Geological stratum

“ And therefore we may state briefly some of the divisions of the series of terrestrial strata which have suggested themselves to geological inquirers. At the outset of such speculations, it was conceived that the lower rocks, composed of granite, slate, and the like, had existed before the earth was peopled with living things; and that these, being broken up into inclined positions, there were deposited upon them, as the sediment of superincumbent waters, strata more horizontal, containing organic remains. ”

“ When we desire to classify, it is necessary to contemplate entire groups of strata in the aggregate; but if we wish to understand the mode of their formation, and to explain their origin, we must think only of the minute subdivisions of which each mass is composed. We must bear in mind how many thin leaf-like seams of matter, each containing the remains of myriads of testacea and plants, frequently enter into the composition of a single stratum, and how vast a succession of these strata unite to form a single group! ”

“ Do you not see that the chief teaching of geology—the piling of stratum upon stratum, the conversion of disrupted rock and decayed plant and animal into rock again—is here exemplified in the history of a domestic toy, which contains already one example of stratification in the silence of watery submergence? ”

“ With regard to the composition of the various strata of the earth, the father of geology {156} considered that if in a layer of rock all the portions are of the same kind there is no reason to deny that such a layer came into existence at the time of creation, when the whole surface of the earth was covered with fluid. If, however, in any one stratum portions of another stratum are found, or if the remains of plants or animals occur, there is no doubt that such a stratum had not its origin at the time of creation, but came into existence later. ”

“ 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. ”

“ Plants and animals flourish luxuriantly near the shore, and animals extend in relatively great abundance down to the lower limits of the region. Here we find now in process of formation deposits which will form rocks similar to those making up the great bulk of continental land. Throughout all geological time the deposits formed in this border or transitional area appear to have been pushed, forced, and folded up into dry land, through the secular cooling of the earth and the necessity of the outer crust to accommodate itself to the shrinking solid nucleus within. ”

“ The flints in general form round or oval masses, but of irregular shape with many projections, and the masses lie in regular bands parallel to the stratification. The tremendous earth movement which has bent the strata into a great curve has compressed the vertical portion into about half its original thickness, and has made the chalk of our downs extremely hard. ”

“ What stratification is to mountains whose geological formation is the same throughout, the superposition of [153] different kinds of rocks is to those of composite materials. The layers may be divided into superior, inferior, and adjacent. These usually vary in respect to age, and may be traced in a regular geological seniority, as for example sandstone, gypsum, limestone, gray-wacke, and granite. ”

“ First, there is no distinction to be made of what is termed primary and secondary mountains, with regard to the position of their strata; every different species of stratum, from the stratified granite and quartzy schistus of the Alps to the oolites of the Jura and Saleve, being found in every respect the same; whether this shall be supposed as arising from their original formation, or, according to the present theory, from a subsequent deplacement of strata formed originally in a horizontal situation. ”

“ 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. ”

“ A succession of strata indicates a succession of time for their formation; and the change from one species of stratum to another placed upon it, indicates a change of cause. Thus is the mass of our continents the product of successive operations, during which the producing causes have undergone successive changes. We see, moreover, that many of these strata contain the remains of animals; and that in some successive strata these organized bodies are of different species. ”

“ With the cretaceous system ends the long series of deposits which are, by general consent, ranked as strata of the secondary periods of geology. In reviewing the successive secondary formations, from the red sandstones to the green sand, and from the mountain limestone to the chalk, it is impossible not to recognise, on a great scale, the gradual change of the physical conditions of the globe which took place during this period. Mineralogically, the rocks successively deposited deviate more and more from the types of the primary strata ”

“ 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. ”

“ When strata are crumpled up they naturally crack across, for they are not elastic. During the great movements which have originated — 52 — all mountains of elevation, it is evident that the strata forming the actual surface of the ground would often be greatly fissured and shattered along the crests of the sharper anticlinal ridges. ”

“ The solution of an old geological problem requires far more caution than the explanation of a modern geographical phenomenon. For in regard to the older events we are seldom aware of all the essential facts, from which not only the nature of the physical agency is to be ascertained, but the measure of its force, the local centre of its effect, and the sudden or gradual, the continuous or interrupted mode of its application. The geographical relations of tertiary strata must be understood before venturing to adopt or to reject the hypothesis of partial elevation and subsidence. ”

“ Now, to support this view, not only the remains of living beings which at present people the globe, might be expected to be found in the oldest secondary strata, but even those of the art of man, the most powerful and populous of its inhabitants, which is well known not to be the case. On the contrary, each stratum of the secondary rocks contains remains of peculiar and mostly now unknown species of vegetables and animals. In those strata which are deepest, and which must consequently 386 be supposed to be the earliest deposited, forms even of vegetable life are rare ”

“ 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. ”

“ 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. ”

“ Stratification.—All rocks formed by strewing materials in water, and their deposition in successive, parallel, horizontal layers, are stratified; and this structure is their stratification. It is the most important of all rock structures; and there is no kind of structure the origin of which is more fully or certainly known. The deposition of sediment in carefully assorted horizontal layers is readily brought within the comprehension of children by simple experiments with sand and clay in water ”

“ 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 the fact of the elevation of the strata, the study of geology is much indebted. The stratified form of a great proportion of the earth's surface, gives to minerals that organization and regularity, which makes their disposition an object of science, and their inclined position serves to bring that organization into view, from far greater depths than we can ever reach by artificial excavations. ”

“ Such great foldings or redoublings of strata are most probably originated during subsidence of a portion of the earth’s crust. While the ground is slowly sinking down, the strata underneath are perforce compelled to occupy less space laterally, and this they can only do by yielding amongst themselves. All folding or contortion on the large scale—that, namely, which has affected areas of strata extending over whole countries—seems to have taken place under great pressure; in other words, to have been produced at considerable depths from the earth’s surface. ”

“ An anticlinal arrangement of strata is a weak structure—it readily succumbs to the attacks of the denuding agents; a synclinal arrangement on the contrary, is a strong structure, which is much less readily broken up. Hence it is that in all regions which have been exposed for prolonged periods to sub-aërial denudation synclinal strata naturally come to form hills, and anticlinal strata valleys or low grounds. In the case of a mountain-chain so recently elevated as that of the Alps, the mountain-ridges, as we have seen, often coincide roughly with the greater folds of the strata. ”

“ But an experienced geologist can rarely be deceived by these exceptional cases. When he finds that the strata are fractured, curved, inclined, or vertical, he knows that the original order of superposition must be doubtful, and he then endeavours to find sections in some neighbouring district where the strata are horizontal, or only slightly inclined. Here, the true order of sequence of the entire series of deposits being ascertained, a key is furnished for settling the chronology of those strata where the displacement is extreme. ”

“ The strata, as upon Earth, were of various character, none of them very thick, seldom reproducing exactly the geology of our own planet, but seldom very widely deviating in character from the rocks with which we are acquainted. ”

“ In all these cases, it is only necessary that there should be some deposition of sedimentary matter, however minute, such as may be supplied by rivers draining a continent, or currents preying on a line of cliffs, in order that stratified formations, hundreds of feet in thickness, and replete with organic remains, should result in the course of ages. ”

“ 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 ”

“ Speculative geology is romance, and does not merit the name of science; yet is science obliged to borrow her theory of soils. The alternation of heat and cold, moisture and dryness, decomposed the mountains of primitive, secondary and tertiary formation; rains, and the laws of gravity, brought these from places of more, to places of less elevation—where, by mechanical mixture and chymical combination, the present substrata were formed. ”