If, with the eye of a geologist, we take but a cursory glance over the Earth, we shall discover that countless ages must have passed during the progress of this planet to its present state. This is a fact written by the finger of nature, in unmistakeable characters, upon the mighty tablets of her mountains.
The superficial crust of the earth,—by which is meant only that film, compared with its diameter, which is represented by a few miles in depth—is composed of distinct mineral masses, exhibiting peculiar physical conditions and a certain order of arrangement. These rocks appear to have resulted from two dissimilar causes; in one class the action of heat is evident, and in the other we have either the slow deposition of matter suspended in water, or crystallization from solution; an aqueous origin is indicated by peculiarities of formation in all the more recent rocks.
There are few branches of science which admit of speculation to the extent to which we find it carried in geology. The consequences of this are shown in the popular character of the science. A few observations are made over a limited area, and certain structural conditions are ascertained, and at once the mind, “fancy free,” penetrates the profound depths of the earth, and imagination, having “ample room and verge enough,” creates causes by which every effect is to be interpreted. Such students, generally ignorant of the first principles of physics, knowing little of mineralogy, and less of chemistry, to say nothing of palæontology, having none of the requisites for an observer, boldly assume premises which are untenable, and think they have explained a phenomenon,—given to the world a truth,—when they have merely promulgated an unsubstantiated speculation, which may have occasional marks of ingenuity, and but little else.
The carefully-made observations of those who, with unwearying industry, have traversed hill and valley, marked and measured the various characters, thicknesses, inclinations, and positions of rocks; who have watched the influences of heat in changing, of water in wearing, and the results of precipitation in forming, strata; who have traced the mechanical effects of earthquake strugglings and of volcanic eruptions, and, reasoning from an immense mass of accumulated facts, deduced certain general conclusions,—are, however, of a totally different character; and it is such observers as these who induced Herschel to say truly, that “geology, in the magnitude and sublimity of the objects of which it treats, undoubtedly ranks, in the scale of the sciences, next to astronomy.”[231]
The origin of this planet is involved in great obscurity, which the powers of the most gifted are unable to penetrate. It stands the work of an Almighty and Eternal mind, the beginning of which we cannot comprehend, nor can we define the period of its termination.
It may, probably, be safe to speculate that there was a time when this globe consisted of only one homogeneous stratum. Whether this remains,—whether, in our plutonic rocks, our granites, or our porphyries, we have any indications of the primitive state of the world, or whether numerous changes took place before even our unstratified formations had birth, are questions we cannot answer. The geologist looks back into the vista of time, and reckons, by phenomena, the progress of the world’s mutations. The stratified formations must have occupied thousands of ages; but before these were, during a period extending over countless thousands, the unstratified rocks may have been variously metamorphosed. It matters not whether we admit the nebular hypothesis or not,—a time must have been when all these bodies which now form the mass of this globe existed in the most simple state. We have already shown that very remarkable changes in external character and in chemical relations are induced, in the same simple element, by its having been exposed to some peculiar and different conditions; and already have we speculated on the probability that the advance of science will enable us to reduce the numerous elements we now reckon, to two or three. It is, therefore, by no means an irrational thought (which must, however, be held in the light of a pure conjecture), to suppose that at the beginning a mighty mass of matter, in the most attenuated state, was produced in space, and was gradually, under the influence of gravitation, of cohesive force, and of chemical aggregation, moulded into the form of a sphere. Ascending to the utmost refinement of physics, we may suppose that this mass was of one uniform character, and that it became in dissimilar parts—its surfaces and towards its centre—differently constituted, under the influences of the same powers which we now find producing, out of the same body, charcoal and the diamond, and creating the multitudinous forms of organized creations. These conditions being established, and carried to an extent of which, as yet, science has afforded us no evidence, chemical intermixture may have taken place, and a new series of compounds have been formed, which, by again combining, gave rise to another and more complex class of bodies.
The foundation of the superficial crust of the earth appears to be formed of a class of rocks which have resulted from the slow cooling of an immense mass of heated matter. These rocks have been called igneous; but are now more generally termed Plutonic (such as granites, syenites, &c.) Immediately above these, we find rocks which have resulted by deposition from water. These masses, having been exposed to the action of the heat below, have been considerably changed in their character, and hence they are often called metamorphic; but metamorphic rocks may, however, be of any age. The rocks formerly termed the transition series—from their forming the connecting link between the earlier formations—are now, from the circumstance of their being fossiliferous, classed under the general term of palæozoic rocks, to distinguish them from the rocks in which no organic remains have been found. Above these are found the secondary strata, and, still more recently produced, we have a class now usually denominated the tertiary formations. “Eternal as the hills” is a poetic expression, implying a long duration; but these must, from the nature of things, eventually pass away. The period of time necessary for the disintegration of a granite hill is vastly beyond the powers of computation, according to our conception of the ordinary bounds of finite things. But a consideration of the results of a few years,—under the influence of the atmosphere and the rains,—as shown in quantity of solid matter carried off by the rivers, and deposited at their mouths, will tend to carry conviction to every mind, that a degrading process is for ever in action on the surface of the earth. The earth itself may be eternal, but the surface is continually undergoing mutation, from various causes, many of which we must briefly consider.[232]
In regarding geological phenomena, the absence of any fossil remains has often been supposed to indicate a period previous to any organic formations. The inorganic constituents of matter are probably of prior origin to the organic combinations; the vessel was constructed, upon which the organic creation was to float in space before any vital organisms were created. The supposed evidences in favour of the assumption that there was no organic life during the formation of the oldest rocks we know, are in some respects doubtful; and we can well understand that changes may have been induced in the earlier rock formations, by heat or by other powers, quite sufficient to destroy all traces of organized forms. It was long thought that phosphoric acid was not to be detected in rocks which are regarded as of igneous origin; and since this acid is peculiarly a constituent of organic bodies, this has been adduced as a proof that the plutonic rocks must have existed previously to the appearance of vegetable or animal life upon the surface of the globe. The researches of modern chemists have, however, shown that phosphoric acid is to be found in formations of granitic origin, in porphyry, basalt, and hornblende rocks.[233] If, therefore, we are to regard this substance as of organic origin, the rational inference is against the speculation; but there is no more necessity for supposing phosphorus to be formed in the animal economy than in the mineral kingdom, from which it will probably be found the animal obtained it.
Without attempting to enter into any account of the apparent progress of life over the earth, it appears desirable that some description should be given of the kinds of plants and animals which we know to have existed at different epochs. We shall thus learn, at least, some of the prevailing characteristics of the earth during its transitions, and be in a better condition for applying our knowledge of physical power to the explanation of the various geological phenomena.
Among the earliest races we have those remarkable forms, the trilobites, inhabiting the ancient ocean.