sheet or bed. If
such an eruption is submarine, or the lava flow is subsequently covered
by the sea, sedimentary deposits are formed over it; and beds of lava
which thus come to lie conformably between sedimentary strata are known
as _cotemporaneous sheets_ or _beds_, because they belong, in order of
time, in the position in which we find them, being, like any member of a
stratified series, newer than the underlying and older than the
overlying strata. Cotemporaneous lava-flows are sometimes repeated again
and again in the same district, and thus important formations are built
up of alternating igneous and aqueous deposits. Evidently, the student
who would read correctly the record of igneous activity in the past must
be able to distinguish intrusive and cotemporaneous beds. The principal
points to be considered in making this distinction are: (1) The
intrusive bed is essentially a dike, dense and more or less crystalline
in texture, altering, and often enclosing fragments of, both the
underlying and overlying strata, and frequently jogging across or
penetrating the sediments. (2) The cotemporaneous bed, on the other
hand, being essentially a lava-flow, is much less dense and crystalline,
being usually distinctly scoriaceous or amygdaloidal, especially at the
borders, and the underlying strata alone showing heat action, or
occurring as enclosures in the lava; for the overlying strata are newer
than the lava, and often consist largely, at the base, of water-worn
fragments of the lava.
AGES OF DIKES.—The ages of dikes may be estimated in several ways. They
are necessarily newer than any stratified formation which they intersect
or of which they enclose fragments; but any formation crossing the top
of a dike must usually be regarded as newer than the dike, especially if
it contains water-worn fragments of the dike rock.
The relative ages of different dikes are determined by their relations
to the stratified formations; and still more easily by their mutual
intersections, on the principle that when two dikes cross each other,
the intersecting must be newer than the intersected dike. It is
sometimes possible, in this way, to prove several distinct periods of
eruption in the same limited district. The textures of dikes also often
afford reliable indications of their ages; for, as we have already seen,
the upper part of a dike, cooling rapidly and under little pressure,
must be less dense and crystalline than the deep-seated portion, which
cools slowly and under great pressure.
Now, the lower, coarsely crystalline part of a dike can usually be
exposed on the surface only as the result of enormous erosion; and
erosion is a slow process, requiring vast periods of time. Hence, when
we see a coarse-grained dike outcropping on the surface, we are
justified in regarding it as very old, for all the fine-grained upper
part has been gradually worn away by the action of the rain, frost, etc.
Other things being equal, coarse-grained must be older than fine-grained
dikes; and the texture of a dike is at once a measure of its age and of
the amount of erosion which the region has suffered since it was formed.
ERUPTIVE MASSES.—In striking contrast with the more or less wall-like
dikes are the highly irregular, and even ragged, outlines of the
eruptive masses; and it is worth while to notice the probable cause of
this contrast. The true dikes are formed, for the most part, of
comparatively fine-grained rocks—the typical “traps”; while the eruptive
masses consist chiefly of the coarse-grained or granitic varieties. Now
we have just seen that the coarse-grained rocks have been formed at
great depths in the earth’s crust, while the fine-grained are
comparatively superficial. But we have good reason for believing that
the joint-structure, upon which the forms of dikes so largely depend, is
not well developed at great depths, where the rocks are toughened, if
not softened, by the high temperature. In other words, trap dikes are
formed in the jointed formations, which break regularly; while the
granitic masses are formed where the absence of joint-structure and a
high temperature combine to cause extremely irregular rifts and cavities
when the crust is broken.
VOLCANIC PIPES OR NECKS.—Every volcano and every lava-flow or volcanic
sheet must be connected with the earth’s interior by a channel or
fissure, which becomes a dike when the lava ceases to flow. But the
converse proposition is not true, for it is probable that many dikes did
not originally reach the surface, but have been exposed by subsequent
denudation. This is conspicuously the case with laccolites and other
forms of intrusive sheets. Volcanic sheets or beds have probably often
resulted from the overflow of the lava at all points of an extensive
fissure or system of fissures; but the vent of the true volcano must be
more circumscribed, an approximately circular opening in the earth’s
crust, although doubtless originating in a fissure or at the
intersection of two or more fissures, the lava continuing to flow at the
widest part of the wound in the crust long after it has congealed in the
narrower parts. Such a tube is known as the neck or pipe of the volcano;
and volcanic necks are a highly interesting class of dikes, since they
determine the exact location of many an ancient volcano, where the
volcanic pile itself has long since been swept away. Necks and dikes are
the downward prolongations or roots of the volcanic cone or sheet, and
cannot be exposed on the surface until the volcanic fires have gone out
and the agents of erosion have removed the greater part of the ejected
materials.
Hence, equally with the dikes which originally failed to reach the
surface, they, wherever open to our observation, testify to extensive
erosion and a vast antiquity.
Original Structures of Vein Rocks.
Many things called veins are improperly so called, such as dikes of
granite and trap, and beds of coal and iron-ore. The smaller, more
irregular, branching dikes, especially, are very commonly called veins,
and to distinguish the true veins from these eruptive masses, they are
designated as _mineral veins_ or _lodes_, although the term _lode_ is
usually restricted to the metalliferous veins.
ORIGIN OF VEINS.—Various theories of the formation of veins have been
proposed, but the most of these are of historic interest merely, for
geologists are now well agreed that nearly all true veins have been
formed by the deposition of minerals from solution in fissures or
cavities in the earth’s crust. In many cases, especially where the veins
are of limited extent, it seems probable that a part or all of the
mineral matter was derived from the immediately enclosing rocks, being
dissolved out by percolating water; and these are known as segregation
or lateral secretion veins. But it is quite certain that as a general
rule the mineral solutions have come chiefly from below, the deep-seated
thermal waters welling up through any channel opened to them, and
gradually depositing the dissolved minerals on the walls of the fissure
as the temperature and pressure are diminished. This case, however,
differs from the first only in deriving the vein-forming minerals from
more remote and deeper portions of the enclosing rocks; and thus we see
that vein-formation, whether on a large or a small scale, is always
essentially a process of segregation.
We know that every volcano and every lava flow must be connected below
the surface with a dike; and it is almost equally certain that the
waters of mineral springs forming tufaceous mineral deposits on the
surface, as in the geyser districts, also deposit a portion of the
dissolved minerals on the walls of the subterranean channels, which are
thus being gradually filled up and converted into mineral veins, which
will be exposed on the surface when erosion has removed the tufaceous
overflow. This connection of vein-formation with the superficial
deposits of existing springs has been clearly proved in several
important instances in Nevada and California.
Veins occur chiefly in old, metamorphic, and highly disturbed
formations, where there is abundant evidence of the former existence of
profound fissures, and in regions similar to those in which thermal
springs occur to-day.
In the supplement to the lithological section the student will find the
formation of a typical vein briefly described and contrasted with that
of a typical dike; also a brief account of the lithological
peculiarities of vein rocks, and general statements concerning their
relative abundance and vast economic importance.
EXTERNAL CHARACTERISTICS OF VEINS.—The typical vein may be described as
a fissure of indefinite length and depth, filled with mineral substances
deposited from solution. Externally, it is very similar to the typical
dike, for the fissures are made in the same way for both. Veins are
normally highly inclined to the horizon; they exhibit in nearly every
respect the same general relations to the structure of the country rock
as dikes; and the ages of veins are determined in the same way as the
ages of dikes.
The extensive mining operations to which veins have been subjected in
all parts of the world, have made our knowledge of their forms below the
surface very full and accurate. It has been learned in this way that
very often the corresponding portions of the walls of a vein do not
coincide in position, but one side is higher or lower than the other,
showing that the walls slipped over each other when the fissure was
formed or subsequently; and this faulting or displacement of the walls
appears to be much more common with veins than with dikes, perhaps
because the fissures remained open much longer. This slipping of the
walls is the principal cause of the almost constant changes in the width
of veins. For, since the walls are never true planes, and are often
highly irregular any unequal movements must bring them nearer together
at some points than at others. As a rule, the enormous friction
accompanying the faulting, either crushes the wall-rock, or polishes and
striates it, producing the highly characteristic surfaces known as
_slicken-sides_. Where the wall is finely pulverized in this way, or is
partially decomposed before or after the filling of the fissure, a thin
layer of soft, argillaceous material is formed, separating the vein
proper from the wall-rock. The miners call this the _selvage_; and it is
a very characteristic feature of the true fissure veins.
Fragments of the wall-rock are frequently enclosed in veins, and the
latter sometimes branch or divide in such a way as to surround a large
mass of the wall, which is known as a “horse.” A similar result is
accomplished when a fissure is re-opened after being filled, if the new
fissure does not coincide exactly with the old. It has been proved that
veins have thus been re-opened and filled several times in succession;
and in this way fragments of the older vein material become enclosed in
the newer.
Although usually determined in direction by the joint-structure of the
country rock, veins are often parallel with the bedding, especially in
highly inclined, schistose formations. Such interbedded veins are
commonly distinctly lenticular in form, occupying rifts in the strata
which thin out in all directions and are often very limited in extent.
Whether conforming with the joint-structure or bedding, veins are
commonly arranged in systems by their parallelism, those of different
systems or directions usually differing in age and composition, and the
older veins being generally faulted or displaced when intersected by the
newer.
INTERNAL CHARACTERISTICS OF VEINS.—Internally, veins and dikes are
strongly contrasted; and it is upon the internal features, chiefly, as
previously explained, that we must depend for their distinction. In
metalliferous veins the minerals containing the metal sought for (the
galenite, sphalerite, etc.) are the _ore_; while the non-metalliferous
minerals (the quartz, feldspar, calcite, etc.) are called the _gangue_
or vein-stone proper. Although the combinations of minerals in veins are
almost endless, yet certain associations of ores with each other and
with different gangue minerals are tolerably constant, and constitute an
important subject for the student of metallurgy and mining.
When a vein is composed of a single mineral, as quartz, it may rival a
dike in its homogeneity. Most important veins, however, are composed of
several or a large number of minerals, which may be sometimes more or
less uniformly mixed with each other, but are usually distributed in the
fissure in a very irregular manner. The great granite veins which are
worked for mica, feldspar and quartz, are good illustrations, on a large
scale, of the structure of veins in which several minerals have been
deposited cotemporaneously. The individual minerals are found to a large
extent, in great, irregular masses, with no order observable in their
arrangement.
When a mineral is deposited from solution, it crystallizes by preference
on a surface of similar composition, thus quartz on quartz, feldspar on
feldspar, and so on; and it seems probable that this selective action of
the wall-rock may be a principal cause of the irregular distribution of
minerals in veins. It has often been observed in metalliferous veins
that the richness varies with the nature of the adjacent country rock.
This dependence of the contents of a fissure upon the wall-rock may be
due in part to the selective deposition of the minerals, and in part to
their derivation from the contiguous portions of the country or
wall-rock, as in the so-called segregated veins. Temperature and
pressure exert an important influence upon chemical precipitation, and
it is, therefore, probable that the composition of many veins varies
with the depth.
[Illustration: Fig. 15.—Ideal section of a vein.]
Frequently, perhaps usually, the minerals of composite veins are
deposited in succession, instead of cotemporaneously, giving rise to the
remarkable banded structure so characteristic of this class of veins.
The first mineral deposited in the fissure forms a layer covering each
wall, and is in turn covered by layers of the second mineral, and that
by the third, and so on, until the fissure is filled, or the solution
exhausted. The distinguishing features of this structure are shown in
Fig. 15, in which _w w_ represents the wall-rock, _a a_, _b b_, _c c_
are successive layers of quartz, fluorite and barite, and the central
band, _d_, is galenite. Since the vein grows from the outside inward,
the outer layers are the oldest, and the central layers are the newest;
again, the layers are symmetrically arranged, being repeated in the
reverse order on opposite sides of the middle of the vein; and, lastly,
in layers composed of prismatic crystals, as quartz (see the figure);
the crystals are perpendicular to the wall and often project into, and
even through, the succeeding layers. Such a crystalline layer is called
a “_comb_” and the interlocking of the layers in this way is described
as the _comb-structure_ of the vein. The banding of veins is thus
strongly contrasted with stratification, and with the structure in dikes
due to the more rapid cooling along the walls. The duplicate layers are
often discontinuous and of unequal thickness, on account of the strong
tendency to segregation in the materials. This is clearly shown in Fig.
16, drawn on a reduced scale from a polished section of a lead vein in
Cumberland, England, contained in the Museum of the Boston Society of
Natural History. In this the gangue minerals are fluorite (_f_) and
barite (_b_). The central band (_f g_) is a darker fluorite containing
irregular masses of galenite. The banded structure of veins is exactly
reproduced in miniature in the banding of agates, geodes, and the
amygdules formed in old lavas. Unfilled cavities frequently remain along
the middle of the vein. When small, these are known as “pockets.” They
are commonly lined with crystals; and when the latter are minute, the
pockets are called druses. In metalliferous veins, the ore is much more
abundant in some parts than in others, and these ore-bodies, especially
when somewhat definite in outline, are known in their different forms
and in different localities, as _courses_, _slants_, _shoots_,
_chimneys_, and _bonanzas_ of ore. The intersections and junctions of
veins are often among the richest parts, as if the meeting of dissimilar
solutions had determined the precipitation of the ore.
[Illustration: Fig. 16.—Section of a lead vein, one-fifth
natural size.]
Metalliferous veins, especially, are usually deeply decomposed along the
outcrop by the action of atmospheric agencies. The ore is oxidized, and
to a large extent removed by solution, leaving the quartz and other
gangue minerals in a porous state, stained by oxides of iron, copper,
and other metals, forming the _gossan_ or _blossom-rock_ of the vein.
PECULIAR TYPES OF VEINS.—In calcareous or limestone formations,
especially, the joint-cracks and bedding-cracks are often widened
through the solution of the rock by infiltrating water, and thus become
the channels of a more or less extensive subterranean drainage, by which
they are more rapidly enlarged to a system of galleries and chambers,
and, in some cases, large limestone caverns. The water dripping into the
cavern from the overlying limestone is highly charged with carbonate of
lime, which is largely deposited on the ceiling and floor of the cavern,
forming stalactitic and stalagmitic deposits. These are masses of
mineral matter deposited from solution in cavities in the earth’s crust,
and are essentially vein-formations. Portions of caverns deserted by the
flowing streams by which they were excavated, are often filled up in
this way, being converted into irregular veins of calcite. But calcite
is not the only mineral found in these cavern deposits, for barite and
fluorite, and various lead and zinc ores, especially the sulphides of
these metals—galenite and sphalerite—have also been leached out of the
surrounding limestone and concentrated in the caverns. The celebrated
lead mines of the Mississippi Valley, and some of the richest
silver-lead mines of Utah and Nevada are of this character. The forms of
these cavern-deposits vary almost indefinitely, and are often highly
irregular. The principal types are known as _gash-veins_, _flats_ and
_sheets_ (Fig. 17), _chambers_ and _pockets_.
Where joints and other cracks have opened slightly in different
directions and become filled with infiltrated ores, we have what the
German miners call a _stock-work_,—an irregular network of small and
interlacing veins.
[Illustration: Fig. 17.—Gash-veins and sheets.]
An _impregnation_ is an irregular segregation of metalliferous minerals
in the mass of some eruptive or crystalline rock. Its outlines are not
sharply defined, but it shades off gradually into the enclosing rock.
_Fahlbands_ are similar ill-defined deposits or segregations in
stratified rocks. An impregnation or vein occurring along the contact
between two dissimilar rocks is called a _contact deposit_. These are
usually found between formations of different geological ages, and
especially between eruptive and sedimentary rocks.
Subsequent Structures produced by Subterranean Agencies.
The subterranean forces concerned in the formation of rocks are chiefly
various manifestations of that enormous tangential pressure developed in
the earth’s crust, partly by the cooling and shrinkage of its interior,
but largely, it is probable, by the diminution of the velocity of the
earth’s rotation by tidal friction, and the consequent diminution of the
oblateness of its form. It is well known that the centrifugal force
arising from the earth’s rotation is sufficient to change the otherwise
spherical form of the earth to an oblate spheroid, with a difference of
twenty-six miles between the equatorial and polar diameters. It is also
well known that while the earth turns from west to east on its axis, the
tidal wave moves around the globe from east to west, thus acting like a
powerful friction-brake to stop the earth’s rotation. Our day is
consequently lengthening, and the earth’s form as gradually approaching
the perfect sphere. This means a very decided shortening and consequent
crumpling of the equatorial circumference, and is equivalent to a marked
shrinkage of the earth’s interior, so far as the equatorial regions are
concerned.
The most important and direct result of the horizontal thrust, whether
due to cooling or tidal friction, is the corrugation or wrinkling of
the crust; and the earth-wrinkles are of three orders of
magnitude,—continents, mountain-ranges, and rock-folds or arches.
Continents and ocean-basins, although the most important and permanent
structural features of the earth’s crust, do not demand further
consideration here, since their forms and relations are adequately
described in the better text-books of physical geography. The forms and
distribution of mountain-ranges might be dismissed in the same way; but,
unlike continents, the structure of mountains, upon which their reliefs
mainly depend, is quite fully exposed to our observation, and is one of
the most important fields of the student of structural geology.
Mountains, however, as previously explained, combine nearly all the
kinds of structure produced by the subterranean agencies, and their
consideration, therefore, belongs at the end rather than the beginning
of this section.
INCLINED OR FOLDED STRATA.—Normally, strata are horizontal, and dikes
and veins are vertical or nearly so. Hence the stratified rocks are more
exposed to the crumpling action of the tangential pressure in the
earth’s crust than the eruptive and vein rocks; and it is for this
reason and partly because the stratified rocks are vastly more abundant
than the other kinds, that the effects of the corrugation of the crust
are studied chiefly in the former. But it should be understood that
folded dikes and veins are not uncommon.
That the stratified rocks have, in many instances, suffered great
disturbance subsequent to their deposition, is very evident; for, while
the strata must have been originally approximately straight and
horizontal, they are now often curved, or sharply bent and contorted,
and highly inclined or even vertical. All inclined beds or strata are
portions of great folds or arches. Thus we may feel sure when we see a
stratum sloping downward into the ground, that its inclination or dip
does not continue at the same angle, but that at some moderate depth it
gradually changes and the bed rises to the surface again. Similarly, if
we look in the opposite direction and think of the bed as sloping
upward—we know that the surface of the ground is being constantly
lowered by erosion, and consequently that the inclined stratum formerly
extended higher than it does now, but not indefinitely higher; for, in
imagination, we see it curving and descending to the level of the
present surface again. Hence it forms, at the same time, part of one
side of a great concave arch, and of a great convex arch, just as every
inclined surface on the ground indicates both a hill and a valley. And
guided by this principle we can often reconstruct with much probability
folds that have been more or less completely swept away by erosion, or
that are buried beyond our sight in the earth’s crust.
The arches of the strata are rarely distinctly indicated in the
topography, but must be studied where the ground has been partly
dissected, as in cliffs, gorges, quarries, etc. They are also, as a
rule, far more irregular and complex than they are usually conceived or
represented. The wrinkles of our clothing are often better illustrations
of rock-folds than the models and diagrams used for that purpose. This
becomes self-evident when we reflect that the earth’s crust is
exceedingly heterogeneous in composition and structure, and must,
therefore, yield unequally to the unequal strains imposed upon it.
The folds or undulations of the strata may be profitably compared with
water-waves. In fact, the comparison is so close that they have been not
inaptly called rock-waves. Folds, like waves, unless very large, rarely
continue for any great distance, but die out and are replaced by others,
giving rise to the _en echelon_ or step-like arrangement. The plan of
both a wave and a fold is a more or less elongated ellipse, each stratum
in a fold being semi-ellipsoidal or boat-shaped. In other words, a
normal fold is an elongated mound of concentric strata, being highest at
the centre, sloping very gradually toward the ends, and much more
abruptly toward the sides.
[Illustration: Fig. 18.—Anticlinal and synclinal folds.]
The imaginary line passing longitudinally through a fold, about which
the strata appear to be bent, is the _axis_; and the plane lying midway
between the two sides of a fold and including the axis is the _axial
plane_. The two principal kinds of folds are the _anticline_ (Fig. 18,
_A_), where the strata dip away from the axis; and the _syncline_ (Fig.
18, _B_), where they dip toward the axis. They are commonly, but not
always, correlative, like hill and valley.
Rock-folds are of all sizes, from almost microscopic wrinkles to great
arches miles in length and breadth, and thousands of feet in height. The
smaller folds, or such as may be seen in hand specimens and even in
considerable blocks of stone, are commonly called contortions, and it is
interesting to observe that they are, in nearly everything except size,
precisely like the large folds, so that they answer admirably as
geological models. Large folds, however, are almost necessarily curves,
but contortions are frequently angular (Fig. 19). With folds, as with
waves, the small undulations are borne upon the large ones; but the
contortions are not uniformly distributed. An inspection of Fig. 18
shows that when the rocks are folded they must be in a state of tension
on the anticlines (_A_), and in a state of compression in the synclines
(_B_), and the latter is evidently the normal position of the puckerings
or contortions of the strata, as shown in Fig. 20. Contortions are also
most commonly found in thin-bedded, flexible rocks, such as shales and
schists. And when we find them in hard, rigid rocks, like gneiss and
limestone, it must mean either that the structure was developed with
extreme slowness, or that the rock was more flexible then and possibly
plastic.
[Illustration: Fig. 19.—Contorted strata.]
[Illustration: Fig. 20.—Contorted syncline.]
[Illustration: Fig. 21.—Section of anticlinal mountains.]
It is very interesting to notice the relations of anticlinal and
synclinal folds to the agents of erosion. At the time the folds are
made, the anticlinals, of course, are ridges, and the synclinals,
valleys, and this relation sometimes continues, as shown in Fig. 21; but
we have seen that the rocks in the trough of the synclinal are
compressed and compacted, _i.e._, made more capable of resisting
erosion, while those on the crest of the anticlinal are stretched and
broken, _i.e._, made more susceptible of erosion. The consequence is
that the anticlinals are usually worn away very much faster than the
synclinals; so much faster that in many cases the topographic features
are completely transposed, and in place of anticlinal ridges and
synclinal valleys (Fig. 21) we find synclinal ridges and anticlinal
valleys (Fig. 22).
[Illustration: Fig. 22.—Section of synclinal mountains.]
[Illustration: Fig. 23.—Monoclinal fold.]
[Illustration: Fig. 24.—Unsymmetrical and inverted folds.]
Besides the anticlinal and synclinal folds already explained, there are
folds that slope in only one direction, one-sided or _monoclinal_ folds
(Fig. 23). Anticlinal and synclinal folds are _symmetrical_ when the dip
or slope of the strata is the same on both sides and the axial plane is
vertical. The great majority of folds, however, are _unsymmetrical_, the
opposite slopes being unequal, and the axial planes inclined to the
vertical (Fig. 24, _A_). This means that the compressing or plicating
force has been greater from one side than from the other, as indicated
by the arrows. It acted with the greatest intensity on the side of the
gentler slope, the tendency evidently having been to crowd or tip the
fold over in the direction of the steep slope. When the steep slope
approaches the vertical, this tendency is almost unresisted, and when it
passes the vertical, gravitation assists in overturning the fold (Fig.
24, _B_). Such highly unsymmetrical folds, including all cases where the
two sides of the fold slope in the same direction, are described as
_overturned_ or _inverted_, although the latter term is not strictly
applicable to the entire fold, but only to the strata composing the
under or lee side of it. Fig. 24, _B_, shows that these beds are
completely inverted, the older, as the figures indicate, lying
conformably upon the newer. This inversion is one of the most important
features of folded strata, and it has led to many mistakes in
determining their order of succession. In the great mountain-chains,
especially, it is exhibited on the grandest scale, great groups of
strata being folded over and over each other as we might fold carpets.
An inverted stratum is like a flattened S or Z, and may be pierced by a
vertical shaft three times, as has actually happened in some coal mines.
Folds are _open_ when the sides are not parallel, and _closed_ when they
are parallel, the former being represented by a half-open, and the
latter by a closed, book. Closed folds are usually inverted, and when
the tops have been removed by erosion (Fig. 25), the repetition of the
strata may escape detection, and the thickness of the section be, in
consequence, greatly overestimated. Thus, a geologist traversing the
section in Fig. 25 would see thirty-two strata, all inclined to the left
at the same angle, those on the right apparently passing below those on
the left, and all forming part of one great fold. The repetition of the
strata in reverse order, as indicated by the numbers, and the structure
below the surface, show, however, that the section really consists of
only four beds involved in a series of four closed folds, the true
thickness of the beds in this section being only one-eighth as great as
the apparent thickness.
[Illustration: Fig. 25.—Series of closed folds.]
The most important features to be noted in observing and describing
inclined or folded strata are the _strike_ and _dip_. The strike is the
compass bearing or horizontal direction of the strata. It is the
direction of the outcrop of the strata where the ground is level. It may
also be defined as the direction of a level line on the surface of a
stratum, and is usually parallel with the axis of the fold.
[Illustration: Fig. 26.—Dip and strike.]
The dip is the inclination of the beds to the plane of the horizon, and
embraces two elements: (_a_) the direction of the dip, which is always
at right angles to the strike, being the line of steepest descent on the
surface of the stratum, and (_b_) the amount of the dip, which is the
value of the angle between the line of steepest descent and the horizon.
In Fig. 26, _s t_ is the direction of the strike, and _d p_ that of the
dip. The strike and direction of the dip are determined with the
compass, and the amount of the dip with the clinometer, an instrument
for measuring vertical angles.
The strike is much less variable than the dip, being often essentially
constant over extensive districts; while the dip, except in very large
or closed folds, is constantly changing in direction and amount.
When the dip and surface breadth of a series of strata have been
measured, it is a simple problem in trigonometry to determine the true
thickness, and the depth below the surface of any particular stratum at
any given distance from its outcrop. When the strata are vertical, the
surface breadth or traverse measure is equal to the thickness.
By the _outcrop_ of a stratum or formation we ordinarily understand its
actual exposure on the surface, where it projects through the soil in
ledges or quarries. But the term is also more broadly defined to mean
the exposure of the stratum as it would appear if the soil were entirely
removed. It is instructive to observe the relations of the outcrop to
the form of the surface. Its breadth varies with its inclination to the
surface, appearing narrow and showing its true thickness where it is
perpendicular to the surface, and broadening out rapidly where the
surface cuts it obliquely. The outcrops of horizontal strata form level
lines or bands along the sides of hills and valleys, essentially contour
lines in the topography; and appear as irregular, sinuous bands
bordering the streams and valleys in the map-view of the country. The
outcrops of vertical strata, dikes, or veins, on the other hand, are
represented by straight lines and bands on the map. While the outcrops
of inclined strata are deflected to the right or left in crossing ridges
and valleys, according to the direction and amount of their inclination.
A geological map shows the surface distribution of the rocks, _i.e._,
gives in one view the forms and arrangement of the outcrops of all the
rocks in the district mapped, including the trend or strike of the
folded strata. The map may be lithological, each kind of rock, as
granite, sandstone, limestone, etc., being represented by a different
color; or, it may be historical, each color representing one geological
formation, _i.e._, the rocks formed during one period of geological
time, without reference to their lithological character. But in the best
maps these two methods are combined. The geological section shows the
arrangement of the rocks below the surface, revealing the dip of the
strata and supplementing the map, both modes of representation, the
horizontal and vertical, being required to give a complete idea of the
geological structure of a country. For a detailed and satisfactory
explanation of the construction and use of geological maps and sections,
students are referred to Prof. Geikie’s “Outlines of Field Geology.”
CLEAVAGE STRUCTURE.—This important structure is now known to be, like
rock-folds, a direct result of the great horizontal pressure in the
earth’s crust. It is entirely distinct in its nature and origin from
crystalline cleavage, and may properly be called lithologic cleavage. It
is also essentially unlike stratification and joint-structure. It agrees
with stratification in dividing the rocks into thin parallel layers, but
the cleavage planes are normally vertical instead of horizontal. And the
cleavage planes differ from joints in running in only one direction,
dividing the rock into layers; while joints, as we shall see, traverse
the same mass of rock in various directions, dividing it into blocks.
[Illustration: Fig. 27.—Slaty cleavage in contorted strata.]
The principal characteristics of lithologic cleavage are: (1) It is
rare, except in fine-grained, soft rocks, having its best development in
the slates, roofing slates and school slates affording typical examples.
Hence it is commonly known as _slaty cleavage_. (2) The cleavage planes
are highly inclined or vertical, very constant in dip and strike, and
quite independent of stratification. (3) It is usually associated with
folded strata, and often with distorted nodules or fossils. The more
important of these characteristics are illustrated by Fig. 27. This
represents a block of contorted strata in which the dark layers are
slate with very perfect cleavage parallel to the left-hand shaded side
of the block; while the white layers are sandstone and quite destitute
of cleavage. Many explanations of this interesting structure have been
proposed, but that first advanced by Sharpe may be regarded as fully
established. He said that _slaty cleavage is always due to powerful
pressure at right angles to the planes of cleavage_. All the
characteristics of cleavage noted above are in harmony with this theory.
Cleavage is limited to fine-grained or soft rocks, because these alone
can be modified internally by pressure, without rupture. Harder and more
rigid rocks may be bent or broken, but they appear insusceptible of
minute wrinkling or other change of structure affecting every particle
of the mass. Since the cleavage planes are normally vertical, the
pressure, according to the theory, must be horizontal. That this
horizontal pressure exists and is adequate in direction and amount, is
proved by the folds and contortions of the cleaved strata; for, as shown
in Fig. 27, the cleavage planes coincide with the strike of the
foldings, and are thus perpendicular to the pressure horizontally as
well as vertically. The distortion of the fossils in cleaved slates is
plainly due to pressure at right angles to the cleavage, for they are
compressed or shortened in that direction, and extended or flattened out
in the planes of cleavage. Again, Tyndall has shown that the magnetism
of cleaved slate proves that it has been powerfully compressed
perpendicularly to the cleavage. And, finally, repeated experiments by
Sorby and others have proved that a very perfect cleavage may be
developed in clay (unconsolidated slate) by compression, the planes of
cleavage being at right angles to the line of pressure. When, however,
Sharpe’s theory had been thus fully demonstrated, the question as to
_how_ pressure produces cleavage still remained unanswered. Sorby held
that clay contains foreign particles with unequal axes, such as
mica-scales, etc., and that these are turned by the pressure so as to
lie in parallel planes perpendicular to its line of action, thus
producing easy splitting or cleavage in those planes. And he proved by
experiments that a mixture of clay and mica-scales does behave in this
way. But Tyndall showed that the cleavage is more perfect just in
proportion as the clay is free from foreign particles, and in such a
perfectly homogeneous substance as beeswax, he developed a more perfect
cleavage than is possible in clay. His theory, which is now universally
accepted, is, that the clay itself is composed of grains which are
flattened by pressure, the granular structure with irregular fracture in
all directions, changing to a scaly structure with very easy and plane
fracture or splitting in one definite direction.
Observations on distorted fossils and nodules have shown that when slaty
cleavage is developed, the rock is, on the average, reduced in the
direction of the pressure to two-fifths of its original extent, and
correspondingly extended in the vertical direction. Thus, whether rocks
yield to the horizontal pressure in the earth’s crust, by folding and
corrugation, or by the flattening of their constituent particles, they
are alike shortened horizontally and extended vertically; and it is
impossible to overestimate the importance of these facts in the
formation of mountains.
FAULTS OR DISPLACEMENTS.—We may readily conceive that the forces which
were adequate to elevate, corrugate, and even crush vast masses of solid
rock were also sufficient to crack and break them; and since the
fractures indicate that the strains have been applied unequally, it will
be seen that unequal movements of the parts must often result. If this
unequal movement takes place, _i.e._, if the rocks on opposite sides of
a fracture of the earth’s crust do not move together, but slip over each
other, a _fault_ is produced. The two sides may move in opposite
directions, or in the same direction but unequally, or one side may
remain stationary while the other moves up or down. It is simply
essential that the movement should be unequal in direction, or amount,
or both; that there should be an actual slip, so that strata that were
once continuous no longer correspond in position, but lie at different
levels on opposite sides of the fracture. The vertical difference in
movement is known as the _throw_, _slip_, or _displacement_ of the
fault. Fault-fractures rarely approach the horizontal direction, but are
usually highly inclined or approximately vertical. When the fault is
inclined, as in Fig. 28, the actual slipping in the plane of the fault
exceeds the vertical throw, for the movement is then partly horizontal,
the beds being pulled apart endwise. The inclination of faults, as of
veins and dikes, should be measured from the vertical and called the
_hade_. Faults are sometimes hundreds of miles in length; and the throw
may vary from a fraction of an inch to thousands of feet.
[Illustration: Fig. 28.—Section of a normal fault.]
[Illustration: Fig. 29.—Section of a reversed fault.]
Transverse sections, such as are represented by Fig. 28 and many
specimens and models, do not give the complete plan or idea of a fault;
but this is seen more perfectly in Fig. 30. We learn from this that a
typical fault is a fracture along which the strata have _sagged_ or
settled down unequally. The most important point to be observed here is
that the strata do not drop bodily, but are merely bent, the throw being
greatest at the middle of the fault and gradually diminishing toward the
ends. In other words, every simple fault must die out gradually; for we
cannot conceive of a fault as ending abruptly, except where it turns
upon itself so as to completely enclose a block of the strata, which may
drop down bodily; but the fault is then really endless. A fault may be
represented on a map by a line; if a simple fault, by a single straight
line. But faults are often compound, and are represented by branching
lines; that is, the earth’s crust has been broken irregularly, and the
parts adjoining the fracture have sagged or risen unequally.
[Illustration: Fig. 30.—Ideal view of a complete fault.]
The rock above an inclined fault, vein, or dike (Fig. 28) is called the
_hanging wall_, and that below the _foot wall_. Now inclined faults are
divided into two classes, according to the relative movements of the two
walls. Usually, the hanging wall slips down and the foot wall slips up,
as in Fig. 28. Faults on this plan are so nearly the universal rule that
they are called _normal_ faults. They indicate that the strata were in a
state of tension, for their broken ends are pulled apart horizontally,
so that a vertical line may cross the plane of a stratum without
touching it.
A few important faults have been observed, however, in which the
foot-wall[**no hyphen before] has fallen and the hanging-wall[**] has
risen (Fig. 29). These are known as _reversed_ faults; and they indicate
that the strata were in a state of lateral compression, the broken ends
of the beds having been pushed horizontally past each other, so that a
vertical line or shaft may intersect the same bed twice, as has been
actually demonstrated in the case of some beds of coal.
[Illustration: Fig. 31.—Explanation of normal faults.]
The usual explanation of normal faults is given in Fig. 31. The inclined
fractures of the earth’s crust must often be converging, bounding, or
enclosing large V-shaped blocks (_A_, _B_). If now, through any cause,
as the folding of the strata, they are brought into a state of tension,
so that the fractures are widened, the V-shaped masses, being
unsupported, settle down, the fractures bounding them becoming normal
faults, as is seen by tracing the bed _X_ through the dislocations. The
single fracture below the block _A_ is inclined, and the stretching has
been accomplished by slipping along it and faulting the bed _Z_ as well
as _X_, the entire section to the right of this fracture being part of a
much larger V-shaped block the right-hand boundary of which is not seen.
But the united fracture below the block _B_ being vertical, any
horizontal movement must widen it into a fissure, which is kept open by
the great wedge above and may become the seat of a dike or mineral vein.
The beds below the V may, in this case, escape dislocation, as is seen
by tracing the bed _Z_ across the fissure. These pairs of converging
normal faults are called _trough_ faults; and this is the only way in
which we can conceive of important faults as terminating at moderate
depths below the surface, and not affecting the entire thickness of the
earth’s crust.
Important reversed faults are believed to occur chiefly along the axes
of overturned anticlines (Fig. 24) where the strata have been broken by
the unequal strains, and those on the upper side shoved bodily over
those on the lower or inverted side.
An extensive displacement of the strata is sometimes accomplished by
short slips along each of a series of parallel fractures, producing a
_step_ fault.
Faults cutting inclined or folded strata are divided into two classes,
according as they are approximately parallel with the direction of the
dip or of the strike. The first are known as _transverse_ or _dip_
faults, and the second as _longitudinal_ or _strike_ faults. The chief
interest of either class consists in their effect upon the outcrops of
the faulted strata, after erosion has removed the escarpment produced by
the dislocation.
[Illustration: Fig. 32.—Plan of a dip fault.]
Dip faults cause a lateral shift or displacement of the outcrops, as
shown in Fig. 32, which represents a plan or map-view of the strata
traversed by the fault _b b_, the down throw being on the right and the
up throw on the left. The dip of the strata is indicated by the small
arrows and the accompanying figures; and it will be observed on tracing
the outcrop of any stratum, _a a_, across the fault that it is shifted
to the right. If the throw of the fault were reversed, the displacement
of the outcrop would be reversed, also. Strike faults are of two kinds,
according as they incline in the same direction as the strata, or in the
contrary direction. The effect of the first kind is to conceal some of
the beds, as shown in Fig. 33, in which beds 5 and 6 do not outcrop, but
we pass on the surface abruptly from 4 to 7. The apparent thickness of
the section is thus less than the real thickness. When the fault
inclines against the strata, on the other hand (Fig. 34), the outcrops
of certain strata are repeated on the surface; and a number of parallel
faults of this kind, a step fault, will, like a series of closed folds
(Fig. 25), cause the apparent thickness of the section to greatly exceed
the real thickness. Repetition of the strata by faulting is
distinguished from repetition by folding by being in the same instead of
the reverse order.
[Illustration: Fig. 33.—Strike fault, concealing strata.]
[Illustration: Fig. 34.—Strike fault, repeating strata.]
Folds and faults are really closely related. In the former the strata
are disturbed and displaced by bending; in the latter by breaking and
slipping; and the displacement which is accomplished by a fold may
gradually change to a fracture and slip. This relation is especially
noticeable with monoclinal folds (Fig. 23), in which the tendency to
shear or break the beds is often very marked.
Important faults are rarely simple, well-defined fractures; but, in
consequence of the enormous friction, the rocks are usually more or less
broken and crushed, sometimes for a breadth of many feet or yards. The
fragments of the various beds are then strung along the fault in the
direction of the slipping, and this circumstance has been made use of in
tracing the continuation of faulted beds of coal. In other cases the
direction of the slip is plainly indicated by the bending of the broken
ends of the strata (Fig. 35), and the beds are sometimes turned up at a
high angle or even overturned in this way.
[Illustration: Fig. 35.—Section of beds distorted by a
fault.]
Since faults are not plane, but undulating and often highly irregular,
fractures, the walls will not coincide after slipping; and if the rocks
are hard enough to resist the enormous pressure, the cavities or
fissures produced in this way may remain open. Now faults are continuous
fractures of the earth’s crust, reaching down to an unknown but very
great depth; and hence they afford the best outlets for the heated
subterranean waters; so that it is common to find an important fault
marked on the surface by a line of springs, and these are often thermal.
The warm mineral waters on their way to the surface deposit part of the
dissolved minerals in the irregular fissures along the fault, which are
thus changed to mineral veins. This agrees with the fact that the walls
of veins usually show faulting as well as crushed rock, slickensides,
and other evidences of slipping.
If the earth’s surface were not subject to erosion, every fault would be
marked on the surface by an escarpment equal in height to the throw of
the fault; and, notwithstanding the powerful tendency of erosion to
obliterate them, these escarpments are sometimes observed, although of
diminished height. Thus, according to Gilbert, the Zandia Mountains in
New Mexico are due to a fault of 11,000 feet, leaving an escarpment
still 7000 feet high. But, as a rule, there is no escarpment or marked
inequality of the surface, the fault, like the fold, not being
distinctly indicated in the topography. In all such cases we must
conclude either that the faults were made a very long time ago, or that
they have been formed with extreme slowness, so slowly that erosion has
kept pace with the displacement, the escarpments being worn away as fast
as formed. These and other considerations make it quite certain that
extensive displacements are not produced suddenly, but either grow by a
slow, creeping motion, or by small slips many times repeated at long
intervals of time.
JOINTS AND JOINT-STRUCTURE.—This is the most universal of all
rock-structures, since all hard rocks and many imperfectly consolidated
kinds, like clay, are jointed. Joints are cracks or planes of division
which are usually approximately vertical and traverse the same mass of
rock in several different directions. They are distinguished from
stratification planes by being rarely horizontal, and from both
stratification and cleavage planes by being actual cracks or fractures,
and by dividing the rock into blocks instead of sheets or layers. The
art of quarrying consists in removing these natural blocks; and most of
the broad flat surfaces of rock exposed in quarries, are the
joint-planes (Fig. 36). Some of the most familiar features of
rock-scenery are also due to this structure, cliffs, ravines, etc.,
being largely determined in form and direction by the principal systems
of joints; and we have already seen that the same is true of veins and
dikes.
Joints are divided by their characteristics and modes of origin into
three classes as follows:—
[Illustration: Fig. 36.—Quarry showing two systems of
parallel joints.]
1. _The parallel and intersecting joints._—This is by far the most
important class, and has its best development in stratified rocks, such
as sandstone, slate, limestone, etc. These joints are straight and
continuous cracks which may often be traced for considerable distances
on the surface. They usually run in several definite directions, being
arranged in sets or systems by their parallelism. Thus in Fig. 36 one
set of joints is represented by the broad, flat surfaces in light, and a
second set crossing the first nearly at right angles, by the narrower
faces in shadow. By the intersections of the different sets of joints
the rock is divided into angular blocks.
Although many explanations of this class of joints have been proposed,
it has long been the general opinion of geologists that they are due to
the contraction of the rocks, _i.e._, that they are shrinkage cracks. We
shall soon see, however, that they lack the most important characters of
cracks known to be due to shrinkage; and the present writer has advanced
the view that movements of the earth’s crust, and especially the swift,
vibratory movements known as earthquakes, are a far more adequate and
probable cause. It is well known that earthquakes break the rocks; and,
if space permitted, it could be shown that the earthquake-fractures must
possess all the essential features of parallel and intersecting joints.
[Illustration: Fig. 37.—Columnar dike.]
2. _Contraction joints or shrinkage cracks._—That many cracks in rocks
are due to shrinkage, there can be no doubt. The shrinkage may result
from the drying of sedimentary rocks; but more generally from the
cooling of eruptive rocks. Every one has noticed in warm weather, the
cracks in layers of mud or clay on the shore, or where pools of water
have dried up; and we have already seen that these sun-cracks are often
preserved in the hard rocks. They have certain characteristic features
by which they may be distinguished from the joints of the first class.
They divide the clay into irregular, angular blocks, which often show a
tendency to be hexagonal instead of quadrangular. The cracks are
continually uniting and dividing, but are not parallel, and rarely cross
each other. Sun-cracks never affect more than a few feet in thickness of
clay, and are an insignificant structural feature of sedimentary rocks.
In eruptive rocks, on the other hand, the contraction joints have a very
extensive, and, in some cases, a very perfect development, culminating
in the prismatic or columnar jointing of the basaltic rocks. This
remarkable structure has long excited the interest of geologists, and,
although the basalt columns were once regarded as crystals, and later as
a species of concretionary structure, it is now generally recognized as
the normal result of slow cooling in a homogeneous, brittle mass. The
columns are normally hexagonal, and perpendicular to the cooling
surface, being vertical in horizontal sheets and lava flows, as in the
classic examples of the Giant’s Causeway and Fingal’s Cave, and
horizontal in vertical dikes (Fig. 37). They begin to grow on the
cooling surface of the mass and gradually extend toward the centre, so
that dikes frequently show two independent sets of columns.
3. _The concentric joints of granitic rocks._—In quarries of granite and
other massive crystalline rocks, it is often very noticeable that the
rock is divided into more or less regular layers by cracks which are
approximately parallel with the surface of the ground, some of the
granite hills having thus a structure resembling that of an onion. The
layers are thin near the surface, become thicker and less distinct
downwards, and cannot usually be traced below a depth of fifty or sixty
feet. These concentric cracks are of great assistance in quarrying, and
are now regarded as due to the expansion of the superficial portions of
the granite caused by the heat of the sun. In reference to this view of
their origin these may be properly called _expansion joints_.
STRUCTURE OF MOUNTAIN-CHAINS.—Mountains are primarily of two
kinds,—volcanic and non-volcanic. The structure of the former belongs
properly with the original structures of the volcanic rocks; but the
latter—the true mountains—owe their internal structure and altitude or
relief almost wholly to the crumpling and mashing together of great
zones of the earth’s crust, being, as already pointed out, the
culminating points of the plication, cleavage, and faulting of the
strata. “A mountain-_chain_ consists of a great plateau or bulge of the
earth’s surface, often hundreds of miles wide and thousands of miles
long. This is usually more or less distinctly divided by great
longitudinal valleys into parallel _ranges_ and _ridges_; and these,
again, are serrated along their crests, or divided into _peaks_ by
transverse valleys. In many cases this ideal chain is far from realized,
but we have instead, a great bulging of the earth’s crust composed on
the surface of an inextricable tangle of ridges and valleys of erosion,
running in all directions. In all cases, however, the erosion has been
immense; for the mountain-chains are the great theatres of erosion as
well as of igneous action. As a general fact, all that we see, when we
stand on a mountain-chain—every peak and valley, every ridge and cañon,
all that constitutes scenery—is wholly due to erosion.”—LE CONTE.
The structure of mountains thus fells under two heads: (1) The internal
structure and altitude, which are due to the action of the subterranean
agencies. (2) The external forms, the actual relief, which are the
product chiefly of the superficial agencies or erosion. The study of
mountains has shown that: (1) They are composed of very thick
sedimentary formations. Thus the sedimentary rocks have a thickness of
40,000 feet in the Alleghanies; of 50,000 feet in the Alps; and of two
to ten miles in all important mountain-chains. Such thick deposits of
sediments, as we have already seen, must be formed on a subsiding
sea-floor, and in many mountain-chains, as in the Alleghanies, the great
bulk of these sediments are still below the level of the sea. Again,
thick sedimentary deposits can only be formed in the shallow, marginal
portions of the sea; and when such a belt of thick shore deposits yields
to the powerful horizontal thrust, and is crumpled and mashed up, it is
greatly shortened in the direction of the pressure and thickened
vertically, so that its upper surface is lifted high above the level of
the sea, and a mountain-chain is formed and added to the edge of the
continent. We thus find an explanation of the important fact that on the
several continents, but notably on the two Americas, the principal
mountain-ranges are near to and parallel with the coast lines.
2. The mountain-forming sediments are usually strongly folded and
faulted, and exhibit slaty cleavage wherever they are susceptible of
that structure; and the older rocks, especially, in mountains are often
highly metamorphosed, and are traversed by numerous veins and dikes, the
infallible signs of intense igneous activity.
“In other words, mountain regions have been the great theatres—(1) of
sedimentation before the mountains were formed; (2) of plication and
upheaval in the formation of the range; and (3) of erosion which
determined the present outline. Add to these the metamorphism, the
faults, veins, dikes, and volcanic outbursts, and it is seen that all
geological agencies concentrate there.”—LE CONTE.
Since mountain-ranges are great up-swellings or bulgings of the strata,
their structure is always essentially anticlinal; and they sometimes
consist of a single more or less denuded anticline (Fig. 38), the oldest
and lowest strata exposed forming the summit of the range. More
commonly, however, the single great arch or uplift is modified by a
series of longitudinal folds, as shown in the section of the Jura
Mountains (Fig. 21). Still more commonly the folds are closely pressed
together, overturned, broken, and almost inextricably complicated by
smaller folds, contortions, and slips.
[Illustration: Fig. 38.—Anticlinal mountain.]
The strata on the flanks of the mountains are usually less disturbed
than those near the axis of the range, and are sometimes seen to rest
unconformably against the latter. In this way it is proved that some
ranges are formed by successive upheavals. But we have still more
conclusive evidence that mountains are formed with extreme slowness in
the fact that rivers sometimes cut directly through important ranges.
This proves, first, that the river is older than the mountains; second,
that the deepening of its channel has always kept pace with the
elevation of the range.
CONCRETIONS AND CONCRETIONARY STRUCTURE.—Folds, cleavage, faults, and
joints—all the subsequent structures considered up to this point—are the
product of mechanical forces. Chemical agencies, although very efficient
in altering the composition and texture of rocks, are almost powerless
as regards the development of rock-structures; and the only important
structure having a chemical origin is that named above.
Concretions are formed by the segregation of one or more of the
constituents of a rock. But there are three distinct kinds of
segregation. If the water percolating through or pervading a rock,
dissolves a certain mineral and afterwards deposits it in cavities or
fissures, _amygdules_, _geodes_, or _veins_ are the result. If the
mineral is deposited about particular points in the mass of the rock, it
may form _crystals_, the rock becoming _porphyritic_; or it may not
crystallize, but build up instead the rounded forms called
_concretions_, the texture or structure of the rock becoming
_concretionary_. A great variety of minerals occur in the form of
concretions, but this mode of occurrence is especially characteristic of
certain constituents of rocks, such as calcite, siderite, limonite,
hematite, and quartz. Concretions may be classified according to the
nature of the segregating minerals; and in each class we may distinguish
the _pure_ from the _impure_ concretions. A pure concretion is one
entirely composed of the segregating mineral. Most nodules of flint and
chert, quartz, geodes, concretions of pyrite, and many hollow iron-balls
are good illustrations of this class. In all these cases the segregating
mineral has been able in some way to remove the other constituents of
the rock, and make room for itself. But in other cases it has lacked
this power, and has been deposited between and around the grains of
sand, clay, etc.; and the concretions are consequently _impure_, being
composed partly of the segregating mineral, and partly of the other
constituents of the rock. The calcareous concretions known as
clay-stones are a good example of this class, being simply discs of
clay, all the minute interstices of which have been filled with
segregated calcite. The solid iron-balls are masses of sand filled in a
similar manner with iron oxides.
Concretions are of all sizes, from those of microscopic smallness in
some oölitic limestones up to those twenty-five feet or more in diameter
in some sandstones.
The point of deposition, when a concretion begins to grow, is often
determined by some concrete particle, as a grain or crystal of the same
or a different mineral, a fragment of a shell, or a bit of vegetation,
which thus becomes the nucleus of the concretion. The ideal or typical
concretion is spherical; but the form is influenced largely by the
structure of the rock. In porous rocks, like sandstone, they are
frequently very perfect spheres; but in impervious rocks, like clay,
they are flat or disc-shaped, because the water passes much more freely
in the direction of the bedding than across it; while the concretions in
limestones, the nodules of flint and chert, are often remarkable for the
irregularity of their forms. In all sedimentary rocks the concretions
are arranged more or less distinctly in layers parallel with the
stratification, which usually passes undisturbed through the impure
concretions. Many silicious and ferruginous concretions are hollow,
apparently in consequence of the contraction of the substance after its
segregation; and the shrinkage due to drying is still further indicated
by the cracks in the septaria stones. The hollow, silicious concretions
are usually lined with crystals (geodes), while the hollow iron-balls
frequently enclose a smaller concretion. Rocks often have a
concretionary structure when there are no distinct or separable
concretions. And the appearance of a concretionary structure
(pseudo-concretions) is often the result of the concentric decomposition
of the rocks by weathering, as explained on page 13.
SUBSEQUENT STRUCTURES PRODUCED BY THE SUPERFICIAL OR AQUEOUS
AGENCIES.—The superficial agencies, as we have seen in the section on
dynamical geology, are, in general terms, water, air, and organic
matter. Geologically considered, the results which they accomplish, may
be summed up under the two heads of deposition and erosion—the formation
of new rocks in the sea, and the destruction of old rocks on the land.
In the rôle of rock-makers they produce the very important original
structures of the stratified rocks; while as agents of erosion they
develop the most salient of the subsequent structures of the earth’s
crust—the infinitely varied relief of its surface. As a general rule, to
which recent volcanoes are one important exception, the original and
subterranean structures of rocks are only indirectly, and often very
slightly, represented in the topography; for this, as we have seen, is
almost wholly the product of erosion. Therefore, what we have chiefly to
consider in this section is to what extent and how erosion is influenced
by the pre-existing structures of rocks.
Horizontal or very slightly undulating strata, especially if the upper
beds are harder than those below, give rise by erosion to flat-topped
ridges or table-mountains (Fig. 39). But if the strata be softer and of
more uniform texture, erosion yields rounded hills, often very steep,
and sometimes passing into pinnacles, as in the Bad Lands of the west.
Broad, open folds, as we have seen, give, normally, synclinal hills and
anticlinal valleys (Fig. 22), when the erosion is well advanced. But in
more strongly, closely folded rocks the ridges and valleys are
determined chiefly by the outcrops of harder and softer strata, as shown
in Fig. 40, the symmetry of the reliefs depending upon the dip of the
strata. This principle of unequal hardness or durability also determines
most of the topographic features in regions of metamorphic and
crystalline rocks, in which the stratification is obscure or wanting.
[Illustration: Fig. 39.—Horizontal strata and
table-mountains.]
[Illustration: Fig. 40.—Ridges due to the outcrops of hard
strata.]
The boldness of the topography, and the relation of depth to width in
valleys, depends largely upon the altitude above the sea; but partly,
also, upon the distribution of the rainfall, the drainage channels or
valleys being narrowest and most sharply defined in arid regions
traversed by rivers deriving their waters from distant mountains. That
these are the conditions most favorable for the formation of cañons is
proved by the fact that they are fully realized in the great plateau
country traversed by the Colorado and its tributaries, a district which
leads the world in the magnitude and grandeur of its cañons. But deep
gorges and cañons will be formed wherever a considerable altitude, by
increasing the erosive power of the streams, enables them to deepen
their channels much more rapidly than the general face of the country is
lowered by rain and frost. This is the secret of such cañons as the
Yosemite Valley, and the gorge of the Columbia River, and probably of
the fiords which fret the north-west coasts of this continent and
Europe. For a full description and illustration of the topographic types
developed by the action of water and ice upon the surface of the land,
and of the various characteristic forms of marine erosion, teachers are
referred to the larger works named in the introduction, especially Le
Conte’s Elements of Geology, and to the better works on physical
geography. We will, in closing this section, merely glance at some of
the minor erosion-forms, which are not properly topographic, but may be
often illustrated by class-room and museum specimens. Mere weathering,
the action of rain and frost, develops very characteristic surfaces upon
different classes of rocks, delicately and accurately expressing in
relief those slight differences in texture, hardness, and solubility,
which must exist even in the most homogeneous rocks. Every one
recognizes on sight the hard, smooth surfaces of water-worn rocks. They
are exemplified in beach and river pebbles, in sea-worn cliffs, and
where rivers flow over the solid ledges. The pot-hole (page 17) is a
well-marked and specially interesting rock-form, due to current or river
erosion.
Ice has also left highly characteristic traces upon the rocks in all
latitudes covered by the great ice-sheet. These consist chiefly of
polished, grooved, and scratched or striated surfaces, the grooves and
scratches showing the direction in which the ice moved.
The organic agencies, as already noted, accomplish very little in the
way of erosion, especially in the hard rocks, but the rock-borings made
by certain mollusks and echinoderms may be mentioned as one unimportant
but characteristic form due to organic erosion.
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