Volume 74 / Part 9 — CORAL RINGS.[5]

CORAL RINGS.^[5]

Montgomery’s well-known lines in praise of the coral polyps have given these animals a tolerable share of poetical celebrity. Mr Darwin’s ingenious researches have invested them with a degree of importance which elevates them to the rank of a great geological power. These minute creatures are now entitled to a larger share of consideration than the greatest and most skilful of quadrupeds can claim. All the elephants and lions which have been quartered in this world since its creation—all the whales and sharks which have prowled about in its waters—have done much less to affect its physical features, and have left far slighter evidences of their existence, than the zoophytes by whose labours the coral formations have been reared. For the most colossal specimens of industry we are indebted to one of the least promising of animated things. Comparing their humble organisation with that of other tribes, we feel pretty much the same sort of surprise as a man might express were he told that the pyramids and temples of antiquity had not been constructed by Egyptians or Romans, but by a race like the Earthmen of Africa, or by a set of pigmies like the Aztecs now exhibiting in London.

Though the works now before us have been long in the hands of the public, the substance of their contents is far from being generally known. Yet the beauty of the results at which their authors have arrived, and the interest with which they have invested the coral reefs, may well recommend these volumes to universal perusal. While Dana, more than all his predecessors, has illustrated the natural history of the little gelatinous creatures by which the coral is secreted, Darwin has described the growth and consolidation of their labours into lofty and extended reefs, and connected these with the broadest and most striking phenomena of physical geology. The toiling of the minute zoophytes in the production of vast masses of coral rock which wall round whole islands, and stretch their mural barriers across deep and stormy seas, he has shown to be successful only through the conjoined operation of those wonderful physical forces which are now lifting and now lowering large areas of the earth’s surface.

Mr Darwin’s views not only exhibit a charming sample of scientific induction, but carry with them such an air of probability, that the most cautious investigators may subscribe to them without any particular demur. Being the result of very extensive inquiries, and confirmed by collating the peculiarities of many reefs, they are grounded upon a sufficient quantity of data to entitle them to reasonable confidence. We propose, in the present article, to indicate some of the principle steps in the theory which this gentleman has propounded; and that the reader may examine them consecutively, we shall imagine an intelligent voyager visiting the Pacific for the first occasion in his life. As he sails across that noble sheet of water, observing with a philosophic eye every object which presents itself to his view, he suddenly perceives in the midst of the sea a long low range of rock against which the surf is breaking with a tremendous roar. He is told that this is a coral reef; and having read a little respecting these curious productions, he resolves to investigate them carefully, in order to fathom, as far as possible, the mystery of their origin. As he approaches, the spectacle grows more interesting at every step. Trees seem to start up from the bosom of the ocean, and to flourish on a beach which is strewed with glistening sand, and washed by the spray of enormous billows. When sufficiently near to survey the phenomenon as a whole, he perceives that he has before him an extensive ring of stone, set in an expanse of waters, and exhibiting the singular form of an annular island. Launching a boat, and following the curve of the shore for some distance, he finds at length an opening through which he penetrates into the interior of the ring. Once entered, he floats smoothly on a transparent lake of bright green water, which seems to have been walled in from the rest of the ocean, as if it were a preserve for some sort of nautical game, or a retreat for the more delicate class of marine divinities. Its bed is partially covered with pure white sand, but partly also with a gay growth of coral—the stems of this zoophyte branching out like a plant, and exhibiting the most brilliant diversities of colour, so that the floor of the lake glows like a sunken grove. All the hues of the spectrum may be seen gleaming below, whilst fishes scarcely less splendid in their tints glide to and fro in search of food amidst this shrubbery of stone. A fringe of trees, consisting principally of graceful palms, decorates the inner portion of the ring, and when surveyed from the centre of the lagoon, this edging of verdure springing up in the midst of the Pacific presents one of the most picturesque sights the voyager can conceive. Indeed, as he contemplates the tranquil lake within, and listens to the dash of the surf without—as he runs over the features of this beautiful oasis in the wilderness of waters, we may pardon him if he almost expects to be accosted by ocean nymphs or startled mermaids, and indignantly expelled from their private retreat.

The whole structure is so striking, that the most careless observer must feel some little curiosity to ascertain its origin. Our voyager regards it with much the same sort of interest as an intelligent wanderer would display, were he to stumble upon a ring of blocks like those at Abury or Stonehenge in some distant desert. In order to pursue his inquiries systematically, he proceeds to note down the principal characteristics of the scene. The first peculiarity which arrests his consideration, is the circular form which the rock assumes. Though far from constituting a smooth and perfect ring, its outline is sufficiently definite to rivet the attention at once. Then he observes that the outer portion of the annulus scarcely rises above the level of the sea, whilst the inner portion—the bank on which the belt of trees is mounted—is not more than ten or twelve feet in height at the utmost. From this he infers that the agency concerned in the formation of the structure was probably restricted in its upward range. Next he notices that the ring itself—that is, the wall of rock enveloping the lake, though by no means uniform in breadth—is not more, perhaps, than three or four hundred yards across in any part of its extent: this seems to say, that the agency was also restrained by circumstances in its lateral expansion. Again, as he runs his eye along the whole sweep of the reef, he remarks that it is not quite continuous, the ring being broken here and there by openings, through one of which he himself passed into the lagoon. If he then endeavours to estimate the size of the whole formation with its included lake, he may find it in this particular case to be eight or ten miles in circumference. Should he stoop down to examine the material of which the reef is composed, he will discover it to be dead coral rock mixed with sand where it is not washed by the sea; but on breaking off a fragment where it is covered with water, he may observe multitudes of little worms, or curiously shaped polyps, which, incompetent as they seem, are in reality the architects of the pile. But perhaps the most significant circumstance to be noticed is the difference in depth between the internal lagoon and the external ocean. If he takes soundings within the reef, he ascertains that the water is comparatively shallow, the slope of the rock beneath the lake being tolerably gentle, and the depth rarely more than thirty or forty fathoms. Let him cross the ring, however, pushing his way through the belt of trees; and on trying the experiment in the contrary direction, seawards, he finds that the ground shelves downwards gradually under the water, until it reaches a depth of five-and-twenty fathoms, after which it plunges precipitously into the abyss. So abrupt, indeed, does the descent become when this point has been attained, that at the distance of a hundred yards from the reef he cannot reach the bottom of the sea with a line of two hundred fathoms. If, then, our explorer were capable of existing under water for a while, and could be lowered to the bed of the ocean, he would see before him an enormous cone or mound of rock shooting upwards through the liquid to a prodigious height, its summit being hollowed into a kind of cup or shallow basin, the rim of this lofty vase just peering above the level of the waves, and its interior being partially inlaid with a gorgeous and flower-like growth of coral.

Now, without glancing at minor details, it must be admitted that our voyager has stumbled upon a fine physical problem. As the Round Towers of Ireland have constituted one of the most perplexing questions on shore, so these coral towers of the tropics seem to present an equally perplexing mystery for the sea. In the course of his researches, however, he detects a circumstance which appears to be perfectly paradoxical. Climbing the cliff from the bottom of the ocean, he perceives that the creatures which produce the coral cannot exist at any greater depths below the surface than from twenty to five-and-twenty fathoms. Within that limit, upwards, the rock is covered with life; below, it is tenantless and dead. Yet, descending as the structure of coral does to immeasurably greater depths, the question naturally arises—how could the animal ever toil where it cannot even live? How has that part of the edifice, which lies buried in a region where no sunbeam ever pierces, been built by architects whose range of activity is comparatively so restricted?

Brooding over an inquiry, which only adds fuel to his curiosity, he proceeds on his cruise. He has already noted the prominent features of one particular reef, which exhibits a coral construction in its simplest shape—namely, as a ring enclosing a lagoon. He now falls in with specimen after specimen of a similar class, and carefully observes the differences in character they present. In point of shape, he finds that some are oval, others greatly elongated, and many very jagged and irregular in their form. Here is one like a bow, and there another like a horse shoe, whilst none can be said to be geometrically round. In regard to size, he meets with reefs which are a single mile only in diameter, and then with others, which amount to as many as fifty, sixty, or even more. If he compares the various rings, he observes that some are perforated by few openings, and in rare cases there are none—the fissures having apparently been filled up with sand or detritus, so as to form a continuous girdle round the lake. But, in other instances, the reef is so freely intersected by these openings, that the ring itself may be said to consist of a series of small islands arranged upon an extensive curve. In general, however, he perceives that the channels connecting the ocean with the lagoon are confined more especially to that side of the structure which is least exposed to the action of the wind; and as he is sailing within the region of the trade-winds, the portion of the reef which fronts the breeze and the billow perpetually, appears to be more lofty and substantial than the other. Glancing, too, at the bank which carries the fringe of trees, he observes that it never seems to rise higher than a certain level in any case whatever; and as he finds that it consists chiefly of sand and sediment, he concludes that it has been heaped up by the waves themselves. The vegetation, indeed, which frequently gives such a gay and graceful aspect to coral rocks, does not always gladden the eye; but where it is wanting, he infers that the circumstances which favour the dissemination of seeds or the growth of plants, have failed to operate as yet, but may, perhaps, in process of time produce their accustomed effects. Comparing also the depth of the lagoons with that of the surrounding ocean, he ascertains that the striking discrepancy which attracted his attention in the first reef he examined, obtains to a considerable degree in every subsequent instance: however shallow the sea may be within the ring, its depth rapidly increases, and frequently becomes quite unfathomable at no great distance without. Finding, then, that though certain differences exist in the formations he has already inspected, yet certain general features of resemblance invariably prevail, he concludes that all of these structures are due to the operation of a kindred agency. But here there arises another perplexing question. If he must admit—and the admission is inevitable—that the coral polyps have been the builders of these piles, how can he suppose that a number of small animals, each labouring separately, as it were, could erect an immense wall of rock, leagues in circumference, which, though far from regular in its composition, shall yet exhibit any marked approach to a circle, an oval, a horse shoe, or any other symmetrical form? Still more, how could they build, not one, but innumerable reefs, differing in various particulars, but all indicating some common principle of construction? How is he to explain the appearance of co-operation, where, from the nature of the creatures, he cannot imagine any intentional co-operation to exist? A troop of moles working beneath a field will never cast up a succession of hillocks in such a way that they will all combine to form a spacious circle, or any other regular and definite figure. If, therefore, he is compelled to believe that a number of insignificant creatures like the coral polyps are capable of executing such prodigious undertakings, wanting, as they do, the intelligence which enables higher beings to carry out a coherent scheme, he must look for an explanation, not in the instincts of the animals, but in the conditions under which they pursue their toils.

Hitherto, however, our voyager has only encountered reefs of one class—namely, “atolls,” or lagoon islands. He looks anxiously, therefore, in the hope of falling in with a specimen of a different description. He knows that if a process is too slow in its action to admit of direct observation, yet its character may probably be ascertained by comparing several cases where the same agency is employed—that is, by criticising the phenomenon in distinct stages of development. He proceeds on his voyage, and at length is fortunate enough to meet with a coral formation which varies in type from those already inspected. There is the same sort of ring springing hastily from the sea; but instead of an internal lagoon, the central space is occupied by a beautiful and populous island, leaving only a belt of water between the reef and the shore. Where all the elements of such a scene are sufficiently defined, a more charming spectacle can hardly be conceived. The land appears like a pleasant picture framed in coral. Round a group of mountains, forming the nucleus of the isle, there runs a verdant zone of soil—next comes a girdle of tranquil water—then a ring of coral—and last, a band of snowy breakers, where the swell of the ocean is shattered into surf. The island of Tahiti, whose mountains rise to the height of seven thousand feet, and whose greatest breadth is about thirty-six miles, is almost encompassed by a reef of this description. When this spot is approached so as to make the separate objects visible, the appearance becomes quite striking. “Even upon the steep surface of the cliff, vegetation abounds; the belt of low land is covered with the tropical trees peculiar to Polynesia, while the high peaks and wall-faced mountains in the rear are covered with vines and creeping plants. This verdure is seen to rise from a quiet girdle of water, which is again surrounded by a line of breakers dashing in snow-white foam on the encircling reefs of coral.”^[6] Perhaps, however, the descent of the waves upon the ring—curling and chafing like coursers suddenly curbed—constitutes the most magnificent feature of the scene. “The long rolling billows of the Pacific, arrested by this natural barrier, often rise ten, twelve, or fourteen feet above its surface, and then, bending over it, their foaming tops form a graceful liquid arch, glittering in the rays of a tropical sun, as if studded with brilliants; but before the eyes of the spectator can follow the splendid aqueous gallery which they appear to have reared, with loud and hollow roar they fall in magnificent desolation, and spread the gigantic fabric in froth and spray upon the horizontal and gently broken surface of the coral.”^[7]

With a reef like this before him our explorer may now collect some additional data which will help him a few steps onward in his inquiry. The distinction between a formation of this class and those of the former description, consists principally in the substitution of an internal island for a lagoon. Were that island pared away or dug out, a simple lake surrounded by a ring of coral rock would be left. The one structure would pass into the other by the erasure of the central land. But here again he has stumbled over a difficulty apparently as great as any he has previously encountered; for it would be preposterous to suppose that large areas or lofty hills could be readily expunged from the surface of the earth. There is a stage, however—call it rather a pause—in the reasoning process, when the great master of inductive logic recommends that, after having arranged all our available facts, and extracted from them all the inferences they can legitimately supply, we should allow the mind to take a little leap forward, just by way of venture, and see what conclusions it will suggest. In short, we are to send for the imagination, yoke it to the materials we have accumulated, and observe in what direction it will conduct us. Our explorer does this. He sets that faculty to work—with due discretion, however—and in a short time it hints to him that islands may possibly sink down slowly in the ocean by the action of the subterranean forces. And if so, would not that explain everything?

He proceeds, therefore, to inquire how this supposition will work; for there are many conditions which it must satisfy, and many puzzles which it must solve, before its probability can be affirmed. In the first place, the coral polyps, as we have seen, can only operate within a limited depth of water, which has been roughly fixed at twenty or five-and-twenty fathoms. Mr Dana, indeed, considers that sixteen fathoms will perhaps measure the whole extent of the region assigned to the principal artificers. Consequently, when the creatures laid the foundation of any particular reef, they must have done so in shoal water, or in the neighbourhood of land. Next, where a small isle issues from a profound sea, it will in general be tolerably regular in shape; because, with relation to the bed of that sea, it must in reality be a kind of mountain: therefore, as the coral builders find the requisite range of water in the zone which encircles the shore, the reef they form will be tolerably regular too. Hence the circular or curvilinear outline which these structures generally assume. Then, if, after the basement of such a ring has been laid, the land should begin to descend slowly, the polyps must proceed to raise the edifice storey after storey, for thus alone can they keep themselves within the region of vitality; and here we have an explanation of the singular fact, that the reef, where it constitutes a true atoll, or coral-lagoon, usually ascends to the level of the sea. A singular fact we call it; because, if we consider how variable are the heights of any series of mountains on land, the equality of stature which distinguishes these marine elevations is certainly a remarkable result. If it were possible for some great giant to run the palm of his hand along the tops of the Andes or Himalayas, it would describe a very irregular sweep, rising or falling with every peak it visited; but were he to draw it over the summits of a succession of atolls, though these might stretch through a space thousands of miles in length, he would scarcely perceive any difference whatever in point of altitude. It will be seen, therefore, that the uniformity characterising these Alps of the ocean is a circumstance which our explorer’s hypothesis readily solves. But in raising their embankment higher, it is clear that the animals must build up vertically, and hence the abrupt or precipitous face which it presents externally towards the deep water. Landwards, again—that is, within the reef—the pigmy architects will labour more feebly, because it is found that the kind of polyps which exist in smooth still water are more delicate in their productions than their gallant little brethren who flourish amongst the breakers. This serves to explain, again, why there is an interval of fluid left between the rising reef and the sinking shore; but as the land subsides, the space which it occupies within the magic ring will obviously diminish, whilst the space covered by water will proportionately increase. The girdle of coral will not maintain its original dimensions, because the polyps will probably incline inwards, instead of building directly upwards; but the contraction of the ring will proceed slowly, because the wall is invariably steep seawards, even if it should not be altogether precipitous. Finally, when the island is fairly drowned, when we have got its whole body well under water, we shall have an enormous mass of coral raised by successive additions of coral skeletons, and resting upon a basis which may be hundreds of feet below the level of the sea. A zone of rock, constituting the rim of the structure, will just show itself above the waves, whilst within this zone sleeps a shallow lake, where the polyps, for various reasons, have not followed the growth of the ring with equal rapidity, or where the sediment deposited has not accumulated in sufficient quantities to fill up the interior. And when the lake is obliterated, as ultimately it may be, either by the labours of the feebler animals, or by the deposition of detritus from the reef, we shall have the platform of a new country where tropical forests may some day flourish, where towns and villages may hereafter arise, and where man may exhibit the strange and mingled play of virtue and vice, which has marked his footsteps from the first. “The calcareous sand lies undisturbed, and offers, to the seeds of trees and plants cast upon it by the waves, a soil upon which they rapidly grow, to overshadow its dazzling white surface. Entire trunks of trees, which are carried by the rivers from other countries and islands, find here, at length, a resting-place, after many wanderings: with these come some small animals, such as insects and lizards, as the first inhabitants. Even before the trees form a wood, the sea-birds nestle here; stray land-birds take refuge in the bushes; and at a much later period, when the work has been long since completed, man appears, and builds his hut on the fruitful soil.”^[8]

Thus, it will be seen that the supposition of a slow descent of the land appears to meet the prominent requirements of the case; and however startling the assumption might seem when first suggested, yet the pressure of certain conditions, which this theory alone can sustain, renders its adoption almost, if not altogether, inevitable. But, says the explorer, if this hypothesis be correct, it should follow that, as the sinking isle may vary in altitude in different parts—as it may have several peaks or elevated districts—all these higher portions must be left projecting out of the water for some time after the lower lands have been entirely submerged. Accordingly, we may expect to discover coral reefs, containing within their circuit several small islands, the relics of some larger district which has died a watery death. And this is just what frequently occurs. The two isles of Raiatea and Tahaa, for example, are included in one reef. The group known as Gambier’s Islands consists of four large and a few smaller islets encircled by a single ring. The reef of Hogoleu, which is one hundred and thirty-five miles in circuit, contains ten or eleven islands in its spacious lagoon.

So, again, says our explorer, as islands are frequently arranged in clusters, it should follow that, if the areas whereon any of these groups were stationed, have subsided, whole archipelagoes of coral reefs ought to exist. And some of these archipelagoes may be expected to exhibit a series of perfect lagoons, where the land has been fairly submerged; whilst others, where the process is less advanced, or the ground more elevated, ought to present a series of reef-encircled islands merely. Here also the theory is fully corroborated by facts. Low Archipelago is composed of about eighty atolls; and of the thirty-two groups examined by Captain Beechy, twenty-nine then possessed the internal lakes which we have seen are characteristic of this class; the remaining three having passed, as he believed, from the same condition originally to the dignity of closed or consolidated reefs. The Society Archipelago, again, consists of tolerably elevated islands, encircled by coral ledges, and lying in a direction almost parallel to the last.

Indeed, it will be readily imagined that the shape and character of the coral formations must be considerably influenced by the nature of the site upon which they are reared. They will assume different aspects according to the physical configuration of the land to be entombed. They must be interrupted where the water is too deep, or the shore too precipitous to permit the artificers to acquire a proper footing. They will exhibit breaches where the descent of cold streams from the mountain heights, or the presence of mud carried down by rivers, rendered it impracticable for the creatures to pursue their avocations. They may also adopt peculiar forms where the lowering of the ground may not have taken place gradually, or where, from some eccentric action of the subterranean force, one portion may have sunk under different circumstances from the rest. A reef may, therefore, be submerged in part, or, as in some instances, throughout its whole extent. Thus, in the Peros Banhos Atoll, forming a member of the Chagos group in the Indian Ocean, a portion of the ring dips under water for a distance of about nine miles. This sunken segment consists of a wall of dead coral rock, lying at an average depth of five fathoms below the surface, but corresponding in breadth and curve with the exposed reef, of which it is obviously the complement. Or a ring may be wholly submarine. The same group affords, amongst others, an admirable example of this in the Speaker’s Bank, which is described as a well-defined annulus of dead coral, let down into the sea to a depth of six or eight fathoms, with a lagoon twenty-two fathoms deep and twenty-four miles across. It is apparently a drowned atoll. Hence from these, or from other causes, such as the action of the sea, the killing of the zoophytes by exposure or otherwise, we may have several modifications of the model reef.

As yet we have only mentioned two principal types of structure—first, the atolls or coral-lagoons; and, second, the encircling reefs. But we may here refer, in a sentence or two, to a third and an important class—namely, the barrier reefs. These are extensive lines of coral masonry, which pursue their course at a considerable distance from the shore, but with a degree of conformity to its outline, sufficient to prove that some relationship subsists between them. They do not, however, surround an island like the encircling reefs. The West Coast of New Caledonia is armed with a reef of this character, 400 miles in length; but in some parts it is sixteen miles distant from the shore, and seldom approaches it nearer than eight miles in any other quarter. This great ledge of coral rock is, moreover, prolonged for 150 miles at the northern extremity of the island; and then, returning in the form of a loop, and terminating on the opposite shore, seems to intimate that, in ancient days, New Caledonia was of much greater extent in this direction than it is at present. There is a still more magnificent specimen of the barrier reef on the north-east of Australia. This noble coral ridge is a thousand miles in length. Its distance from the coast is generally between twenty and thirty miles, but occasionally as much as seventy. The depth of the sea within the barrier is from ten to twenty-five fathoms, but at the southern extremity it increases to forty, or even sixty. On the other side, without the barrier, the ocean is almost unfathomable. The breadth of this embankment varies from a few hundred yards to a mile, and it is only at distant intervals that it is intersected by channels through which vessels may enter. It is a causeway for giants, and yet the architects were mere polyps!

It is time, however, that our voyager should proceed to verify the supposition his fancy suggested. As yet he has adduced no proof that subsidence is, or has been, the order of the day where its results are supposed to appear. He knows that mountains and islands must not be sunk by a mere assumption, however plausibly that assumption may seem to solve the mystery of the reefs. Now, it is an admitted fact that, in certain parts of the globe, extensive regions have been hoisted up, some suddenly, some slowly; whilst others have gone down in the world just as suddenly or as slowly. The coast of Chili and the adjoining district, as is well known, were once elevated several feet, throughout an area of perhaps 100,000 square miles, in the course of a single night. Sweden has long been rising in its northern portion, and sinking in its southern, as if it were playing at see-saw on a magnificent scale. But we want evidence from the coral localities themselves. Of course, from the nature of the case, the testimony must necessarily be somewhat limited; because the question relates to a tardy movement, operating through ages, and occurring in regions which may be wholly uninhabited, or else peopled by tattooed and unphilosophical savages. But there seems to be tolerable proof for the purpose in hand. For instance, in an island called Pouynipate, in the Caroline Archipelago, one voyager describes the ruins of a town which is now accessible only by boats, the waves reaching to the steps of the houses. Of course, it is not likely that the founders of that place would build their habitations in the water; and, therefore, it must be inferred that this spot is in course of depression. Such, according to theory, should be its condition, because it consists of land encircled by a reef—that is, of land which must all vanish before the formation can be converted into a true coral-lagoon. At Keeling Island, again, Mr Darwin observed a storehouse, the basement of which was originally above highwater, but which was then daily washed by the tide. Many other instances of the same sort might be advanced; but there is still more striking evidence on this point, perhaps, in the existence of certain reefs which may now be introduced as links in the theory, or rather as tests by which its validity may be tried. These have been styled “shore” or “fringing” reefs. They differ from the other classes in the shallowness of the foundation on which they rest, and in the closeness of their approach to the land—either lining the shore itself, or, if separated, leaving a channel of no great depth between the coral bank and the coast. Wherever these exist, it is clear that the soil is stationary, or that it must be in course of elevation. It cannot be undergoing depression, because the coral beds would increase in thickness, and graduate into another class of structure. And in many instances where these fringes abound, there is the clearest proof, derived from organic remains, and other geological evidences, that the land has been actually upraised. A resident at Oahu, one of the Sandwich Islands (which are all fringed), stated that, from changes effected within a period of sixteen years only, he was satisfied that the work of elevation was proceeding at a very perceptible rate. Indeed, in numerous cases of this kind, coral deposits are found at a height where it is as certain that the polyps could never have toiled, as it is certain that fishes could never have lived. But elevation in one quarter implies depression in another. And, accordingly, it has been shown that the Pacific and Indian Oceans might almost be divided into a series of great bands, where the bed of the sea has alternately risen and sunk—just as if in one band the crust of the earth had been heaped up into a great solid wave, and in the next had subsided into a huge submarine trough or valley. For it happens that the reefs abounding over one of these areas belong almost universally to the class of formation which, according to theory, indicates that the ground is subsiding, whilst those which distinguish the next area are quite of the opposite description, and intimate that the crust is rising. Thus, for example, if we select the broadest illustration available, it will be seen, on referring to a map of the Pacific, that there is an extensive chain of islands, beginning to the west of the Caroline Archipelago, and running through Low Archipelago—a distance of several thousand miles—the whole family of which belong to the type denoting depression; whilst there is another long chain of islands, corresponding or parallel, in some measure, with the first, and extending, say from Sumatra to the south-east of the Friendly Isles, most of which indicate, by their reefs, that they belong to the type denoting elevation.

The general coincidence, therefore, of fringing reefs with raised or stationary districts, and of atolls or lagoons with regions which appear to be subsiding, affords considerable support to the theory our voyager is maturing. But there is another remarkable criterion, which in due time he contrives to discover. In the districts where fringing reefs occur, or where the coral has been plainly uplifted, active volcanoes are frequently established. But where reefs of the contrary character prevail, these agents are rarely, if ever, to be found. Of course, where a volcano presents itself in any particular locality, and especially if it happens to be a volcano in a state of activity, this shows that the subterranean forces are disposed to upheave the soil above them; whereas, if volcanoes are wanting in another quarter, or if, being there, their activity has ceased, the conclusion is, that in this region no upward tendency at present exists. Now, this test, too, is in striking accordance with geographical fact. The two great chains of reefs already mentioned may again be adduced. In the series of atolls or subsiding islands extending from Caroline Archipelago to Low Archipelago, not a single working volcano is to be detected within several hundred miles of any moderate cluster; whereas, in the band or series of isles which are characterised by fringes, numbers of these powerful agents are busily engaged; and in some of them, as, for instance, in Java, the subterranean forces are known to be intensely energetic. In fact, it may be stated as a pretty authentic conclusion, that whilst volcanoes frequently appear in those areas where the crust of the earth is now, or has recently been, in upward motion, “they are invariably absent in those where the surface has lately subsided, or is still subsiding.”^[9]

At the same time, it may be interesting to remark, that whilst busy volcanoes are thus shown to be irreconcilable with the presence of true atolls, yet at one period the theory most in fashion assumed that all coral-lagoons were mere submarine craters, whose rims had been coated with calcareous matter by the coral polyps. However plausible this hypothesis might seem when applied to a few particular cases, its insufficiency was soon discovered when a considerable number of reefs had been compared, and when the order of transition from one type to another was clearly understood. The vast size of some of these atolls—the elongated shape which many assume—the mode in which they are frequently clustered—the precipitousness of their flanks, rendered it difficult, if not impossible, to treat them as drowned Etnas or Heclas. Then the equal altitudes they must have attained as submarine mounts, is totally inexplicable, if the fact of the limited operations of the polyps be admitted; for it would be preposterous to imagine that thousands of volcanic cones could all rise to the surface of the sea, or within a range of five-and-twenty fathoms, and yet never overtop the waves to a greater height than a dozen feet. But, above all, the existence of coral rings, with land in the interior—where, if the theory were correct, a large cavity should have taken the place of primitive rocks, exhibiting no signs of volcanic action—has proved utterly fatal to the theory. It is manifest that Tahiti, for example, with its lofty mountains, could never have been the centre-piece of a huge crater; and it is certain that a volcanic vent would not assume the shape of a mere moat, like the girdle of water which encompasses an ancient castle.

Combining, then, the various data already adduced, and observing that there is a general harmony in the results, our voyager may reasonably conclude that his theory has now been mounted upon a tolerably fair basis of facts. He has explained the seeming paradoxes which thrust themselves upon his view at the earlier stages of the inquiry. He has brought all the different varieties of coral formations under the grasp of one law, and shown how, by the continued operation of a subsiding force and the continued addition of coral skeletons, the “fringing” reef would pass into an “encircling” reef, and this again would graduate into a perfect “atoll.” It is true that in doing this he has been compelled to draw a pretty picture of the fluctuations to which the earth’s crust is exposed. Large areas are supposed to sink in one quarter, and to rise in another. Here and there a spot which has once been lowered may again be uplifted; and this fitful movement may, in the course of ages, be repeated, as if to show what “ups-and-downs” a poor island may be called upon to endure. He knows, indeed, that his theory trenches upon the marvellous. Were it not for the light which geology has latterly thrown upon the pranks played by the Earth in its youthful days, he is aware that his hypothesis would be condemned as a thing far too romantic for belief.

But perhaps the most surprising circumstance, after all, is, that such stupendous structures should really be fashioned by such puny artificers. When he turns his attention to the builders themselves, he finds that they are little better than lumps of jelly.^[10] The workmen, who far surpass, in the vastness of their erections, all the proud masonry of man, belong to the lowest classes of animated things. They are half-plant, half-animal. Until the commencement of the last century, indeed, their pretensions to a higher dignity than that of marine vegetables was denied; and when a certain M. Peyssonel interested himself on their behalf, and endeavoured to raise them to a higher position in the scale of organisation, his proposal was treated with much the same sort of derision as if he had demanded the admission of monkeys into the ranks of humanity. These zoophytes consist, in the main, of a mere visceral cavity, containing no distinct system of vessels, exhibiting no decided appearance of nerves, possessing no other senses than an imperfect touch and taste, and certainly manifesting no distinction of sex. They are simply digestive sacs, for which a troop of tentacles are continually foraging: they eat, drink, secrete coral, throw off young polyps, and die, without in general wandering an inch from the place where they were produced.

Of all living things we should least expect that creatures so imbecile as these would be able to run up great embankments capable of repelling billows which sometimes roll along in an unbroken ridge of a mile or two in length, or of resisting a surf whose roar may be heard at the distance of eight or nine miles. That a feeble zoophyte should have the power of breasting the waves of the Pacific, did we not know it to be a fact, would appear a more preposterous notion than that of the memorable lady who attempted to keep the Atlantic out of her dwelling with a mop. No other animals seem to possess a faculty at all approaching to this: none exhibit a constructive propensity which leads to such massive results. The bee, for example, produces more geometrical works, but we cannot conceive of a honeycomb as large as a county, or a mountain of cells as tall as Skiddaw or Snowdon. It would be absurd to dream of fabricating a reef of sponge, though, if its animal character be admitted, this creature will almost hold as high a rank in life as the coral polyp; nor would it be pardonable to imagine that such a miserable material could ever become the basis of a new island. The beaver, it is true, executes very extensive dams; he is an excellent carpenter—perhaps the most skilful four-footed artisan with which we are acquainted; but put him in the midst of a boisterous sea, to erect a great circular rampart fifty or a hundred miles in diameter, with the billows tumbling about his ears continually, and he might just as well have contracted to build the Plymouth Breakwater, or the Eddystone Lighthouse. In fact, if we consider what difficulty men have in achieving their simplest specimens of marine architecture, it may be said that, were a whole nation of human beings set to work in the Pacific, they could not accomplish one of the colossal enterprises which these morsels of pulp silently effect.

What renders the undertaking more surprising is, that these soft-bodied things have to make rock for themselves; they have to provide the very stone which constitutes the edifice they build; they have not only to find straw to produce their bricks, as it were, but to procure the clay itself. The hard coral composing their edifices is the internal skeleton of the animals, and appears to be a secretion from their own tissues. Chemical analysis has shown that it consists principally of carbonate of lime—upwards of 95 parts out of every 100—including also small quantities of silica, alumina, magnesia, iron, fluorine, and phosphoric acid. It is remarkable, however, that this secreted matter is harder than calcareous spar or common marble—much harder, indeed, says Mr Dana, than its peculiar chemical composition will explain. “Using an iron mortar,” observes Mr B. Silliman, junior, “in the earlier trials, the iron pestle was roughened and cut under the resistance of the angular masses of coral, to a degree quite remarkable, considering the nature of the substance operated on. So much iron was communicated to the powder from this source, that recourse was had to a mortar of porcelain; and even this was not proof against wear, the porcelain pestle being pitted by the repeated blows. The more porous species, of course, were crushed with less difficulty.” Whence, then, do the animals procure the materials which they fashion into such dense and enormous piles? Here are millions of tons of calcareous matter heaped up by their agency, and yet there is no visible storehouse from which they can obtain any solid supplies. For as the land subsides, the builders of the reef are cut off from the shore: there is little but coral beneath them—there is nothing but water around them. It must therefore be from the billows of the ocean that the creatures possess the power of picking out the small quantity of carbonate of lime which the fluid contains. Their food may, of course, contribute to the supply; but from what source again did the minute animals they devour procure their stock of salts and earths?

It is singular, too, to observe how limited is the sphere of activity assigned to these creatures. In order to complete a reef, it is not sufficient that one tribe or species alone should be employed; the Madrepores, Astræas, and Gemmipores are the principal masons engaged; but each structure exhibits considerable diversity of workmen. There are some polyps, as we have seen, which love the contention of the surf, and thrive only when exposed to the play of the waves; there are others which covet a more tranquil life, and prosper only in the peaceful lagoon. Neither could change places with safety, any more than the reindeer could barter climates with the camel. A reef might almost be divided into a number of zones, in each of which a particular sort of coral polyp finds its appropriate habitat. The sea-front of the ring appears to be partitioned into belts, like the vegetable regions on the slope of a mountain. “The corals on the margin of Keeling Island,” says Mr Darwin, “occurred in zones: thus the Porites and Millepora complanata grow to a large size only where they are washed by a heavy sea, and are killed by a short exposure to the air; whereas three species of Nullipora also live amidst the breakers, but are able to survive uncovered for a part of each tide. At greater depths a strong Madrepora and Millepora alcicornis are the commonest kinds, the former appearing to be confined to this part. Beneath the zone of massive corals, minute encrusting corallines and other organic bodies live.” Thus, even in the limited range allotted to these zoophytes, we have a minute illustration of the law which has been so admirably developed by Professor Edward Forbes—that the bed of the sea exhibits a series of regions, each peopled, according to its depth, by its peculiar inhabitants.

But if the creatures which are employed in the erection of the reefs are restricted to so narrow a field of exertion, a very peculiar provision has fitted them for the work they have to perform. This consists in what is called their acrogenous mode of increase. If, for example, the zoophytes assume the form of a plant, it is not the whole mass which is alive, but only a very small portion at the summit and at the extremities of the branches. All the remainder of the stem and boughs has been converted into dead coral. To grow, with them, is therefore to mount. The skeleton of the young animal is hoisted upon that of its defunct predecessor. Some zoophytes, like the Goniopores, spring up in columns to the height of two or three feet; and to each of these coral pillars a capital of live polyps, two or three inches in extent, is affixed. Or if the creatures assume a more clustered or globular form, as is the case with many of the Astrææ, Porites, and others, the depth of life in the mass is extremely small. A dome of Astræas, twelve feet in diameter, is supposed to consist of a thin film of living polyps, extending not more than half or three quarters of an inch below the surface—a solid nucleus of coral being, in fact, merely coated with vitality. It is to this property of upward and outward growth that we must ascribe the prodigious power these animals possess. Their labours are cumulative; and hence, though in themselves the most insignificant of creatures, they are enabled to heap up tier after tier of skeletons, until the mountain which has sunk in the waters is rivalled by the monument they erect upon its site.

If we wish, however, to form some conception of the marvels which these zoophytes accomplish, we have only to remember that the coral formations in the Pacific occupy an area of four or five thousand miles in length, and then to imagine what a picture that ocean would exhibit were it suddenly drained. We should walk amongst huge mounds which had been cased and capped with the stone these animals had secreted. Prodigious cones would rise from the ground, all towering to the same altitude, and reflecting the light of the sun from their white summits with dazzling intensity. Here and there we should come to a huge platform, once a large island, whose peaks, as they sank, were clothed in coral, and then prolonged upwards until they rose before us like the columns of some huge temple which had been commenced by the Anakims of an antediluvian world. If, as Champollion has said, the edifices of ancient Egypt seem to have been designed by men fifty feet high, here, whilst wandering amongst these strange monuments, we might almost fancy that beings hundreds of yards in stature had been planting the pillars of some colossal city, which they never lived to complete. But the builders, as we have seen, were mere worms; the quarry from which they dug their masonry was the limpid wave; and the vast structures which have been calmly upreared in the midst of a tempestuous sea, are the workmanship of creatures which possess neither bodily strength nor high animal instinct. That duties so important should have been assigned to beings so lowly, is one of the finest moral facts science has unfolded. It is the function of the coral polyp, under the present geological dispensation, to counteract the distant volcano, and to repair in some degree the ravages of the subterranean fires. Its task is to fasten upon a sinking island, and keep its top on a level with the sea. The haughtiest of physical forces—that which sometimes shakes great continents—which lifts or lowers whole regions in a night—is often kept in check by the industry of these diminutive things. When the earth’s crust is collapsing, and it becomes necessary to fill up the vacancy, the commission is not given to any gigantic workmen, but a number of mere polyps are bid to labour upon the subsiding soil, as if to show that the Creator could employ the humblest of His creatures in executing the largest of physical undertakings.

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