THE ROCKS OF THE EARTH’S SURFACE SHELL
=The processes by which rocks are formed.=—Rocks may be formed in
any one of several ways. When a portion of the molten lithosphere,
so-called _magma_, cools and consolidates, the product is _igneous_
rock. Either igneous or other rock may become disintegrated at the
earth’s surface, and after more or less extended travel, either in the
air, in water, or in ice, be laid down as a sediment. Such sediments,
whether cemented into a coherent mass or not, are described as
_sedimentary_ or _clastic_ rocks. If the fluid from which they were
deposited was the atmosphere, they are known as _subaërial_ or _eolian_
sediments; but if water, they are known as _subaqueous_ deposits. Still
another class are ice-deposited and are known as _glacial_ deposits.
[Illustration:
FIG. 16.—Laminated structure of sedimentary rock, Western Kansas
(after a photograph by E. S. Tucker).]
But, as we have learned, rocks may undergo transformations through
mineral alteration, in which case they are known as _metamorphic_
rocks. When these changes consist chiefly in the production of
more soluble minerals at the surface, accompanied by thorough
disintegration, due to the direct attack of the atmosphere, the
resulting rocks are called _residual_ rocks.
=The marks of origin.=—Each of the three great classes of rocks,
the igneous, sedimentary, and metamorphic, is characterized by both
coarser and finer structures, in the examination of which they may be
identified. The igneous rocks having been produced from magmas, which
are essentially homogeneous, are usually without definite directional
structures due to an arrangement of their constituents, and are said
to have a _massive_ structure. Sedimentary rocks, upon the other hand,
have been formed by an assorting process, the larger and heavier
fragments having been laid down when there was high velocity of either
wind or water current, and the smaller and lighter fragments during
intermediate periods. They are therefore more or less banded, and are
said to have a _bedded_ or _laminated_ structure (Fig. 16).
Again, igneous rocks, being due to a process of crystallization, are
composed of mineral individuals which are bounded either by crystal
planes or by irregular surfaces along which neighboring crystals have
interfered with each other; but in either case the grains possess
sharply angular boundaries. Quite different has been the result of
the attrition between grains in the transportation and deposition of
sediments, for it is characteristic of the sedimentary rocks that their
constituent grains are well rounded. Eolian sediments have usually more
perfectly rounded grains than subaqueous deposits.
Glacial deposits, if laid down directly by the ice, are unstratified,
relatively coarse, and contain pebbles which are both faceted
and striated. Such deposits are described as till or tillite. If
glacier-derived material is taken up by the streams of thaw water and
is by them redeposited, the sediments are assorted or stratified, and
they are described as _fluvio-glacial_ deposits.
=The metamorphic rocks.=—Both the coarser structures and the finer
textures of the metamorphic rocks are intermediate between those of
the igneous and the sedimentary classes. A metamorphosed sedimentary
rock, in proportion to its alteration, loses the perfect lamination
and the rounded grain which were its distinguishing characters; while
an igneous rock takes on in the process an imperfect banding, and
the sharp angles of its constituent grains become rounded off by a
sort of peripheral crushing or granulation. Metamorphic rocks are
therefore characterized by an imperfectly banded structure described
as _schistosity_ or _gneiss banding_, and the constituent grains may
be either angular or rounded. If the metamorphism has not been too
intense or too long continued, it is generally possible to determine,
particularly with the aid of the polarizing microscope, whether
the original rock from which it was derived was of igneous or of
sedimentary origin. There are, however, many examples which have defied
a reliable verdict concerning their origin.
=Characteristic textures of the igneous rocks.=—In addition to the
massiveness of their general aspect and the angular boundaries of
their constituents, there are many additional textures which are
characteristic of the igneous rocks. While those that have consolidated
below the earth’s surface, the _intrusive_ rocks, are notably compact,
the magmas which arrive at the surface of the lithosphere before their
consolidation reveal special structures dependent either upon the
expansion of steam and other gases within them, or upon the conditions
of flow over the earth’s surface. Magmas which thus reach the surface
of the earth are described as _lavas_, and the rocks produced by their
consolidation are _extrusive_ or _volcanic_ rocks. The steam included
in the lava expands into bubbles or vesicles which may be large or
small, few or many. According to the number and the size of these
cavities, the rock is said to have a _vesicular_, _scoriaceous_, or
_pumiceous_ texture.
Most lavas, when they arrive at the earth’s surface, contain crystals
which are more or less disseminated throughout the molten mass. The
tourist who visits Mount Vesuvius at the time of a light eruption
may thrust his staff into the stream of lava and extract a portion
of the viscous substance in which are seen beautiful white crystals
of the mineral leucite, each bounded by twenty-four crystal faces.
It is clear that these crystals must have developed by a slow growth
within the magma while it was still below the surface, and when the
inclosing lava has consolidated, these earlier crystals lie scattered
within a _groundmass_ of glassy or minutely crystalline material.
This scattering of crystals belonging to an earlier generation within
a groundmass due to later consolidation is thus an indication of
interruption in the process of crystallization, and the texture which
results is described as _porphyritic_ (Fig. 17 _b_). Should the lava
arrive at the surface before any crystals have been generated and
consolidate rapidly as a rock glass, its texture is described as
_glassy_ (Fig. 17 _c_).
When the crystals of the earlier generation are numerous and
needle-like in form, as is very often the case, they arrange themselves
“end on” during the rock flow, so that when consolidation has occurred,
the rock has a kind of puckered lamination which is the characteristic
of the _fluxion_ or _flow_ texture. This texture has sometimes been
confused with the lamination of the sedimentary rocks, so that wrong
conclusions have been reached regarding origin. At other times the same
needle-like crystals within the lava have grouped themselves radially
to form rounded nodules called spherulites. Such nodules give to the
rock a _spherulitic_ texture, which is nowhere better displayed than
in the beautiful glassy lavas of Obsidian Cliff in the Yellowstone
National Park.
[Illustration:
FIG. 17.—Characteristic textures of igneous rocks. _a_, granitic
texture characteristic of the deep-seated intrusive rocks; _b_,
porphyritic texture characteristic of the extrusive and of the
near-surface intrusive rocks; _c_, glassy texture of an extrusive rock.]
Those intrusive rocks which consolidate deep below the earth’s surface,
part with their heat but slowly, and so the process of crystallization
is continued without interruption. Starting from many centers,
the crystals continue to grow until they mutually intersect in an
interlocking complex known as the _granitic_ texture (Fig. 17 _a_).
=Classification of rocks.=—In tabular form rocks may thus be
classified as follows:—
{_Intrusive._ Granitic or porphyritic
{ texture.
_Igneous._ Massive and {_Extrusive._ Glassy or porphyritic
with sharply angular { texture; often also with vesicular,
grains. { scoriaceous, pumiceous, fluxion,
{ or spherulitic textures.
{_Subaërial._ Sands and loess.
{_Subaqueous._ (See below.)
_Sedimentary._ Laminate { _Glacial._ Coarse, unstratified
and with rounded { deposits with faceted pebbles. Till
grains. { and tillite.
{_Fluvio-glacial._ Stratified sands
{ and gravels with “worked over”
{ glacial characters.
_Metamorphic._ Schistose {_Metamorphic proper._ Due to below
and with grains either { surface changes.
angular or rounded. {_Residual._ Disintegrated at or near
{ surface.
=Subdivisions of the sedimentary rocks.=—While the eolian sediments
are all the product of a purely mechanical process of lifting,
transportation, and deposition of rock particles, this is not always
the case with the subaqueous sediments, since water has the power of
dissolving mineral substance, as it has also of furnishing a home
for animal and vegetable life. Deposited materials which have been
in solution in water are described as _chemical_ deposits, and those
which have played a part in the life process as _organic_ deposits. The
organic deposits from vegetable sources are peat and the coals, while
limestones and marls are the chief depositories of the remains of the
animal life of the water. The tabular classification of the sediments
is as follows:—
_Classification of Sediments._
{ _Subaqueous_ Conglomerate, sandstone
{ Deposited by water. and shale.
{ _Subaërial_ or _Eolian_ Sandstone and loess.
_Mechanical_ { Deposited by wind.
{ _Glacial_ Till and tillite.
{ Deposited by ice.
{ _Fluvio-glacial_ Sands and gravels.
{ Glacier-water deposits.
{ Calcareous tufa Deposited in springs
{ and rivers.
_Chemical_ { Oölitic limestone Deposited at the
{ mouths of rivers
{ between high and
{ low tide.
{ Formed of plant remains. Peats and coals.
_Organic_ { Formed of animal remains. Limestones and
{ marls.
Winds are under favorable conditions capable of transporting both dust
and sand, but not the larger rock fragments. The dust deposits are
found accumulating outside the borders of deserts as the so-called
_loess_ (Fig. 216), though the sand is never carried beyond the desert
border, near which it collects in wide belts of ridges described as
dunes. When this sand has been cemented into a coherent mass, it is
known as eolian sandstone. A section of the appendix (B) is devoted to
an outline description of some of the commoner rock types.
=The different deposits of ocean, lake, and river.=—Of the
subaqueous sediments, there are three distinct types resulting: (1)
from sedimentation in rivers, the _fluviatile_ deposits; (2) from
sedimentation in lakes, the _lacustrine_ deposits; and (3) from
sedimentation in the ocean, _marine_ deposits. Again, the widest
range of character is displayed by the deposits which are laid down
in the different parts of the course of a stream. Near the source of
a river, coarse river gravels may be found; in the middle course the
finer silts; and in the mouth or delta region, where the deposits
enter the sea or a lake, there is found an assortment of silts and
clays. Except within the delta region, where the area of deposition
begins to broaden, the deposits of rivers are stretched out in long and
relatively narrow zones, and are so distinguished from the far more
important lacustrine and marine deposits.
Lakes and oceans have this in common that both are bodies of standing
as contrasted with flowing water; and both are subject to the
periodical rhythmic motions and alongshore currents due to the waves
raised by the wind. About their margins, the deposits of lake and ocean
are thus in large part wrested by the waves from the neighboring land.
Their distribution is always such that the coarsest materials are laid
down nearest to the shore, and the deposits become ever finer in the
direction of deeper water. Relatively far from shore may be found the
finest sands and muds or calcareous deposits, while near the shore are
sands, and, finally, along the beach, beds of beach pebbles or shingle.
When cemented into coherent rocks, these deposits become shales or
limestones, sandstones, and conglomerates, respectively.
As regards the limestones, their origin is involved in considerable
uncertainty. Some, like the shell limestone or coquina of the Florida
coast, are an aggregation of remains of mollusks which live near
the border of the sea. Other limestones are deposited directly from
carbonate of lime in solution in the water. A deposit of this nature is
forming in southern Florida, both as a flocculent calcareous mud and as
crystals of lime carbonate upon a limestone surface. Again, there is
the reef limestone which is built up of the stony parts of the coral
animal, and, lastly, the calcareous ooze of the deep-sea deposits.
The marine sediments which are derived from the continents, the
so-called _terrigenous_ deposits, are found only upon the continental
shelf and upon the continental slope just outside it. Of these
terrigenous deposits, it is customary to distinguish: (1) _littoral_
or alongshore deposits, which are laid down between high and low tide
levels; (2) _shoal water_ deposits, which are found between low-water
mark and the edge of the continental shelf; and (3) aktian or offshore
deposits, which are found upon the continental slope. The littoral and
shoal water deposits are mainly gravels and sands, while the offshore
deposits are principally muds or lime deposits.
=Special marks of littoral deposits.=—The marks of ripples are often
left in the sand of a beach, and may be preserved in the sandstone
which results from the cementation of such deposits (pl. 11 A). Very
similar markings are, however, quite characteristic of the surface
of wind-blown sand. For the reason that deposits are subject to many
vicissitudes in their subsequent history, so that they sometimes stand
at steep angles or are even overturned, it is important to observe the
curves of sand ripples so as to distinguish the upper from the lower
surface.
In the finer sands and muds of sheltered tidal flats may be preserved
the impressions from raindrops or of the feet of animals which have
wandered over the flat during an ebb tide. When the tide is at flood,
new material is laid down upon the surface and the impressions are
filled, but though hardened into rock, these surfaces are those upon
which the rock is easily parted, and so the impressions are preserved.
In the sandstones of the Connecticut valley there has been preserved
a quite remarkable record in the footprints of animals belonging to
extinct species, which at the time these deposits were laid down must
have been abundant upon the neighboring shores.
Between the tides muds may dry out and crack in intersecting lines
like the walls of a honeycomb, and when the cracks have been filled at
high tide, a structure is produced which may later be recognized and
is usually referred to as “mud-crack” structure. This structure is of
special service in distinguishing marine deposits from the subaërial or
continental deposits.
A variation in the direction of winds of successive storms may be
responsible for the piling up of the beach sand in a peculiar “plunge
and flow” or “cross-bedded” structure, a structure which is extremely
common in littoral deposits, though simulated in rocks of eolian origin.
=The order of deposition during a transgression of the sea.=—Many
shore lines of the continents are almost constantly migrating either
landward or seaward. When the shore line advances over the land, the
coast is sinking, and marine deposits will be formed directly above
what was recently the “dry land.” Such an invasion of the land by
the sea, due to a subsidence of the coast, is called a transgression
of the sea, or simply a _transgression_. Though at any moment the
littoral, shoal water, and offshore deposits are each being laid down
in a particular zone, it is evident that each must advance in turn in
the direction of the shore and so be deposited above the zones nearer
shore. Thus there comes to be a definite series of continuous beds,
one above the other, provided only that the process is continued (Fig.
18). At the very bottom of this series there will usually be found a
thin bed of pebbly beach materials, which later will harden into the
so-called _basal conglomerate_. If the size of the pebbles is such as
to make possible an identification, it will generally be found that
these represent the ruins of the rock over which the sea has advanced
upon the land.
[Illustration: FIG. 18.—Diagram to show the order of the sediments
laid down during a transgression of the sea.]
Next in order above the basal conglomerate, will follow the coarser and
then the finer sands, upon which in turn will be laid down the offshore
sediments—the muds and the lime deposits. Later, when cemented
together, these become in order, coarser and finer sandstones, shales,
and limestones. The order of superposition, reading from the bottom to
the top, thus gives the order of decreasing age of the formations.
A subsequent uplift of the coast will be accompanied by a recession
of the sea, and when later dissected by nature for our inspection,
the order of superposition and the individual character of each of
the deposits may be studied at leisure. From such studies it has been
found that along with the inorganic deposits there are often found the
remains of life in the hard parts of such invertebrate animals as the
mollusks and the crustacea. These so-called _fossils_ represent animals
which were gradually developed from simpler to more and more complex
forms; and they thus serve the purpose of successive page numbers in
arranging the order of disturbed strata, at the same time that they
supply the most secure foundation upon which rests the great doctrine
of evolution.
=The basins of earlier ages.=—It was the great Viennese geologist,
Professor Suess, who first pointed out that in mountain regions there
are found the thickest and the most complete series of the marine
deposits; whereas outside these provinces the formations are separated
by wide gaps representing periods when no deposits were laid down
because the sea had retired from the region. The completeness of the
series of deposits in the mountain districts can only be interpreted to
mean that where these but lately formed mountains rise to-day, were for
long preceding ages the basins for deposition of terrigenous sediments.
It would seem that the lithosphere in its adjustment had selected these
earlier sea basins with their heavy layers of sediment for zones of
special uplift.
=The deposits of the deep sea.=—Outside the continental slope, whose
base marks the limit of the terrigenous deposits, lies the deeper sea,
for the most part a series of broad plains, but varied by more profound
steep-walled basins, the so-called “deeps” of the ocean. As shown by
the dredgings of the _Challenger_ expedition and others of more recent
date, the deposits upon the ocean floor are of a wholly different
character from those which are derived from the continents. Except in
the great deeps, or between depths of five hundred and fifteen hundred
fathoms, these deposits are the so-called “ooze”, composed of the
calcareous or chitinous parts of algæ and of minute animal organisms.
The pelagic or surface waters of the ocean are, as it were, a great
meadow of these plant forms, upon which the minute crustacea, such
as globigerina, foraminifera, and the pteropods, feed in countless
myriads. The hard parts of both plant and animal organisms descend to
the bottom and there form the ooze in which are sometimes found the ear
bones of whales and the teeth of sharks.
In the deeps of the ocean, none of these vegetable or animal deposits
are being laid down, but only the so-called “red clay”, which is
believed to represent decomposed volcanic material deposited by the
winds as fine dust on the surface of the ocean, or the product of
submarine volcanic eruption. From the absence of the ooze in these
profound depths, the conclusion is forced upon us that the hard parts
of the minute organisms are dissolved while falling through three or
four miles of the ocean water.
READING REFERENCES FOR CHAPTER IV
J. S. DILLER. The Educational Series of Rock Specimens collected and
distributed by the United States Geological Survey, Bull. 150 U. S.
Geol. Surv., 1898, pp. 1-400.
L. V. PIRSSON. Rocks and Rock Minerals. Wiley, New York, 1908.
SIR JOHN MURRAY. Deep-sea Deposits, Reports of the _Challenger_
expedition, Chapter iii.
L. W. COLLET. Les dépôts marins. Doin, Paris, 1907 (Encyclopédie
Scientifique).