Cast (geology)

“ All the fossiliferous formations may be studied by the geologist in two distinct points of view: first, in reference to their position in the series, their mineral character and fossils; and, secondly, in regard to their physical geography, or the manner in which they now enter, as mineral masses, into the external structure of the earth; forming the bed of lakes and seas, or the surface and foundation of hills and valleys, plains and table-lands. Some account has already been given on the first head of the Tertiary, the Cretaceous, and Wealden strata ”

“ In surveying the different systems of strata in succession, we readily perceive that, independent of the local influence of particular masses of igneous rocks, whose influence extends only a few yards at most from their bounding surfaces, the formations of different ages are unequally indurated,—the oldest being by far the most consolidated, while the newest appear but little harder than the analogous deposits which at this day are known to be produced in freshwater lakes, at the mouths of rivers, on the sea coast, or on the bed of the ocean. ”

“ We must inquire to what regions the ruins of the white chalk, greensand, Wealden, and other strata which have disappeared were transported. We are then led at once to the examination of all the deposits newer than the chalk, and first to the oldest of these, the Lower Eocene, and its sand, shingle, and clay. In them, so largely developed in the immediate neighbourhood of the denuded area, we discover the wreck we are in search of, regularly stratified, and inclosing, in some of its layers, organic remains of a littoral, and sometimes fluviatile character. What more can we desire? ”

“ A species which has once died out does not seem ever to have reappeared.
4. When the order of succession of organic remains among the stratified rocks has been determined, they become an invaluable guide in the investigation of the relative age of rocks and the structure of the land. Each zone and formation, being characterized by its own species or genera, may be recognized by their means, and the true succession of strata may thus be confidently established even in a country wherein the rocks have been shattered by dislocation, folded, inverted or metamorphosed. ”

“ Where the formation is extensive, or covers a large area, as the new red sandstones and coal measures do in this country,—the mountain limestones in Ireland, and the granites in the north of Scotland—the transported sand, gravel, or clay, strewed over one part of the formation, has not unfrequently been derived from the rocks of another part of the same formation, so that, after all, the soils may be said to be produced from the rocks on which they rest, and may be judged of from the known constitution of these rocks. ”

“ Yet many geologists, when they behold the spoils of the land heaped in successive strata, and blended confusedly with the remains of fishes, or interspersed with broken shells and corals; when they see portions of erect trunks of trees with their roots still retaining their natural position, and one tier of these preserved in a fossil state above another, imagine that they are viewing the signs of a turbulent instead of a tranquil and settled state of the planet. ”

“ It is a softer stone than marble, and more easily wrought.
Forms of stratification.—A series of strata sometimes consists of one of the above rocks, sometimes of two or more in alternating beds. Thus, in the coal districts of England, for example, we often pass through several beds of sandstone, some of finer, others of coarser grain, some white, others of a dark colour, and below these, layers of shale and sandstone or beds of shale, divisible into leaf-like laminæ, [p.14] and containing beautiful impressions of plants. ”

“ Many, indeed most, of these strata contain the remains of animals which were living in the water at or before the time of the deposition of the rocks, and several are full of plants which were swept down from the dry land on which they grew into the ancient ocean, and then entombed in the strata at that epoch in progress of formation. ”

“ That they should retain such markings when again upraised above the sea ought not to surprise us, when we remember that rippled sands, and the cracks in clay dried between high and low water, and the foot-tracks of animals and rain-drops impressed on mud, and other superficial markings, are all found fossil in rocks of various ages. ”

“ This kind of observation will enable us to find proofs everywhere that the surface of the land has not always been what it is to-day. In some districts, for example, when the dark layer of soil in which vegetables grow is turned up, there may be found beneath it sand and gravel full of smooth, well-rounded stones. Such materials are to be seen in course of formation where water keeps them moving to and fro, as on the beds of rivers, the margins of lakes, or the shallow shores of the sea. ”

“ No reasonable doubt can therefore remain that the loose beds of gravel, sand, and clay, which, as we have already seen, are deposited from day to day, and from year to year, and from century to century, beneath the waters of the ocean, may be converted in the course of time by natural agents into solid rocks of Conglomerate, of Sandstone, and of Shale. ”

“ At one level, for example, univalve shells of some one or more species predominate; at another, bivalve shells; and at a third, corals; while in some formations we find layers of vegetable matter, commonly derived from land plants, separating strata.
It may appear inconceivable to a beginner how mountains, several thousand feet thick, can have become full of fossils from top to bottom; but the difficulty is removed, when he reflects on the origin of stratification, as explained in the last chapter, and allows sufficient time for the accumulation of sediment. ”

“ Such a system of classification is not only recommended by its greater clearness and facility of application, but is also best fitted to strike the imagination by bringing into one view the contemporaneous revolutions of the inorganic and organic creations of former times. For the sedimentary formations are most readily distinguished by the different species of fossil animals and plants which they inclose, and of which one assemblage after another has flourished and then disappeared from the earth in succession. ”

“ They are found to constitute a very large part, perhaps we might say the larger part, of the stratified rocks in every country that has hitherto been explored by Geologists. Wherever we go we are met by the same familiar appearances;—beds of Conglomerate, Sandstone, Clay, Marl, Shale, recurring again and again through a series of many hundred strata, sometimes in one order, and sometimes in another; sometimes without any formation of a different kind intervening, and sometimes alternating with limestone or other rocks of which we shall speak hereafter. ”

“ But it is mainly by the remains of plants and animals imbedded in the rocks that the geologist is guided in unravelling the chronological succession of geological changes. He has found that a certain order of appearance characterizes these organic remains, that each great group of rocks is marked by its own special types of life, and that these types can be recognized, and the rocks in which they occur can be correlated even in distant countries, and where no other means of comparison would be possible. ”

“ We find the fossil bones of some of the dwellers in these woods; the rocks also retain their footprints. The great creatures, marching over the soft earth and mud, made deep tracks, and these remained like casts or moulds as the soil hardened; then sand and other sediment were deposited over them, and so the footprints were encased. The deposit, hardened into rock, has preserved the footprints for thousands of years. Strange that a thing so fleeting as a footprint on sand should remain to tell a story for ages! ”

“ 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. ”

“ To a geologist the most important river deposits are those which gather in areas of gradual subsidence; they are often of vast extent and immense thickness, and such deposits of past geological ages have not infrequently been preserved, with all their records of the times in which they were built, by being carried below the level of the sea, to be brought to light by a later uplift. ”

“ If one desires to know the history of a region, one turns naturally to its mountain ranges, for here may be found the upturned and dissected strata, a study of whose kinds, thickness, and fossils throws light upon past events, while their foldings and dislocations show the nature and results of those great dynamic agencies which, from time to time, have operated upon the outer portion of the earth, and given to it the broad distinctive features which characterize it to-day. ”

“ Independently of those ascending and descending movements which have given rise to the superposition of the limestones and clays, and by which the position of land and sea are made in the course of ages to vary, the geologist has the difficult task of allowing for the contemporaneous thinning out in one direction and thickening in another, of the successive organic and inorganic deposits of the same era. ”

“ Every successive stratum of sedimentary rock, sand, or gravel, is a proof that changes of level have taken place; and the different species of animals and plants, whose remains are found in these deposits, prove that corresponding changes did occur in the organic world. ”

“ 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. ”

“ The plants and animals found in these strata have been already described somewhat in detail, and we have turned aside to look at the processes whereby such masses of sedimentary rock came to be accumulated. But we shall probably better understand the habits of the animals and the general aspect of the vegetation, as well as the agencies at work in depositing vast beds of mineral matter, if we take a coal-field and analyse it stratum by stratum, marking as we go their varied and ever-changing character, and the corresponding diversity of the included organic remains. ”

“ It has been shown in the third chapter that there is such a difference between land, fresh-water, and marine fossils as to enable the geologist to determine whether particular groups of strata were formed at the bottom of the ocean or in estuaries, rivers, or lakes. ”

“ In this manner the geologist may account, perhaps, for the circumstance that in Scandinavia, Scotland, and other countries where erratics are met with, the beds of sand, loam, and gravel are often vertical, bent, and contorted into the most complicated folds, while the underlying strata, although composed of equally pliant materials, are horizontal. ”

“ Every geologist will at once perceive, that forests sunk by such subterranean movements may become imbedded in subaqueous deposits, both fluviatile 762 and marine, and the trees may still remain erect, or sometimes the roots and part of the trunks may continue in their original position, while the current may have broken off, or levelled with the ground, their upper stems and branches. ”

“ That there has been land as well as sea at all former geological periods, we know from the fact, that fossil trees and terrestrial plants are imbedded in rocks of every age. Occasionally lacustrine and fluviatile shells, insects, or the bones of amphibious or land reptiles, point to the 189 same conclusion. The existence of dry land at all periods of the past implies, as before mentioned, the partial deposition of sediment, or its limitation to certain areas ”

“ In many places where a vertical section is exposed to view, the phenomenon of stratification may be distinctly recognized. The upper part of the deposit belonging to each year is composed of earth of a lighter color than the lower part; and the whole forms a distinct layer of hardened clay, which may be easily separated from those above and below. This formation, therefore, corresponds exactly with those strata of shale which we so often meet with in the Crust of the Earth. ”

“ The further fact that the several stratified rocks are remarkably constant in their mineral character, renders this knowledge of the order of relative superposition still more valuable to the agriculturist. Thousands of different beds are known to geologists to occur on various parts of the earth’s surface—each occupying its own unvarying place in the series. ”

“ Its outcrop was carefully examined for many miles along the mountain sides with the hope of finding a locality where conditions had been favorable for (a) the presence of life during deposition of sediments, (b) the subsequent changing of sediments into rock, and (c) the preservation of the rock during the vicissitudes consequent on mountain building, so that the preservation of the life of the epoch would be as complete as possible. ”