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.

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.

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:—

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.

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.

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).