Marine sedimentary rock

“ The sedimentary strata alternating with the lavas on the crest and western side, are of an almost infinitely varying nature; but a large proportion of them closely resemble those already described on the eastern flank: there are white and brown, indurated, easily fusible tuffs,—some passing into pale blue and green semi-porcellanic rocks,—others into brownish and purplish sandstones and gritstones, often including grains of quartz,—others into mudstone containing broken crystals and particles of rock, and occasionally single large pebbles. ”

“ 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. ”

“ Shallow-water deposits often contain fossils of organisms which live in waters of slight depth. These characteristics are sufficient to differentiate sedimentary formations made in shallow water from those made in deep water, even after they have been converted into solid rock and after the rock has emerged from the sea. Many of these characteristics are, however, shared by deposits made by streams on the land. Subaërial and lacustrine sediments are usually distinguishable from those made in the sea by their fossils, and sometimes by their distribution. ”

“ In some of these cases, this foliation followed planes of original stratification, and we are forced to conclude that true sedimentary structure may after all control the features of a gnarled and contorted fundamental gneiss. We are still far from discovering the primitive crust formed about a molten globe, and the brilliant proofs of evolution in the organic world are unmatched by any evidence of the evolution of rock-types during geological time. ”

“ 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. ”

“ With the strata of sedimentary origin are frequently associated igneous rocks, and in many continental regions, as in the majority of oceanic islands, the crust of the earth consists wholly of volcanic material or of rocks produced by the gradual solidification of molten magmas. Rocks composed mainly of carbonate of lime, such as limestones and chalk, bear witness to ocean beds or to sediments deposited on the floors of inland seas beyond the reach of land detritus where coral reefs were reared or the shells and other calcareous skeletons of animals supplied the material for future land. ”

“ In the sedimentary rocks one is apt to find remains of some of the animals and plants that lived at the time the rock was forming. While the soft parts of animals decompose rapidly, shells and bones are likely to be buried in the sediments, and if the conditions have been favorable, these remains may be preserved more or less perfectly. All through the millions of years that sedimentary rocks have been forming in the sea, in lakes, on river flood plains and in wind swept deserts, there was an abundance of life, as much as there is today ”

“ In these parts of our country they form the surface-rock, and whether they have been always raised beyond the reach of sedimentary deposit or have been scoured and relieved by frost and flood of superincumbent strata, whether their present extent is conterminous with their limits, as once revealed above the level of primeval seas, or whether shrunken by subsidence and partially obliterated by later formations, they are at any rate outcrops of the vast bedding on which ocean and continent alike repose. ”

“ In the same manner as aqueous and fossiliferous strata are now formed in certain seas or lakes, while in other places volcanic rocks break out at the surface, and are connected with reservoirs of melted matter at vast depths in the bowels of the earth, so, at every era of the past, fossiliferous deposits and superficial igneous rocks were in progress contemporaneously with others of subterranean and Plutonic origin, and some sedimentary strata were exposed to heat, and made to assume a crystalline or metamorphic structure. ”

“ It is, moreover, liable to wide-spread inroads of the sea, which sometimes covers cultivated districts to a depth of several feet, laying waste the fields and destroying the inhabitants. These and other features help us to understand the origin of such vast masses of sedimentary strata as those of our coal-fields, where terrestrial, fluviatile, and marine remains alternate in rapid sequence, or sometimes occur together.
The origin of the constant succession of coal seams, sandstones, and shales, of the Edge series may be thus accounted for. ”

“ 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. ”

“ Ample proofs of these reiterated revolutions will be given in the sequel, and it will be seen that many distinct sets of sedimentary strata, each several hundreds or thousands of feet thick, are piled one upon the other in the earth's crust, each containing peculiar fossil animals and plants which are distinguishable with few exceptions from species now living. The mass of some of these strata consists almost entirely of corals, others are made up of shells, others of plants turned into coal, while some are without fossils. In one set of strata the species of fossils are marine ”

“ Since the conditions of deposition may remain constant for a long time in open seas, and since stratification cannot be obvious until these conditions change, limestones may have a massive character that is exceptional among sedimentary rocks. ”

“ Now, the sandstones, clays, gravels, and pebble-beds all represent, as will presently be explained, so much material worn away from the surface of the land and swept into the ocean (or in some cases into inland seas and lakes) by streams and rivers, which are the great transporting agents of the world. Hence such deposits of débris, supplied by the constant wear and tear of all rocks exposed to the atmosphere, are truly sedimentary and have a purely mechanical origin. ”

“ A series of specimens may be collected showing unaltered or at least quite recognizable sedimentry rocks at the one end, and thoroughly crystalline igneous rocks at the other. Thus the remarkable fact is brought home to the mind that ordinary sandstones, shales, and other sedimentary materials may in the course of ages be converted by underground changes into crystalline granite. The framework of the land, besides being knit together by masses of igneous rock intruded from below, has been strengthened by the welding and crystallization of its lowest rocks. ”

“ No part of the results obtained by the Challenger Expedition has a profounder interest for geologists and geographers than the proof which they furnish that the floor of the ocean-basins has no real analogy among the sedimentary formations which form most of the framework of the land. We now know by actual dredging and inspection that the ordinary sediment washed off the land sinks to the sea-bottom before it reaches the deeper abysses, and that, as a rule, only the finer particles are carried more than a few score of miles from the shore. ”

“ Beneath the current of the river, and at the bottom of a lake or sea, these sediments (mud, etc.) accumulate one on the top of the other in regular series called strata, and then the weight and pressure acting with the soluble mineral deposits always washing down, consolidate and bind the loose sand-grains into stone, which, in the course of ages, hardens. The stones thus formed from sediments such as gravel, mud, and sand, are termed Sedimentary Rocks; they have become rocks by enormous and continuous pressure. ”

“ It is true that at each oscillation of level the solid rocks composing the original continent suffer some fresh denudation, and do not remain unimpaired like the wooden and glass framework of the hour-glass, still the wear and tear suffered by the larger area exposed to subaërial denudation consists either of loose drift or of sedimentary strata, which were thrown down in seas near the land, and subsequently upraised, the same continents and oceanic basins remaining in existence all the while. ”

“ That the sea was of very early Paleozoic (i.e., Cambrian) age has been proved by fossils in associated strata. Long after their deep burial and consolidation within the earth, the strata were subjected to tremendous mountain-making pressure, notably altered to a rock called “Quartzite,” raised high above sea level, and tilted almost vertically. Then through long ages (millions of years) overlying rocks of great thickness have been cut away (eroded) by weathering and stream action, laying bare the ledge as we see it to-day. ”

“ Now, limestones are not formed in the same way in which we have seen that sandstones and shales are formed. The last two mentioned owe their origin to their deposition as sediment in seas, estuaries or lakes, but the masses of limestone which are found in the various geological formations owe their origin to causes other than that of sedimentary deposition.
In carboniferous times there lived numberless creatures which we know nowadays as encrinites. These, when growing, were fixed to the bed of the ocean, and extended upward in the shape of pliant stems composed of limestone joints or plates ”

“ In like manner the beds of superficial sand, clay, and gravel, with recent shells, on the coasts of Norway and Sweden, where the land has risen in Post-tertiary times, are so thin and scanty as to incline us to admit a similar proposition. We may in fact assume that in all cases where the bottom of the sea has been undergoing continuous elevation, the total thickness of sedimentary matter accumulating at depths suited to the habitation of most of the species of shells can never be great, nor can the deposits be thickly covered by superincumbent matter, so as to be consolidated by pressure. ”

“ It is also observed that strata, in proportion as they are of newer date, bear the nearest resemblance in mineral character to those which are now in the progress of formation in seas or lakes, the newest of all consisting principally of soft mud or loose sand, in some places full of shells, corals, or other organic bodies, animal or vegetable, in others wholly devoid of such remains. The farther we recede from the present time, and the higher the antiquity of the formations which we examine, the greater are the changes which the sedimentary deposits have undergone. ”

“ Evidently the development of sediments from igneous rocks has involved the destruction of much of the feldspars and ferro-magnesian silicates, and the building from the elements of these destroyed minerals of more quartz, white mica, clay, dolomite, calcite, chlorite and limonite. The composition of the minerals of the sedimentary rocks is such as to indicate that the constituents of the air and water have been added in important amounts to accomplish this change of mineral character. ”

“ The thickness of the Mississippian deposit indicated that at this time the earth had again sunk beneath the waters to a considerable depth. The shallow sea which had not offered sediment to a greater depth than a few feet was replaced by active currents which carried heavier sedimentary materials from great distances, laying them down on the floor of the sea in various strata to a depth of several hundred feet. Finally, after an unknown number of millions of years, but perhaps during the 24 Triassic period, the land again rose above the level of the waters. ”

“ Rivers would begin to arise, and they would carry on the work of erosion. Other causes tending to break up the rocks would be the gases in the air—the excessive quantities of carbonic acid and oxygen would be active chemical agents in this work. Before the close of this era the limestones and iron carbonates began to form; sediments arose in the lifeless oceans, and thus began the first formation of those sedimentary rocks and strata which have been dealt with in the earlier chapters. ”

“ Thus, as the current sweeps along in its course, the sediment which it bears away from the land will be in a manner sorted, and three distinct layers of different materials will be deposited in the bed of the ocean;—first, nearest to the shore, a layer of pebbles and coarse gravel, then a layer of sand, and last of all a layer of fine mud or clay. This is the first step in the construction of stratified rock. To complete the work nothing more is necessary than the consolidation of these loose and incoherent materials. ”