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served, which, in mineral composition, had a good deal of resemblance to those below them, but contained rounded fragments of other rocks: and, when these fragments were examined, they were found to be identical with the rocks composing the lower strata. This second series was observed to be covered by another group of strata, which contained shells and corals, bodies that had never been seen in any of the lower strata. Thus it was clear, as the including substance must necessarily be formed subsequently to the pebble or shell it contains, that, previous to the formation of this third group, there had existed rocks to supply the imbedded fragments, and to contain the waters of the ocean, in which the animals that once inhabited the shells must have lived. Ascending still higher, that is, observing the strata as they lay one above another towards the surface, it was found, that many were entirely composed of the fragments of pre-existing rocks, either in the form of pebbles, or of sand cemented together: that there was a vast increase in the number and variety of the imbedded shells, the latter forming very often entire beds of rock, many feet in thickness; and that the remains of plants began to appear.
In this manner certain great divisions of the strata were established, by very clear and infallible distinctive characters. But it was reserved for an English practical mineral surveyor to make a discovery, which gave a new direction to geological inquiries, and which, in the course of a few years, introduced into the science a degree of precision and certainty, that was formerly unknown. About thirty-five years ago, Mr. William Smith, of Churchill in Oxfordshire, by an extensive series of observations in different parts of England, ascertained that particular strata were characterized by the presence of certain fossil or petrified shells, which were either confined to them exclusively, or in predominating quantity, or were of rare occurrence in other strata: and he was thus enabled to identify two rocks at distant points as belonging to one stratum, when mere mineral characters would either have left
him in uncertainty, or have entirely failed in deciding the question. When this discovery became known to geologists, numerous observations were made in other countries, which completely proved, that the principle was not only applicable in those places, which Mr. Smith had had an opportunity of observing, but that it held good generally, and throughout the whole series of strata, from the lowest, in which organic remains are found, to those nearest the surface. Under the direction of this guide, geologists have been enabled to discover lines of separation in the great divisions, which, as already mentioned, had been established by prior observations, pointing out distinct epochs of deposition, and revealing a succession of changes in the organic and inorganic creation, in a determinate chronological order. This more accurate knowledge of the structure of the crust of the globe is of the highest interest and importance; not only as a matter of speculative science, but as regards the practical advantages in common life, that have been derived from it.
An examination of the phenomena, exhibited by the internal structure of this series of superimposed rocks, has established this farther principle that all the strata must have been deposited on a level foundation that is, on pre-existing ground, that was either horizontal or nearly so, at the bottom of a fluid holding their materials either in suspension, or in solution, or partly both. Now, as we know of no fluid in which this could have taken place except water, geologists have come to the conclusion, that the chief part of all the strata, however elevated they may now be above the level of the sea, were gradually deposited at the bottom of the ocean; and the remainder of them at the bottom of inland seas, or lakes. But if this be so, what mighty revolutions must have taken place to cause rocks, formed in the depths of the ocean, to occupy the summits of the highest mountains! By what known agency can so extraordinary a change of position have been effected! That the fact of elevation is indisputable, is proved by the shells imbedded in stratified rocks at the
greatest elevations; and geologists, who have endea voured to discover by what cause this change in the relative position of the rock and the sea has been brought about, have, by an attentive observation of the phenomena of earthquakes and volcanos, and the resemblance between the products of the latter and certain parts of the earth's structure, which we have yet to notice, arrived at a very probable solution of the problem.
Although the strata were originally deposited in a horizontal position, and are often found so, especially as regards the inferior members of the series, they are not uniformly so, but are frequently inclined, more or less; and they have been seen, not only at every angle of inclination, but very often in a vertical position. When a vertical section of a mountain is exposed, as is often the case in valleys or the deep bed of a river, such an appearance as that represented here is not un
common; and if the stratum a be composed of rounded blocks of stone surrounded by fine sand or clay, and if the stratum b contain a layer of shells lying parallel to the sides of the stratum, and if they be unbroken although of the most delicate texture, it is manifest, that these strata could not have been deposited in their present vertical position, but upon a level ground. Sometimes they are not only disturbed from their horizontality, but are bent and contorted in the most extraordinary way, as if they had been acted upon by some powerful force while they were yet in a soft flexible
state. This appearance, very common in the slate rocks of the north coast of Devon, is shown in the diagram.
This seeming disorder and confusion is evidently a part of the order and harmony of the universe, a proof of design in the structure of the globe, and one of the progressive steps, by which the earth seems to have been prepared as a fit habitation for man. For if all the strata had remained horizontal, that is, parallel to the surface of the globe, if they had enveloped it like a shell, or to use a familiar example, had they surrounded it like the coats of an onion, it is clear that we should never have become acquainted with any other than the upper members of the series; and that the beds of coal and salt, and the ores of the metals, all of which are confined to the inferior strata, could never have been made available for the purposes of man. Without this elevation of the strata, the earth would have presented a monotonous plain, unbroken by the beautiful forms of hill and valley, or the majestic scenery of mountains. With these inequalities of the surface are intimately connected all the varieties of climates, and the diversified products of animal and vegetable life dependent thereon; as well as the whole of what may be termed the aqueous machinery of the land-the fertilizing and refreshing rains, the sources of springs, inland lakes, and the courses of rivers and brooks in their endless ramifications. Throughout all this there reigns such a harmony of purpose, that the conclusion is irresistible, that the breaking up of the earth's crust is not an irregular disturbance, but a work of design, in perfect accordance with the whole economy of nature.
We have said, that if we dig through the superficial covering of sand and clay, we usually come upon stone
disposed in layers; but there are many places, where we should find a rock without any such arrangement, which would continue of the same uniform texture, and without any parallel rents dividing it into beds, however deeply we might penetrate into it. Such unstratified rocks, although of limited extent in proportion to the stratified rocks, constitute a considerable portion of the crust of the earth, and in all parts of it they generally rise above the surface in huge unshapen masses, surrounded by the stratified rocks; and sometimes they occupy districts of great extent, where none of the latter rocks can be seen. In mineral composition they are essentially different from the other class; never consisting of limestone, or sandstone, or clay, and never containing rounded pebbles, shells, or the remains of any other kind of organized matter. Their elementary constituent parts are simple mineral substances, which, although sometimes found in the stratified rocks, are always, in the rocks we now speak of, in different combinations: they are always in that particular state called crystalline; and when the parts are large enough to be distinguished, they are seen to interlace each other, and by this arrangement they form a very hard tough stone, very difficult to break into regular squared forms, or to work with the chisel, and they are very often capable of receiving a high polish. The substances most familiar to us in common life, which belong to this class of rocks, are granite, whinstone, and basalt. Ibid.
IV. MINERAL KINGDOM.
We have shown, that the crust of the globe is composed of two great classes of rocks, one of which consists of a series of beds of stone of different kinds, lying upon one another in a certain determinate order of succession, called the Stratified Rocks, or the