Sedimentary rock

“ 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 ”

“ They cannot be derived from the underlying rock by weathering, but have been brought from elsewhere. Thus where a region is underlain by sedimentary rocks, as is the drift-covered area from the Hudson River to the Missouri, the drift contains not only fragments of limestone, sandstone, and shale of local derivation, but also pebbles of many igneous and metamorphic rocks, such as granites, gneisses, schists, dike rocks, quartzites, and the quartz of mineral veins, whose nearest source is the Archean area of Canada and the states of our northern border. ”

“ All igneous and sedimentary rocks, and earlier-formed metamorphic rocks too, can be changed, without being moved to some other place, 13 into new and different rocks. As they are changed, they may become harder, new minerals may form, and they may look entirely different. For example, granite, an igneous rock, can be changed into the metamorphic rock known as gneiss; limestone, a sedimentary rock, can be changed into marble; shale, a sedimentary rock, can be changed into slate. These changes occur because the earth is a big and complex chemical system. ”

“ These sediments were buried beneath thousands of feet of other sediments, cemented and hardened into layers of sedimentary rock and later squeezed, crushed, and elevated by slow, ceaselessly working earth forces that produced mountains even in that ancient time. ”

“ But they are always intimately connected, as their mineral composition indicates, with the rock mass in which they lie, and they represent merely the last part of the magma to consolidate. The fissures they occupy are presumably due to contraction, seeing that they are not accompanied by displacement, brecciation or faulting.
Veins of sedimentary rock are few and of little importance. They occur where sediment has gathered in cavities of other rocks. ”

“ Concretionary structure.—Various sedimentary formations contain nodules or irregularly shaped masses of mineral matter unlike the rock in which they occur. When these nodules consist of matter aggregated about some center, they are called concretions. They are common in sedimentary rocks, and here it may sometimes be seen that the aggregation has taken place about a shell, a leaf, or some other organic relic. The nuclei are, however, not always organic. ”

“ 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. ”

“ Sometimes a huge mass of crystalline rock, glowing from the furnaces of the deep, has, by its irresistible expansion, slowly forced its way through the superincumbent sedimentary formations, which, yielding to the pressure from below, now form vast mountain slopes, or vertical rock walls, or have even been so totally inverted that strata of a more ancient formation now overlie those of more modern date, and excite the wonder of the puzzled geologist. ”

“ Very early in the science of geology it was recognized that in many places one can tell the comparative ages of rocks by their relations to one another. For example, most sedimentary rocks are consolidated accumulations of large or small rock fragments and were deposited as nearly horizontal 46 layers of gravel, sand, or mud. ”

“ 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. ”

“ Particles of quartz, kaolin, and lime, separately, or mixed, loose or more or less cemented, with accompanying impurities, make up the great bulk of the sedimentary rocks. They are usually arranged in layers, of varying thickness, as they were laid down by water or the wind. In the same way layered accumulations which are either products of plants or animals, or parts of the plants or animals, are considered sedimentary, as for instance, coal, chalk, petroleum, etc. ”

“ 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. ”

“ 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. ”

“ It would be most evident that the mountains consist of strata, that is sedimentary rocks, such as sandstone, shale and limestone, which were deposited under water. A few measurements would reveal the fact that thousands of feet in thickness of strata are represented. Careful measurements by geologists have, in fact, shown that the strata were originally piled up layer upon layer to a thickness of 25,000 to 30,000 feet. The fact that they are strata of such great thickness proves that sediments must there have accumulated under water for some millions of years at least. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ Great absorption48 and slow conduction favor disruption. A given range of temperature is unequally effective on rocks of different mineral composition. In general crystalline rocks (igneous and metamorphic) are more subject to disruption by this means than sedimentary rocks, partly because they are more compact, but especially because they are made up of aggregates of crystals of different minerals which, under changes of temperature, expand and contract at different rates, while the common sedimentary rocks are made up largely of numerous particles of one mineral. ”

“ 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. ”

“ Economic Importance of the Sedimentary Terranes.—The sedimentary rocks are composed principally of fragmental material derived from the disintegration of older rocks transported and deposited mechanically, and resulting in the formation of sandstone, shale, etc., and of organically concentrated material, such as shells and corals, which form limestones. The deposits originating in these ways furnish excellent building stones, the principal classes being sandstones and limestones. These occur widely throughout North America, and in formations of all ages subsequent to the Archean. ”

“ The system has been somewhat improved since its earliest use, and one of its latest applications gives us an age, for known sedimentary rocks, of nearly half a billion years, this being based on a total thickness of one hundred miles and an average rate of 880 years for the building up of one foot of sediments. Its greatest weakness is due to the absence of a reliable factor to take care of long stretches of time in which the sedimentary rocks are known to have been subjected to destructive processes. ”

“ The warm waters farthest from the pole would bear only the stable constituents brought down by the Arctic current, so that, to continue our comparison, the river, which has gradually eaten away the granite block, finally transports the last remains of the rock in the form of sands and clays, which are destined to accumulate in the sedimentary deposits. ”

“ Our deepest wells and mines go down about a mile, then digging stops, on account of the excessive heat. But the crumpling of the crust, and the wearing away of the folded strata by wind and running water, have laid bare rocks several miles in thickness on the slopes of mountains, and exposed the underlying granite, on which the first sedimentary rocks were deposited. ”

“ Where deposited under water, soil materials may be brought above the water by physiographic changes, and exposed at the surface in condition for immediate use. Or, they may become buried by other sediments and not be exposed again until after they have been pretty well hardened and cemented,—in which case they must again undergo the softening processes of weathering before they become available for use. Where soils become buried under other rocks and become hardened, they are classed as sedimentary rocks and form a part of the geologic record. ”

“ 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. ”

“ If we approximate the average composition of each group and the average composition of the igneous rocks from which they are ultimately derived, it can be calculated that sedimentary rocks must form in the proportions of 82 per cent shale, 12 per cent sandstone, and 6 per cent limestone. ”