We now pass to the third division of Aqueous Rocks, those, namely, which are believed to have come into existence chiefly through the agency of animal and vegetable life, and are therefore called Organic. The study of these rocks has been prosecuted with no inconsiderable ardor during the last thirty years; and the facts which have been brought to light are certainly amongst the most curious and interesting in the whole range of physical science. Indeed we are convinced that a simple narrative of the researches which have recently been made upon this subject, and the discoveries to which these researches have led, would be no less attractive, and scarcely less wonderful, than a fairy tale. But it is not for us to wander at large over this vast and tempting field of inquiry. We must be content with one or two examples, which may help to illustrate the process of inductive reasoning upon which the general principles of geological science are founded.
It is argued, then, that the present operations of Nature afford the best key for the interpretation of her works in bygone times. We observe various beds of rocks now in course of formation on the surface of the Earth; and within the Crust of the Earth we discover corresponding strata of the self-same rock already complete, and laid by, as it were, in Nature’s storehouse. Side by side, therefore, we may study and compare the finished work and the work that is yet in progress; and if, on a close examination, they are found to agree in all essential characters, we have doubtless a strong presumption, that the same causes which are now producing the one, must in former times have produced the other. This line of argument we have already considered in reference to those two classes of Aqueous Rocks, which are said to be respectively of Mechanical and of Chemical origin. We now proceed to show that it is no less applicable to those which are called Organic. And although we may not hope to unfold all the secret wonders of Nature’s laboratory, that have come to light in recent times, yet we may afford a passing glimpse at her operations, which can scarcely fail to be interesting and instructive.
We have shown how strata of solid rock are sometimes formed in lakes by the precipitation of lime from a state of solution. Now this process cannot take place in the sea; for though lime is present in the sea, the quantity of carbonic acid with which it is there associated, is far more than sufficient to render its precipitation impossible.[49] But Nature has another contrivance for gathering together the solid elements of her building. The depths of the ocean are teeming with life; and countless tribes of minute animals are furnished with the power of extracting the lime from the waters they inhabit, and of reproducing it under a new form. Sometimes, through this mysterious operation of organic life, the lime is converted into a calcareous shell, like that of the oyster; sometimes into a stony skeleton, as in the case of the numerous families of coral-producing animalcules. After death the soft, fleshy substance of these animals melts away and disappears; but the limestone shells and skeletons remain, accumulating during the long course of ages to an almost incredible extent. And, if we are to believe Geologists, out of these accumulated materials, sometimes preserving their original form and structure, sometimes altered more or less by chemical action, sometimes broken up into fragments by mechanical force, has been produced a very large proportion of the limestone rocks which occur so abundantly in the Crust of the Earth.
No better illustration can be found than the white earthy limestone, familiar to every one under the name of chalk. An undulating stratum of Chalk Rock, attaining not unfrequently a thickness of one thousand feet, may be said, speaking roughly, to underlie the southeastern half of England. Sometimes it appears at the surface: sometimes it dips downward, and forms a kind of great basin, over which are regularly spread out various other groups of Stratified Rocks. On the southern coast it rises to a height of several hundred feet above the level of the sea in a line of perpendicular cliffs, conspicuous from a distance by their dazzling whiteness. But the White Chalk of England is only an insignificant part of a great rock-formation, which may be traced over extensive areas throughout all Europe, from Ireland to the Crimea, from the Baltic Sea to the Bay of Biscay; and which everywhere preserves in a remarkable degree the same mineral character, and presents to the eye the same general appearance.
Now it had often been suggested by Geologists that this wide-spread formation derived its existence chiefly from the accumulated remains of organic life. For in many instances the broken shells of minute animalcules could be distinctly observed to constitute a part of the rock. And even where the organic structure could not be so clearly traced, the carbonate of lime composing the Chalk presented just that appearance which would naturally result from the decomposition of such shells. This theory, however, was long put forward with diffidence and received with incredulity. Even scientific men found it hard to persuade themselves that a solid rock of such great extent and thickness could have been the work of agents apparently so insignificant. But it has been confirmed and illustrated in a very interesting and unexpected manner within the last few years.
When the project of connecting Europe and America by a telegraph cable was first set on foot, it became necessary to ascertain, as far as possible, the general configuration of the ocean bottom and the exact nature of the bed on which the cable was to lie. Accordingly in the year 1857 an expedition was fitted out for this purpose under the command of Captain Dayman; and a careful series of soundings was taken between Valentia, on the West Coast of Kerry, and Trinity Bay on the shores of Newfoundland. It was found that the floor of the ocean between Ireland and America is a vast irregular plain, and that by far the greater part is covered over with a kind of soft mud or ooze. Samples of this ooze were scooped up, even at the most profound depths, by means of an ingenious apparatus attached to the sounding-lines, and brought undisturbed to the surface. Afterward they were carried home to England and submitted for examination to Professor Huxley. The result has been to show that the materials of a limestone rock, resembling in every essential feature the White Chalk of Europe, are being spread out at the present day over an area of immense extent on the floor of the Atlantic Ocean.
With the permission of our readers we shall allow Professor Huxley, as far as may be, to tell his own story.[50] As to the ocean floor itself, “It is,” he says, “a prodigious plain—one of the widest and most even plains in the world. If the sea were drained off, you might drive a wagon all the way from Valentia to Trinity Bay. And, except upon one sharp incline about two hundred miles from Valentia, I am not quite sure that it would even be necessary to put the skid on, so gentle are the ascents and descents upon that long route. From Valentia the road would lie down hill for about two hundred miles to the point at which the bottom is now covered by 1700 fathoms of sea-water. Then would come the central plain, more than a thousand miles wide, the inequalities of the surface of which would be hardly perceptible, though the depth of water upon it now varies from 10,000 to 15,000 feet; and there are places in which Mont Blanc might be sunk without showing its peak above water. Beyond this the ascent on the American side commences, and gradually leads for about three hundred miles, to the Newfoundland shore.”
The central plain here described, which has been since found to extend many hundred miles north and south of the cable line, is covered almost everywhere by that soft, mealy sort of mud of which we have already spoken; and this, it is now confidently believed, is nothing else than a stratum of Chalk Rock in an early stage of formation. When thoroughly dried it assumes a whitish color, and exhibits a texture which even to the superficial observer appears closely to resemble fine chalk. Nay, we are told that if so disposed, one may take a bit of it in his fingers and write with it upon a blackboard. Like chalk, too, when chemically analyzed it is found to be almost pure carbonate of lime.
But there is a yet more striking analogy between the mud of the Atlantic and the White Chalk of Europe. Both have been submitted to the magnifying power of the Microscope; and, after an examination conducted with scrupulous care, a wonderful and almost startling identity of mineral, or rather we should say of organic, composition has been established between them. To the naked eye Chalk is simply a soft, earthy sort of stone. But when a thin transparent slice is placed under the Microscope, the general mass is found to be made up of very minute particles, in which are embedded a vast number of other bodies possessing a well-defined form and structure. These are of various sizes, but on a rough average may be said not to exceed a hundredth of an inch in diameter. Hundreds of thousands of them are sometimes contained in a cubic inch of Chalk, together with countless millions of the more minute granules.
Professor Huxley succeeded in separating these bodies from the mass of granules in which they were embedded, and by examining them apart, he has ascertained still more fully their exact structure and composition. “Each one of them,” he says, “is a beautifully constructed calcareous fabric, made up of a number of chambers communicating freely with one another. They are of various forms. One of the commonest is something like a badly-grown raspberry, being formed of a number of nearly globular chambers of different sizes congregated together. It is called Globigerina; and some specimens of Chalk consist of little else than Globigerinæ and granules.”