§ II. CHAPTER II.

STRUCTURE OF THE EARTH.

DESCENDING from the contemplation of the celestial order, in the composition of which our globe is only an insignificant element, we turn our attention to the massive structure of that globe itself. We carry our illustrative survey from the vast regions and unnumbered worlds, lying all around us in space, and with which we are only enabled dimly to converse, to the bosom of that familiar earth on which we dwell, and which everywhere invites our inspection.

We are prepared to trace order here, as in the far-off regions we have been traversing. To the untutored eye, the mass of our earth may seem a mere vast conglomeration, even as the heavens seem a mere mazy dance of sparkling lights; but as science has disclosed the magnificent system of the one, so has it unfolded the special structure of the other. As in the heavens we still read in the blaze of modern astronomy the glory of God, so in the crust of the earth do we read, in the light of modern geology, the impress of Divine power and wisdom. As we confine our 104attention here to the massive construction of this crust, a few words will suffice to bring before us the facts which the subject involves.

The component rocks of the earth are divided into two great classes—stratified and unstratified. The latter represent the oldest, and, so to speak, the original material of the earth. They constitute its solid basement. The foundations of the structure are laid in granite. The hard and agglutinated character of these rocks favours the supposition that they were originally in a state of fusion. There cannot, at least, be any doubt that they are of igneous production. Their unworn and angular crystals clearly point to such a mode of production.

The stratified rocks, in all their varieties, present different peculiarities of formation. Those which lie immediately above the unstratified granitic mass, closely resemble the latter in character: they are in fact composed of the same constituents, different only in the form and proportion in which they are aggregated. Their crystalline texture betrays the same fiery agency which discovers itself in the parent rock. At the same time, they bear marks of distinctive origin. Their crystals are worn and abraded by the action of atmospheric and aqueous influences. Yet the igneous character is here still predominant; and, as might be expected, in the fire-locked embrace of these primary rocks there is to be found no trace of organic existence.

Above what we may call this hard and unfossiliferous basis, the fossiliferous rocks rise in an ascending series, comprehending various systems which geologists have grouped 105into three great periods or epochs, successively called Palæozoic, Secondary, and Tertiary. The Palæozoic group, which is next in age to the metamorphic rocks, comprehends the vast systems of the lower and upper Silurian, the Old red sandstone, and the Coal-measures. The crystalline texture of the previous rocks disappears, save among the lowest of these strata, and a clayey or sandy texture takes its place, discovering the more powerful working of those atmospheric and aqueous influences which we have mentioned. Here, also—as the name of the group implies—in the Llandeilo flags of the lower Silurian, we find the first traces of organic being, which henceforth multiply, in endless and marvellous forms, in the onward course of the earth’s growth. In the great carboniferous system we perceive in a very large degree the operation of a further influence in the formation of the earth’s crust—the submersion and depression, namely, of organic remains. This, in the ascending history of our globe, is one of the most extensive of all the causes contributing to the earth’s formation, in respect not merely of vegetable, but also of animal remains. The former, it is well known, are the peculiar ingredient of the vast coal-measures. In them we behold the deposition of the enormous vegetation which, in the carboniferous era, must have overspread the earth—vegetation in comparison with which, it has been said, the existing jungle of the tropics is mere barrenness.

In the secondary period we have, as in the Palæozoic, three great systems, the New red sandstone, the Oolitic, and the Chalk—the Oolitic being especially remarkable as the era of 106those gigantic reptiles, whose strange and fearful forms at once amaze the ignorant and interest the curious.

With the tertiary period—with whose subdivisions, as laid down by Lyell, and generally accepted by geologists, we need not here concern ourselves—we approach our own era. We meet with animals of dimensions, indeed, far exceeding any with which we are now familiar, but in structure allied to existing species. We are carried forwards to an arrangement of physical conditions not differing widely from the present.

Such is a brief statement of the successive materials, so to speak, which compose the structure of the earth. Imperfect as it is, it is sufficiently complete for our purpose. In the mere facts thus disclosed, there seems already evidence of the order for which we seek. The actual structure of the earth, however, is something very different from that now suggested. It is not built up in the manner we have described, with the successive systems regularly laid upon one another, as they were progressively formed—the earliest everywhere lowest, and the latest highest. If such had been its actual construction, that construction would probably have for ever remained a secret to us. We could not have penetrated to its deep and hidden foundations. As it is, however, we are enabled to explore the whole structure, and find order and beauty in it, through means which might have seemed only destined to insure its destruction. Its foundations have been laid bare to us; while its later architecture lies equally exposed, not in mere disrupted fragments, but in vast and orderly terraces. The fact is, that in the process of 107the earth’s formation, during the long periods which had been employed in the gradual deposition of the various strata in the order of time we have described, those igneous agencies concerned in the production of the earliest rocks continued at work, breaking up and dislocating the incumbent strata, and forcing the granite upwards in all directions. To the same causes the different species of trap-rocks, piercing upwards in great veins, owe their elevation causes which we still see in some degree active in our volcanoes. Whatever theory may be held as to the special intensity of these causes in the past periods of the earth’s history—whether we adopt a catastrophic or a uniformitarian hypothesis—the result is the same. The granite, which is everywhere the base of the earth’s crust, has yet been elevated far above all the posterior strata. It is no longer merely the impenetrable foundation or central abutment of the rocky systems; but it stretches upwards in vast branches, forming, so to speak, a skeleton framework for the earth. Somewhat as the bony skeleton in the living body everywhere ramifies it, giving strength and consistency to all its parts, so the granitic framework pierces on all sides throughout the earth’s crust, compacting and consolidating it into its present state. And even somewhat as the muscular tissues and folds of flesh overlie the bony skeleton, and find in it their ultimate points of support, so do the various rocky tissues, the successive folds of softer material, rest against the mountain masses. We must surely in all this trace evidence of special arrangement. “It is not,” as Dr Chalmers has said, “from some matter being harder than others that 108we infer design; but when we see the harder placed just where it is most needed, the inference seems irresistible.” And in the present case it is surely impossible to contemplate the peculiar disposition of the granite in our earth, without recognising that so it must have been placed. The very terms which we are compelled to use in speaking of it, after the least theological fashion, imply so much. That so it is by any mere accident, is altogether inconceivable. The enormous agencies concerned in the elevation of the granite—could we have seen them operating—might have seemed merely blind and lawless; but the result is order, and we cannot help concluding that some presiding mind has been at work. The granite has been upheaved, it may be, by convulsive agencies of a magnitude and intensity far beyond any of which we have now experience; the superimposed strata have been rent, and tossed hither and thither. The vast process by which this was accomplished might have seemed mere wild confusion. But pierce and bore the earth in all directions, there is really nothing like confusion. The term is indeed unknown to science, and to no science more than to geology, immense and catastrophic, according to the most common opinion, as are the changes with which it has to do. Let the granite, for example, rise to whatever heights—let it tower in whatever alpine magnitudes—we never find that its proper, or what we might call its constitutional position, is altered: the foundations are still granite, if the granitic mass yet stretch in cleaving branches through the sedimentary strata, and far overreach their roof.

And even so of all the different strata over the diversified surface of the earth; they all of them lie, as we have mentioned, severally exposed,—characterising, in their distribution, different countries and localities. The old red sandstone and carboniferous systems of the palæozoic era, for instance, form the immediate platform of large tracts of our island. The oolitic system of the reptilean era marks its eastern seaboard, while the chalk extends on the south and south-east. The whole economy of the terrene architecture is thus laid bare. It is spread out for our inspection; but, while all the various depositions thus appear on the surface, there is no confusion in their relative positions. They are never found at random—one set of strata being now below and then above another set—but always occupying the same relation to one another. If we find, for example, the lower silurian formation exposed in Wales, it is everywhere found to rest directly on the granite; if we find the old red sandstone in Devonshire, it again rests on the silurian, and the carboniferous system again on it. We never find the silurian imposed on the old red sandstone, nor the chalk below the oolitic. A set structure is surely here in the clearest manner discernible. We cannot well conceive any higher idea of structure than just such a special distribution of parts,—the parts of the same character being always found in the same place, in relation to the others.

The order, indeed, which the mass of our earth discovers, is on a vast and comprehensive scale, which may not very readily fall in with our preconceptions or fancies. Man’s 110feebleness is apt everywhere not merely to limit, but to spoil his judgments, so that order is perhaps more easily seen by him in mere neatness and formality, than in the bursting and glorious fulness of Nature’s own form. Could the crust of our earth, for example, have preserved that appearance of uniform regularity which would have followed from the continuance of the sedimentary strata in the successive positions of the order of their formation—had it been a granite nucleus surrounded, in the words of Dr Buckland, “by entire concentric coverings of stratified rocks like the coats of an onion,” and could we have been cognisant of this regularity, it might, we dare say, have impressed many more than the actual structural appearance which it presents. The order in the one case might have seemed more direct and apparent than in the other. But as it is, it is undoubtedly a far more glorious order—the product of a boundlessly comprehensive Plasticity, moulding the most mighty and apparently lawless agencies to the most magnificent, yet most exquisite results, and the more perfect just as it may transcend our feebleness and awaken our wonder.

Apart from the disruptive movements of which our earth has been the scene, it would not have presented any of its characteristic and beautiful variety of hill and valley, of glen and stream. Its surface would have been a mere uniform level, without life or picturesqueness; its rivers mere sluggish canals; its whole aspect destitute of that interchangeable sweetness and grandeur, softer loveliness and rugged magnificence, which now makes it so glorious 111a mirror of Power and Wisdom and Goodness. To the same causes obviously does it also owe its peculiar fitness as the abode of human life. Tor otherwise the metals, without some knowledge of which man has never been able to rise above barbarism, would have been for ever concealed in their native crypts. Coal would have been sunk at an impenetrable depth, which no eye could have seen, and no skill have reached. And where, again, would have been our oceans, with no vast hollows to repose in? But it is needless, and even absurd, to make such suppositions. We have only done so for a moment, in order to make it clear how the mighty agencies which have been concerned in the present structure of the globe, wild and convulsive as they may have been, have been directed by the most far-reaching foresight to purposes of human improvement and happiness. They were only the tools in the Divine hand for the construction of man’s abode. Far from being, in any sense, interferences with the terrene architecture, they were the very means by which it has been built up into the special order, at once most beautiful and most appropriate for him.

In contemplating the great movements which geology reveals, it is important to observe further how completely dependent they appear. In those disruptive agencies, as well as in the various atmospheric, aqueous, and organic influences, under the operation of which the earth has assumed its present structure, it seems impossible that any one could for a moment find the ultimate explanation of the phenomena 112presented. If there are minds content to linger among the ultimate harmonies of astronomy, which stand forth so palpably to the intellectual view, we cannot yet imagine any abiding by the final agencies of geology, as if they carried with them any self-sustaining or efficient energy. They appear in the highest degree to be simply instrumental,—the merely blind agencies of a creative and designing Mind.