Part 5 (1/2)

CHAPTER V.--THEORY OF THE FORMATION OF THE DIFFERENT CLa.s.sES OF CORAL-REEFS.

The atolls of the larger archipelagoes are not formed on submerged craters, or on banks of sediment.--Immense areas interspersed with atolls.--Their subsidence.--The effects of storms and earthquakes on atolls.--Recent changes in their state.--The origin of barrier-reefs and of atolls.--Their relative forms.--The step-formed ledges and walls round the sh.o.r.es of some lagoons.--The ring-formed reefs of the Maldiva atolls.--The submerged condition of parts or of the whole of some annular reefs.--The disseverment of large atolls.--The union of atolls by linear reefs.--The Great Chagos Bank.--Objections from the area and amount of subsidence required by the theory, considered.--The probable composition of the lower parts of atolls.

The naturalists who have visited the Pacific, seem to have had their attention riveted by the lagoon-islands, or atolls,--those singular rings of coral-land which rise abruptly out of the unfathomable ocean--and have pa.s.sed over, almost unnoticed, the scarcely less wonderful encircling barrier-reefs. The theory most generally received on the formation of atolls, is that they are based on submarine craters; but where can we find a crater of the shape of Bow atoll, which is five times as long as it is broad (Plate I., Figure 4); or like that of Menchikoff Island (Plate II., Figure 3.), with its three loops, together sixty miles in length; or like Rimsky Korsacoff, narrow, crooked, and fifty-four miles long; or like the northern Maldiva atolls, made up of numerous ring-formed reefs, placed on the margin of a disc,--one of which discs is eighty-eight miles in length, and only from ten to twenty in breadth? It is, also, not a little improbable, that there should have existed as many craters of immense size crowded together beneath the sea, as there are now in some parts atolls.

But this theory lies under a greater difficulty, as will be evident, when we consider on what foundations the atolls of the larger archipelagoes rest: nevertheless, if the rim of a crater afforded a basis at the proper depth, I am far from denying that a reef like a perfectly characterised atoll might not be formed; some such, perhaps, now exist; but I cannot believe in the possibility of the greater number having thus originated.

An earlier and better theory was proposed by Chamisso (Kotzebue's ”First Voyage,” volume iii., page 331.); he supposes that as the more ma.s.sive kinds of corals prefer the surf, the outer portions, in a reef rising from a submarine basis, would first reach the surface and consequently form a ring. But on this view it must be a.s.sumed, that in every case the basis consists of a flat bank; for if it were conically formed, like a mountainous ma.s.s, we can see no reason why the coral should spring up from the flanks, instead of from the central and highest parts: considering the number of the atolls in the Pacific and Indian Oceans, this a.s.sumption is very improbable. As the lagoons of atolls are sometimes even more than forty fathoms deep, it must, also, be a.s.sumed on this view, that at a depth at which the waves do not break, the coral grows more vigorously on the edges of a bank than on its central part; and this is an a.s.sumption without any evidence in support of it. I remarked, in the third chapter, that a reef, growing on a detached bank, would tend to a.s.sume an atoll-like structure; if, therefore, corals were to grow up from a bank, with a level surface some fathoms submerged, having steep sides and being situated in a deep sea, a reef not to be distinguished from an atoll, might be formed: I believe some such exist in the West Indies. But a difficulty of the same kind with that affecting the crater theory, runners, as we shall presently see, this view inapplicable to the greater number of atolls.

No theory worthy of notice has been advanced to account for those barrier-reefs, which encircle islands of moderate dimensions. The great reef which fronts the coast of Australia has been supposed, but without any special facts, to rest on the edge of a submarine precipice, extending parallel to the sh.o.r.e. The origin of the third cla.s.s or of fringing-reefs presents, I believe, scarcely any difficulty, and is simply consequent on the polypifers not growing up from great depths, and their not flouris.h.i.+ng close to gently shelving beaches where the water is often turbid.

What cause, then, has given to atolls and barrier-reefs their characteristic forms? Let us see whether an important deduction will not follow from the consideration of these two circ.u.mstances, first, the reef-building corals flouris.h.i.+ng only at limited depths; and secondly, the vastness of the areas interspersed with coral-reefs and coral-islets, none of which rise to a greater height above the level of the sea, than that attained by matter thrown up by the waves and winds. I do not make this latter statement vaguely; I have carefully sought for descriptions of every island in the intertropical seas; and my task has been in some degree abridged by a map of the Pacific, corrected in 1834 by MM. D'Urville and Lottin, in which the low islands are distinguished from the high ones (even from those much less than a hundred feet in height) by being written without a capital letter; I have detected a few errors in this map, respecting the height of some of the islands, which will be noticed in the Appendix, where I treat of coral formations in geographical order. To the Appendix, also, I must refer for a more particular account of the data on which the statements on the next page are grounded. I have ascertained, and chiefly from the writings of Cook, Kotzebue, Bellinghausen, Duperrey, Beechey, and Lutke, regarding the Pacific; and from Moresby (See also Captain Owen's and Lieutenant Wood's papers in the ”Geographical Journal”, on the Maldiva and Laccadive Archipelagoes. These officers particularly refer to the lowness of the islets; but I chiefly ground my a.s.sertion respecting these two groups, and the Chagos group, from information communicated to me by Captain Moresby.) with respect to the Indian Ocean, that in the following cases the term ”low island” strictly means land of the height commonly attained by matter thrown up by the winds and the waves of an open sea. If we draw a line (the plan I have always adopted) joining the external atolls of that part of the Low Archipelago in which the islands are numerous, the figure will be a pointed ellipse (reaching from Hood to Lazaref Island), of which the longer axis is 840 geographical miles, and the shorter 420 miles; in this s.p.a.ce (I find from Mr. Couthouy's pamphlet (page 58) that Aurora Island is about two hundred feet in height; it consists of coral-rock, and seems to have been formed by the elevation of an atoll. It lies north-east of Tahiti, close without the line bounding the s.p.a.ce coloured dark blue in the map appended to this volume. Honden Island, which is situated in the extreme north-west part of the Low Archipelago, according to measurements made on board the ”Beagle”, whilst sailing by, is 114 feet from the SUMMIT OF THE TREES to the water's edge.

This island appeared to resemble the other atolls of the group.) none of the innumerable islets united into great rings rise above the stated level.

The Gilbert group is very narrow, and 300 miles in length. In a prolonged line from this group, at the distance of 240 miles, is the Marshall Archipelago, the figure of which is an irregular square, one end being broader than the other; its length is 520 miles, with an average width of 240; these two groups together are 1,040 miles in length, and all their islets are low. Between the southern end of the Gilbert and the northern end of Low Archipelago, the ocean is thinly strewed with islands, all of which, as far as I have been able to ascertain, are low; so that from nearly the southern end of the Low Archipelago, to the northern end of the Marshall Archipelago, there is a narrow band of ocean, more than 4,000 miles in length, containing a great number of islands, all of which are low. In the western part of the Caroline Archipelago, there is a s.p.a.ce of 480 miles in length, and about 100 broad, thinly interspersed with low islands. Lastly, in the Indian Ocean, the archipelago of the Maldivas is 470 miles in length, and 60 in breadth; that of the Laccadives is 150 by 100 miles; as there is a low island between these two groups, they may be considered as one group of 1,000 miles in length. To this may be added the Chagos group of low islands, situated 280 miles distant, in a line prolonged from the southern extremity of the Maldivas. This group, including the submerged banks, is 170 miles in length and 80 in breadth.

So striking is the uniformity in direction of these three archipelagoes, all the islands of which are low, that Captain Moresby, in one of his papers, speaks of them as parts of one great chain, nearly 1,500 miles long. I am, then, fully justified in repeating, that enormous s.p.a.ces, both in the Pacific and Indian Oceans, are interspersed with islands, of which not one rises above that height, to which the waves and winds in an open sea can heap up matter.

On what foundations, then, have these reefs and islets of coral been constructed? A foundation must originally have been present beneath each atoll at that limited depth, which is indispensable for the first growth of the reef-building polypifers. A conjecture will perhaps be hazarded, that the requisite bases might have been afforded by the acc.u.mulation of great banks of sediment, which owing to the action of superficial currents (aided possibly by the undulatory movement of the sea) did not quite reach the surface,--as actually appears to have been the case in some parts of the West Indian Sea. But in the form and disposition of the groups of atolls, there is nothing to countenance this notion; and the a.s.sumption without any proof, that a number of immense piles of sediment have been heaped on the floor of the great Pacific and Indian Oceans, in their central parts far remote from land, and where the dark blue colour of the limpid water bespeaks its purity, cannot for one moment be admitted.

The many widely-scattered atolls must, therefore, rest on rocky bases. But we cannot believe that the broad summit of a mountain lies buried at the depth of a few fathoms beneath every atoll, and nevertheless throughout the immense areas above-named, with not one point of rock projecting above the level of the sea; for we may judge with some accuracy of mountains beneath the sea, by those on the land; and where can we find a single chain several hundred miles in length and of considerable breadth, much less several such chains, with their many broad summits attaining the same height, within from 120 to 180 feet? If the data be thought insufficient, on which I have grounded my belief, respecting the depth at which the reef-building polypifers can exist, and it be a.s.sumed that they can flourish at a depth of even one hundred fathoms, yet the weight of the above argument is but little diminished, for it is almost equally improbable, that as many submarine mountains, as there are low islands in the several great and widely separated areas above specified, should all rise within six hundred feet of the surface of the sea and not one above it, as that they should be of the same height within the smaller limit of one or two hundred feet. So highly improbable is this supposition, that we are compelled to believe, that the bases of the many atolls did never at any one period all lie submerged within the depth of a few fathoms beneath the surface, but that they were brought into the requisite position or level, some at one period and some at another, through movements in the earth's crust. But this could not have been effected by elevation, for the belief that points so numerous and so widely separated were successively uplifted to a certain level, but that not one point was raised above that level, is quite as improbable as the former supposition, and indeed differs little from it.

It will probably occur to those who have read Ehrenberg's account of the Reefs of the Red Sea, that many points in these great areas may have been elevated, but that as soon as raised, the protuberant parts were cut off by the destroying action of the waves: a moment's reflection, however, on the basin-like form of the atolls, will show that this is impossible; for the upheaval and subsequent abrasion of an island would leave a flat disc, which might become coated with coral, but not a deeply concave surface; moreover, we should expect to see, in some parts at least, the rock of the foundation brought to the surface. If, then, the foundations of the many atolls were not uplifted into the requisite position, they must of necessity have subsided into it; and this at once solves every difficulty (The additional difficulty on the crater hypothesis before alluded to, will now be evident; for on this view the volcanic action must be supposed to have formed within the areas specified a vast number of craters, all rising within a few fathoms of the surface, and not one above it. The supposition that the craters were at different times upraised above the surface, and were there abraded by the surf and subsequently coated by corals, is subject to nearly the same objections with those given above in this paragraph; but I consider it superfluous to detail all the arguments opposed to such a notion. Chamisso's theory, from a.s.suming the existence of so many banks, all lying at the proper depth beneath the water, is also vitally defective. The same observation applies to an hypothesis of Lieutenant Nelson's (”Geolog. Trans.” volume v., page 122), who supposes that the ring-formed structure is caused by a greater number of germs of corals becoming attached to the declivity, than to the central plateau of a submarine bank: it likewise applies to the notion formerly entertained (Forster's ”Observ.” page 151), that lagoon-islands owe their peculiar form to the instinctive tendencies of the polypifers. According to this latter view, the corals on the outer margin of the reef instinctively expose themselves to the surf in order to afford protection to corals living in the lagoon, which belong to other genera, and to other families!), for we may safely infer, from the facts given in the last chapter, that during a gradual subsidence the corals would be favourably circ.u.mstanced for building up their solid frame works and reaching the surface, as island after island slowly disappeared. Thus areas of immense extent in the central and most profound parts of the great oceans, might become interspersed with coral-islets, none of which would rise to a greater height than that attained by detritus heaped up by the sea, and nevertheless they might all have been formed by corals, which absolutely required for their growth a solid foundation within a few fathoms of the surface.

It would be out of place here to do more than allude to the many facts, showing that the supposition of a gradual subsidence over large areas is by no means improbable. We have the clearest proof that a movement of this kind is possible, in the upright trees buried under the strata many thousand feet in thickness; we have also every reason for believing that there are now large areas gradually sinking, in the same manner as others are rising. And when we consider how many parts of the surface of the globe have been elevated within recent geological periods, we must admit that there have been subsidences on a corresponding scale, for otherwise the whole globe would have swollen. It is very remarkable that Mr. Lyell (”Principles of Geology,” sixth edition, volume iii., page 386.), even in the first edition of his ”Principles of Geology,” inferred that the amount of subsidence in the Pacific must have exceeded that of elevation, from the area of land being very small relatively to the agents there tending to form it, namely, the growth of coral and volcanic action. But it will be asked, are there any direct proofs of a subsiding movement in those areas, in which subsidence will explain a phenomenon otherwise inexplicable?

This, however, can hardly be expected, for it must ever be most difficult, excepting in countries long civilised, to detect a movement, the tendency of which is to conceal the part affected. In barbarous and semi-civilised nations how long might not a slow movement, even of elevation such as that now affecting Scandinavia, have escaped attention!

Mr. Williams (Williams's ”Narrative of Missionary Enterprise,” page 31.) insists strongly that the traditions of the natives, which he has taken much pains in collecting, do not indicate the appearance of any new islands: but on the theory of a gradual subsidence, all that would be apparent would be, the water sometimes encroaching slowly on the land, and the land again recovering by the acc.u.mulation of detritus its former extent, and perhaps sometimes the conversion of an atoll with coral islets on it, into a bare or into a sunken annular reef. Such changes would naturally take place at the periods when the sea rose above its usual limits, during a gale of more than ordinary strength; and the effects of the two causes would be hardly distinguishable. In Kotzebue's ”Voyage”

there are accounts of islands, both in the Caroline and Marshall Archipelagoes, which have been partly washed away during hurricanes; and Kadu, the native who was on board one of the Russian vessels, said ”he saw the sea at Radack rise to the feet of the cocoa-nut trees; but it was conjured in time.” (Kotzebue's ”First Voyage,” volume iii., page 168.) A storm lately entirely swept away two of the Caroline islands, and converted them into shoals; it partly, also, destroyed two other islands. (M.

Desmoulins in ”Comptes Rendus,” 1840, page 837.) According to a tradition which was communicated to Captain Fitzroy, it is believed in the Low Archipelago, that the arrival of the first s.h.i.+p caused a great inundation, which destroyed many lives. Mr. Stutchbury relates, that in 1825, the western side of Chain Atoll, in the same group, was completely devastated by a hurricane, and not less than 300 lives lost: ”in this instance it was evident, even to the natives, that the hurricane alone was not sufficient to account for the violent agitation of the ocean.” (”West of England Journal”, No. I., page 35.) That considerable changes have taken place recently in some of the atolls in the Low Archipelago, appears certain from the case already given of Matilda Island: with respect to Whitsunday and Gloucester Islands in this same group, we must either attribute great inaccuracy to their discoverer, the famous circ.u.mnavigator Wallis, or believe that they have undergone a considerable change in the period of fifty-nine years, between his voyage and that of Captain Beechey's.

Whitsunday Island is described by Wallis as ”about four miles long, and three wide,” now it is only one mile and a half long. The appearance of Gloucester Island, in Captain Beechey's words (Beechey's ”Voyage to the Pacific,” chapter vii., and Wallis's ”Voyage in the 'Dolphin',” chapter iv.), has been accurately described by its discoverer, but its present form and extent differ materially.” Blenheim reef, in the Chagos group, consists of a water-washed annular reef, thirteen miles in circ.u.mference, surrounding a lagoon ten fathoms deep: on its surface there were a few worn patches of conglomerate coral-rock, of about the size of hovels; and these Captain Moresby considered as being, without doubt, the last remnants of islets; so that here an atoll has been converted into an atoll-formed reef. The inhabitants of the Maldiva Archipelago, as long ago as 1605, declared, ”that the high tides and violent currents were diminis.h.i.+ng the number of the islands” (See an extract from Pyrard's Voyage in Captain Owen's paper on the Maldiva Archipelago, in the ”Geographical Journal”, volume ii., page 84.): and I have already shown, on the authority of Captain Moresby, that the work of destruction is still in progress; but that on the other hand the first formation of some islets is known to the present inhabitants. In such cases, it would be exceedingly difficult to detect a gradual subsidence of the foundation, on which these mutable structures rest.

Some of the archipelagoes of low coral-islands are subject to earthquakes: Captain Moresby informs me that they are frequent, though not very strong, in the Chagos group, which occupies a very central position in the Indian Ocean, and is far from any land not of coral formation. One of the islands in this group was formerly covered by a bed of mould, which, after an earthquake, disappeared, and was believed by the residents to have been washed by the rain through the broken ma.s.ses of underlying rock; the island was thus rendered unproductive. Chamisso (See Chamisso, in Kotzebue's ”First Voyage,” volume iii., pages 182 and 136.) states, that earthquakes are felt in the Marshall atolls, which are far from any high land, and likewise in the islands of the Caroline Archipelago. On one of the latter, namely Oulleay atoll, Admiral Lutke, as he had the kindness to inform me, observed several straight fissures about a foot in width, running for some hundred yards obliquely across the whole width of the reef. Fissures indicate a stretching of the earth's crust, and, therefore, probably changes in its level; but these coral-islands, which have been shaken and fissured, certainly have not been elevated, and, therefore, probably they have subsided. In the chapter on Keeling atoll, I attempted to show by direct evidence, that the island underwent a movement of subsidence, during the earthquakes lately felt there.

The facts stand thus;--there are many large tracts of ocean, without any high land, interspersed with reefs and islets, formed by the growth of those kinds of corals, which cannot live at great depths; and the existence of these reefs and low islets, in such numbers and at such distant points, is quite inexplicable, excepting on the theory, that the bases on which the reefs first became attached, slowly and successively sank beneath the level of the sea, whilst the corals continued to grow upwards. No positive facts are opposed to this view, and some general considerations render it probable. There is evidence of change in form, whether or not from subsidence, on some of these coral-islands; and there is evidence of subterranean disturbances beneath them. Will then the theory, to which we have thus been led, solve the curious problem,--what has given to each cla.s.s of reef its peculiar form?

(PLATE: WOODCUT NO. 4.

AA--Outer edge of the reef at the level of the sea.

BB--Sh.o.r.es of the island.

A'A'--Outer edge of the reef, after its upward growth during a period of subsidence.

CC--The lagoon-channel between the reef and the sh.o.r.es of the now encircled land.

B'B'--The sh.o.r.es of the encircled island.

N.B.--In this, and the following woodcut, the subsidence of the land could only be represented by an apparent rise in the level of the sea.

PLATE: WOODCUT NO. 5.