Nature of organic rocks—Carbonate of lime extracted from the sea by the intervention of minute animalcules—Chalk rock—Its vast extent—Supposed to be of organic origin—A stratum of the same kind now growing up on the floor of the Atlantic ocean—Coral reefs and islands—Their general appearance—Their geographical distribution—Their organic origin—Structure of the zoophyte—Various illustrations—Agency of the zoophyte in the construction of coral rock—How the sunken reef is converted into an island and peopled with plants and animals—Difficulty proposed and considered—Hypothesis of Mr. Darwin—Coral limestone in the solid crust of the earth.

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.”

Previous to 1857 the GlobigerinÆ of the Chalk were a matter of no small controversy among Geologists and Naturalists. Some contended that they were the organic remains—the shells or skeletons—of ancient animalcules. Others were disposed to regard them simply as aggregations of lime, which, so to speak, chanced to assume the form of these little chambered bodies; though it was not easy to explain, on this hypothesis, how these chance concretions, however much they varied in size, preserved over the whole of Europe the same exact form and structure. But the controversy is now at an end. The specimens of the Atlantic ooze brought home by Captain Dayman, when examined under the higher powers of the Microscope, are found, like Chalk, to be composed almost entirely of GlobigerinÆ. And that no doubt may remain as to their organic origin, a portion of the fleshy integument of the little animalcules is seen, in many cases, still adhering to the calcareous skeleton.

“GlobigerinÆ of every size,” we are told, “from the smallest to the largest, are associated together in the Atlantic mud, and the chambers of many are filled by a soft animal matter. This soft substance is, in fact, the remains of the creature to which the Globigerina shell, or rather skeleton, owes its existence—and which is an animal of the simplest imaginable description. It is, in fact, a mere particle of living jelly, without defined parts of any kind—without a mouth, nerves, muscles, or distinct organs; and only manifesting its vitality to ordinary observation by thrusting out and retracting, from all parts of its surface, long filamentous processes which serve for arms and legs. Yet this amorphous particle, devoid of everything which, in the higher animals we call organs, is capable of feeding, growing, and multiplying; of separating from the ocean the small proportion of carbonate of lime which is dissolved in sea-water; and of building up that substance into a skeleton for itself, according to a pattern which can be imitated by no other known agency.”

That the same process is going on in other parts of the ocean appears by observations made by Sir Leopold M’Clintock during the cruise of the Bulldog in 1860. He discovered that a calcareous ooze having the consistency of putty is spread out over extensive areas between the Faroe Islands and Iceland, and also between Iceland and Greenland. Of this mud about ninety-five per cent. is composed of GlobigerinÆ, which in some instances were brought up actually living to the surface, and busily engaged in secreting, by their vital powers, carbonate of lime from the waters of the sea.51

Professor Huxley goes yet one step further in following out the resemblance between the Chalk Rock that exists in the Crust of the Earth and the stratum of Chalk that is now growing up in the depths of the Atlantic. Not only are the GlobigerinÆ, of which the one is in great part composed, identical with the animalcules that make up about nine-tenths of the other, but even the minute granules that constitute the residue of each formation, correspond in a very remarkable manner. “In working over the soundings collected by Captain Dayman, I was surprised to find that many of what I have called the Granules of that mud were not, as one might have been tempted to think at first, the mere powder and waste of GlobigerinÆ, but they had a definite form and size. I termed these bodies Coccoliths, and doubted their organic nature. Doctor Wallich verified my observation, and added the interesting discovery that, not unfrequently, bodies similar to these Coccoliths were aggregated together into spheroids, which he termed Coccospheres. So far as we knew, these bodies, the nature of which is extremely puzzling and problematical, were peculiar to the Atlantic soundings.

“But a few years ago Mr. Sorby, in making a careful examination of the Chalk by means of thin sections and otherwise, observed, as Ehrenberg had done before him, that much of its granular basis possesses a definite form. Comparing these formed particles with those in the Atlantic soundings, he found the two to be identical; and thus proved that the Chalk, like the soundings, contains these mysterious Coccoliths and Coccospheres. Here was a further and a most interesting confirmation, from internal evidence, of the essential identity of the Chalk with modern deep-sea mud.”

We may, therefore, set it down as certain, first, that the formation of Chalk Rock is going on very extensively at the present day; and secondly, that the chief agency employed in its production is no other than the vital action of minute animalcules. This is no longer merely a plausible theory or an ingenious hypothesis: it is simply a matter of fact ascertained by direct observation. If then it is just and philosophical to ascribe like effects to like causes, the conclusion is plain that the White Chalk of Europe came into existence in some far distant age by just such a process as that which is now in operation on the bed of the Atlantic Ocean.

From the Chalk mud of the Atlantic we will now pass to the Coral Reefs that are growing up beneath the waters of the Pacific and the Indian Oceans. Every one has heard of Coral Reefs and Coral Islands; yet we fancy many persons have but vague and indefinite notions about them. We shall, therefore, in the first place, give a brief account of their general appearance, their extent, and their geographical distribution. Afterward we shall give some of the evidence which goes to show that these huge masses of rock owe their existence to the organic powers of minute living animalcules.

The Coral Reef is familiar to the navigator of tropical seas under a great variety of forms, and in many different stages of development. In one case it is a chain of hidden rocks rising not quite to the level of the sea; in another it appears just above the waters, but is washed over by each returning tide; while in another it rises up beyond the reach of the waves, is clothed with luxuriant vegetation, and inhabited by various species of animals, even by man himself. Again there is great diversity of outline among these rocks, whether they are sunk beneath the surface of the waters or lifted above them. But all may be reduced to four classes, of which we propose to give a short description.

First is the Atoll, or lagoon island. It is a circular strip of limestone rock enclosing a shallow lake within, and surrounded by a deep and often unfathomable ocean without. The scene presented by some of these circular reefs is described by travellers as equally striking for its singularity and its beauty. “A strip of land a few hundred yards wide is covered by lofty cocoa-nut trees, above which is the blue vault of heaven. This band of verdure is bounded by a beach of glittering white sand, the outer margin of which is encircled with a ring of snow-white breakers, beyond which are the dark heaving waters of the ocean. The inner beach encloses the still clear water of the lagoon, resting in its greater part on white sand, and, when illuminated by a vertical sun, of a most vivid green.”

These lagoon islands are often found in groups stretching, with little interruption, for many hundred miles across the ocean. The Maldives, for example, which lie a little distance to the southwest of Hindostan, form a continuous chain, running due north and south, four hundred and seventy miles in length and fifty miles in breadth. Each successive link in this chain does not consist, as might be supposed, of a single circular reef, but it is rather a ring of small coral islets, sometimes more than a hundred in number, each of which is itself a perfect Atoll or lagoon island such as we have just described. Of these miniature islets many are from three to five miles in diameter; while the larger rings of which they form a part are from thirty to fifty. The Laccadive islands, a little more to the north, exhibit a similar arrangement, and indeed would seem to be a continuation of the same group. In the Pacific are found some chains of coral islands yet more extensive; as for instance the Dangerous Archipelago, which is upward of eleven hundred miles in length, and from three to four hundred in breadth; but the islands within these spaces are thinly scattered, and insignificant in size.

Sometimes the annular strip of coral rock encloses within itself a lofty island, which rises up from the centre of the lagoon. In this case it is called an Encircling Reef; the lagoon being simply a broad channel surrounding the island in the centre, and encompassed itself by the coral rock. An example occurs in the island of Vanikoro, celebrated for the shipwreck of La Peyrouse, where the Encircling Reef runs at a distance of two or three miles from the shore, the channel between it and the land having a general depth of between two and three hundred feet. The well-known mountainous island of Tahiti in the South Pacific Ocean is also encompassed by an Encircling Reef, from which it is separated by a broad belt of tranquil water.

A third class of Coral Reefs consists of those which run parallel to the shores of continents or great islands, from which they are cut off by a broad channel, to which the sea has free access through certain open passages in the rock. They are called Barrier Reefs; and differ from the former only in this, that they do not surround the land, but run parallel to it at a distance of some miles. The Great Barrier Reef of Australia offers a noble example. It has been described as a huge, massive, submarine wall or terrace, fronting the northeastern coast of that continent, varying from ten to ninety miles in breadth, and extending, with some trifling interruptions, to a length of 1250 miles. Another reef of the same kind, 400 miles in length, faces the western coast of the long narrow island of New Caledonia.

When a chain of Coral rocks approaches close to the shore, so as to leave no intervening channel of deep water, they are called Fringing Reefs; and these constitute the fourth and last class of the Coral formation. They prevail everywhere in tropical regions, and appear as banks of Coral encrusting the rocky shores of islands and continents.

As regards the geographical distribution of Coral Reefs, the first circumstance that claims our notice, is that they are exclusively confined to the warmer regions of the globe. They exist in great profusion within the tropics, and are rarely to be found beyond the thirtieth parallels of latitude on each side of the Equator. The only remarkable exception is in the case of the Bermuda Islands in 32° north latitude; but here, it is to be observed, the ocean is warmed by the waters of the Gulf Stream. Another singular fact is the almost total absence of Coral Reefs from the Atlantic Ocean. In fact, the Bermudas, we believe, constitute here again the only exception. The Pacific, on the contrary, is wonderfully productive of coral; also the Indian Ocean, the Persian and Arabian Gulfs, and the Red Sea.

It may gratify, perhaps, the curiosity of some readers, if we add a word on the Red Coral which is now so favorite an ornament in the fashionable world. Though it never attains to the magnitude of those reefs and islands we have been describing, it partakes nevertheless of the same peculiar structure; and no doubt is entertained that, like them, it derives its existence from animal life, in the manner we shall presently explain. It is produced chiefly in the Mediterranean, in the Red Sea, and in the Persian Gulf; and is brought up from the great depths by means of a grappling apparatus attached to boats. The largest pieces have a shrub-like branching form, and are supposed to grow to the height of one foot in about eight years.52

So much for the existence of the Coral Formation. Next comes the question of its origin, with which, of course, we are chiefly concerned. It is now the received belief of all distinguished Naturalists, that these huge and wide-spread masses of limestone rock, against which the breakers of the ocean are ever thundering in vain, are the work of tiny marine animalcules, and chiefly of those seemingly insignificant creatures known by the name of Polyps or Zoophytes. The Zoophyte, they tell us, is a mason who himself produces the stones that he employs in his building. “He has neither plane, nor chisel, nor trowel; there is no sound of hammer in his city. He erects mighty and enduring edifices, yet has no mechanical power by which to raise his rocks to their summits. He can answer thee nothing—no tongue, no eyes, no hands, no brains has he—yet from the caves of old ocean has he raised that which fills you with admiration.”53 Surely if all this be true, these countless myriads of animalcules call aloud to us from the depths of the ocean in language that cannot be mistaken: “Know ye that the Lord He is God; it is He that hath made us, and not we ourselves.”54

The Zoophyte belongs to the simplest form of the animal creation. Its body consists merely of a pouch or stomach, with tentacles arranged round the margin, which it can extend at pleasure to supply itself with food. In many species the individuals grow together on a common stem, from which new members are constantly shooting forth like buds from the branches of a tree. Hence the origin of the name Zoophyte, which literally means a plant-like animal. The common stem on which they grow is sometimes composed of a horny substance, but more generally it is pure carbonate of lime, which they secrete by the powers of organic action from the waters of the sea. It forms, therefore, a kind of internal skeleton or framework, to which the soft, gelatinous parts of the animal adhere, pretty much as, in the case of other animals, the flesh adheres to the bones. Thus we have, as it were, a community of living creatures, growing together upon one common stony framework, called a Polypidom or Polyp edifice, which they themselves build by the very fact of living.

Fig. 4.—Campanularia Gelatinosa.