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at which the corals are able to live in the places mentioned by Ehrenberg, to the greater quantity of sediment there; and the situations, where they were flourishing at the depth of 25 fathoms, were protected, and the water was extraordinarily limpid. On the leeward side * Ehrenberg über die Natur, &c. p. 50. of Maurit... |
coral growing at a somewhat greater depth than at Keeling atoll, the sea, owing apparently to its tranquil state, was likewise very clear. Within the lagoons of some of the Marshall atolls, where the water can be but little agitated, there are, according to Kotzebue, living beds of coral in 25 fathoms. From these facts... |
which the beds of clean coral off Mauritius, Keeling Island, the Maldiva and Chagos atolls, graduated into a sandy slope, it appears very probable that the depth, at which reef-building polypifers can exist, is partly determined by the extent of inclined surface, which the currents of the sea and the recoiling waves ha... |
MM. Quoy and Gaimard* believe that the growth of coral is confined within very limited depths; and they state that they never found any fragment of an Astræa, (the genus they consider most efficient in forming reefs) at a depth above 25 or 30 feet. But we have seen that in several places the bottom of the sea is paved |
with massive corals at more than twice this depth; and at 15 fathoms (or thrice this depth) off the reefs of Mauritius, the arming was marked with the distinct impression of a living Astræa. Millepora alcicornis lives in from 0 to 12 fathoms, and the genera Madrepora and Seriatopora from 0 to 20 fathoms. Capt. Moresby ... |
specimen of Sideropora scabra (Porites of Lamarck) brought up alive from 17 fathoms. Mr. Couthouy† states that he has dredged up on the Bahama banks considerable masses of Meandrina from 16 fathoms, and he has seen this coral growing in 20 fathoms. A Caryophyllia, half an inch in diameter, was dredged up alive from 80 ... |
33° S.) by Capt. P. P. King:‡ this is the most remarkable fact with which I am acquainted, shewing the depth at which a genus of corals often found on reefs, * Annales des Sci. Nat. tom. vi. † Remarks on Coral Formations, p. 12. ‡ I am indebted to Mr. Stokes for having kindly communicated this fact to me, |
together with much other valuable information. can exist.* We ought, however, to feel less surprise at this fact, as Caryophyllia alone of the lamelliform genera, ranges * I will record in the form of a note all the facts that I have been able to collect on the depths, both within and without the tropics, at which thos... |
Ellis (Nat. Hist. of Coralline, p. 96) states that Ombellularia was procured in lat. 79° N. sticking to a line from the depth of 236 fathoms; far beyond the tropic; it is found in Zetland* in Lat. 60° N. in deep water, and I procured a small species from Tierra del Fuego in Lat. 53° S. Capt. Beechey informs me, |
that branches of pink and yellow coral were frequently brought up from between 20 and 25 fathoms off the Low atolls; and Lieut. Stokes writing to me from the N.W. coast of Australia, says that a strongly branched coral was procured there from 30 fathoms: unfortunately it is not known to what genera these corals belong.... |
depth, at which each particular kind of coral ceases to exist, is far from being accurately known; yet when we bear in mind the manner in which the clumps of coral gradually became infrequent at about the same depth, and wholly disappeared at a greater depth than 20 fathoms, on the slope round Keeling atoll, on the lee... |
the Mauritius, and at rather less depth, both without and within the atolls of the Maldiva and Chagos Archipelagoes; and when we know that the reefs round these islands do not differ from other coral formations in their form and structure, we may, I think, conclude that in ordinary cases, reef-building polypifers do no... |
20 and 30 fathoms. hence this coral either must have been floating loose, or was entangled in stray line at the bottom. Off Keeling atoll a compound Ascidia (Sigillina) was brought up from 39 fathoms, and a piece of sponge, apparently living, from 70, and a fragment of Nullipora also apparently living from 92 fathoms. ... |
90 fathoms off this coral-island, the bottom was thickly strewed with joints of Halimeda and small fragments of other Nulliporæ, but all dead. Capt. B. Allen, R.N., informs me that in the survey of the West Indies it was noticed, that between the depth of 10 and 200 fathoms, the sounding-lead very generally came up, co... |
of a Halimeda, of which he shewed me specimens. Off Pernambuco, in Brazil, in about 12 fathoms, the bottom was covered with fragments dead and alive of a dull red Nullipora, and I infer from Roussin's chart, that a bottom of this kind extends over a wide area. On the beach, within the coral-reefs of Mauritius, vast qua... |
of Nulliporæ were piled up. From these facts it appears, that these simply organized bodies are amongst the most abundant productions of the sea. * Fleming's British Animals, genus Caryophyllia. It has been argued* that reefs may possibly rise from very great depths through the means of small corals, first making a pla... |
This, however, is an arbitrary supposition: it is not always remembered, that in such cases there is an antagonist power in action, namely, the decay of organic bodies, when not protected by a covering of sediment, or by their own rapid growth. We have, moreover, no right to calculate on unlimited time for the accumula... |
great masses. Every fact in geology proclaims that neither the land, nor the bed of the sea retain for indefinite periods the same level. As well might it be imagined that the British Seas would in time become choked up with beds of oysters, or that the numerous small corallines off the inhospitable shores of Tierra de... |
time form a solid and extensive coral-reef. * Journal of the Royal Geograph. Soc. 1831, p. 218. THEORY OF THE FORMATION OF THE DIFFERENT CLASSES 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... |
theory, considered—The probable composition of the lower parts of atolls. THE naturalists who have visited the Pacific, seem to have had their attention rivetted by the lagoon-islands, or atolls,—those singular rings of coral-land which rise abruptly out of the unfathomable ocean,—and have passed over, almost unnoticed... |
of atolls, is that they are based on submarine craters: but where we can find a crater of the shape of Bow atoll, which is five times as long as it is broad, (Plate I. fig. 4); or like that of Menchicoff Island, (Plate II. fig. 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 disk,—one of which disks 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... |
together beneath the sea, as there are now in some parts atolls. But this theory lies under a great 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 re... |
a perfectly characterized 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;* he supposes that as the more massive kinds of corals prefer the surf, the outer portions, in a... |
would first reach the surface and consequently form a ring. But on this view it must be assumed, that in every case the basis consists of a flat bank; for if it were conically formed, like a mountainous mass, 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 assumption is very improbable. As the lagoons of atolls are sometimes even more than forty fathoms deep, it must, also, be assumed on this view, that at a depth at which the waves do not break, the coral grows more vigorously on the edge... |
bank than on its central part: and this is an assumption without any evidence in support of it. I remarked in the third chapter, that a reef, growing on a detached bank, would tend to assume on an atoll-like structure; if, therefore, corals were to grow up from a bank with a level surface some fathoms submerged, having... |
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, renders, as we shall presently see, this view inapplicable to the greater number of atolls. No theory wo... |
notice has been advanced to account for those barrier-reefs, which encircle islands of moderate * Kotzebue's First Voyage, vol. iii. p. 331. 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 t... |
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 flourishing close to gently shelving beaches where the water is often turbid. What cause, then, has given to atolls and barrier-reefs their characteristic forms? ... |
from the consideration of these two circumstances,—first, the reef-building corals flourishing 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 ... |
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 M.M. 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... |
written without a capital letter: I have detected a few errors in this map, respecting the heights 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... |
are grounded. I have ascertained, and chiefly from the writings of Cook, Kotzebue, Bellinghausen, Duperrey, Beechey, and Lutké, regarding the Pacific; and from Moresby* with respect to the Indian Ocean, that in the fol- * See also Capt. Owen's and Lieut. Wood's papers in the Geographical Journal, on the Maldiva and Lac... |
of the islets; but I chiefly ground my assertion respecting these two groups, and the Chagos group, from information communicated to me by Capt. Moresby. lowing 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 li... |
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 space,* none of the innumerable is... |
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:... |
two groups together are 1040 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 Archipela... |
to the northern end of the Marshall Archipelago, there is a narrow band of ocean, more than 4000 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 space of 480 miles in length, and about 100 broad, thinly interspersed with low island... |
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 consi- * I find from Mr. Couthouy's pamphlet, p. 58, that Aurora island is about 200 feet in height; it consists |
of coral rock, and seems to have been formed by the elevation of an atoll. It lies N.E. of Tahiti, close without the line bounding the space coloured dark blue in the map appended to this volume. Honden island, which is situated in the extreme N.W. part of the Low Arch., according to measurements made on board the Beag... |
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. dered as one group of a thousand 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 1500 miles long. I ... |
justified, in repeating, that enormous spaces, 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 mu... |
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 accumulation of great banks of sediment, which owing to the action of superficial currents, (aided possibly by th... |
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 assumption without any proof, that a number of immense piles of sediment have been heaped on the floor of the great P... |
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... |
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 bread... |
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 assumed that they can flourish at a depth of even 100 fathoms, yet the we... |
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 600 feet of the surface of the sea and not one above it, as that they should be of the same height wi... |
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 ... |
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 ... |
probably occur to those who have read Ehrenberg's account of the reefs of the Red Sea, that many points in these great areas 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 bason-like form of the atolls, wil... |
is impossible; for the upheaval and subsequent abrasion of an island would leave a flat disk, 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... |
the requisite position, they must of necessity have subsided into it; and this at once solves every difficulty,* for we may safely infer, from the facts given in the last chapter, that during a gradual subsidence the corals would be favourably circumstanced for building up their solid frameworks and reaching the surfac... |
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 gro... |
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 strata many thousand * T... |
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 ... |
the surface, and were there abraded by the surf and subsequently coated by corals, is subject to nearly the same objections with those given at the bottom of the last page; but I consider it superfluous to detail all the arguments opposed to such a notion. Chamisso's theory, from assuming the existence of so many banks... |
proper depth beneath the water, is also vitally defective. The same observation applies to an hypothesis of Lieut. Nelson's (Geolog. Trans. vol. v. p. 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 s... |
notion formerly entertained, (Forster's Observ. p. 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... |
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 c... |
the whole globe would have swollen. It is very remarkable that Mr. Lyell,* 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,... |
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 civilized, to detect a movement, the... |
affected. In barbarous and semi-civilized nations how long might not a slow movement, even of elevation such as that now affecting Scandinavia, have escaped attention! Mr. Williams† insists strongly that the traditions of the natives, which he has taken much pains in collecting, do not indicate the appearance of any ne... |
all that would be apparent would be, the water sometimes encroaching slowly on the land, and the land again recovering by the accumulation 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... |
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 * Principles of Geology, sixth edition, vol. iii. p. ... |
p. 31. 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."* A storm lately entirely swept away two of the Caroline islands, and converted them into shoals; it partly,... |
destroyed two other islands.† According to a tradition which was communicated to Capt. FitzRoy, it is believed in the Low Archipelago, that the arrival of the first ship caused a great inundation, which destroyed many lives. Mr. Stuchbury relates, that in 1825, the western side of Chain Atoll, in the same group, was co... |
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."‡ That considerable changes have taken place recently in some of the atolls in the Low Archipelago, appears certain from the case already given ... |
Whitsunday and Gloucester Islands in this same group, we must either attribute great inaccuracy to their discoverer, the famous circumnavigator Wallis, or believe that they have undergone a considerable change in the period of fifty-nine years, between his voyage and that of Capt. Beechey's. Whitsunday Island is descri... |
is only one mile and a half long. The appearance of Gloucester Island, in Capt. Beechey's words,§ "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 circumference, surroundin... |
few worn patches of conglomerate coral-rock, of about the size of hovels; and these * Kotzebue's First Voyage, vol. iii. p. 168. † M. Desmoulins in Comptes Rendus, 1840, p. 837. ‡ West of England Journal, No. 1, p. 35. § Beechey's Voyage to the Pacific, chap. vii., and Wallis's Voyage in the Dolphin, chap. iv. Capt. Mo... |
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 always diminishing the number of the islands:"* and I have already shown, on th... |
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-i... |
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 ... |
to have been washed by the rain through the broken masses of underlying rock: the island was thus rendered unproductive. Chamisso† 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 ato... |
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, ... |
subsided. In the chapter on Keeling atoll, I attempted to show by direct evidence, that * See an extract from Pyrard's Voyage in Capt. Owen's paper on the Maldiva Archipelago, in the Geographical Journal, vol. ii. p. 84. † See Chamisso, in Kotzebue's First Voyage, vol. iii. p. 182 and 136. the island underwent a moveme... |
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 poi... |
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... |
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 class of reef its peculiar form? Let us in imagination place within one of the subsiding areas, an island surrounded by a "fringing reef... |
the explanation of its origin. Let the unbroken lines and the oblique shading in the woodcut A A—Outer edge of the reef at the level of the sea. B B—Shores of the island. A'A'—Outer edge of the reef, after its upward growth during a period of subsidence. C C—The lagoon-channel between the reef and the shores of the now... |
land. B'B'—The shores of the encircled island. N.B. In this, and the following wood-cut, the subsidence of the land could only be represented by an apparent rise in the level of the sea. (No. 4.) represent a vertical section through such an island; and the horizontal shading will represent the section of the reef. Now,... |
either a few feet at a time or quite insensibly, we may safely infer from what we know of the conditions favourable to the growth of coral, that the living masses bathed by the surf on the margin of the reef, will soon regain the surface. The water, however, will encroach, little by little, on the shore, the island bec... |
lower and smaller, and the space between the edge of the reef and the beach proportionally broader. A section of the reef and island in this state, after a subsidence of several hundred feet, is given by the dotted lines: coral-islets are supposed to have been formed on the new reef, and a ship is anchored in the lagoo... |
section is in every respect that of an encircling barrier-reef; it is, in fact, a section taken* E. and W. through the highest point of the encircled island of Bolabola; of which a plan is given in Plate I. fig. 5. The same section is more clearly shown in the following woodcut (No. 5) by the unbroken lines. The width |
of the reef, and its slope both on the outer and inner side, will have been determined by the growing powers of the coral, under the conditions, (for instance the force of the breakers and of the currents) to which it has been exposed; and the lagoon-channel will be deeper or shallower, in proportion to the growth of t... |
branched corals within the reef, and to the accumulation of sediment, relatively, also, to the rate of subsidence and the length of the intervening stationary periods. It is evident in this section, that a line drawn perpendicularly down from the outer edge of the new reef to the foundation of solid rock, exceeds by as... |
been feet of subsidence, that small limit of depth at which the effective polypifers can live,—the corals having grown up, as * The section has been made from the chart given in the Atlas of the Voyage of the Coquille. The scale is .57 of an inch to a mile. The height of the island, according to M. Lesson, is |
4026 feet. The deepest part of the lagoon-channel is 162 feet; its depth is exaggerated in the woodcut for the sake of clearness. the whole sank down, from a basis formed of other corals and their consolidated fragments. Thus the difficulty on this head, which before seemed so great, disappears. As the space between th... |
continued to increase in breadth and depth, and as the injurious effects of the sediment and fresh water borne down from the land were consequently lessened, the greater number of the channels, with which the reef in its fringing state must have been breached, especially those which fronted the smaller streams, will ha... |
coral: on the windward side of the reef, where the coral grows most vigorously, the breaches will probably have first been closed. In barrier-reefs, therefore, the breaches kept open by draining the tidal waters of the lagoon-channel, will generally be placed on the leeward side, and they will still face the mouths of ... |
influence of their sediment and fresh-water;—and this, it has been shown, is commonly the case. Referring to the following diagram, in which the newly-formed barrier-reef is represented by unbroken lines, instead of by dots as in the former woodcut, let the work of subsidence go on, and the doubly-pointed hill will for... |
the level of the sea. The cocoa-nut trees represent coral-islets formed on the reef. B'B'—The shores of the island, generally formed of low alluvial land and of coral detritus from the lagoon-channel. A"A"—The outer edges of the reef now forming an atoll. C'—The lagoon of the newly-formed atoll. According to the scale,... |
lagoon-channel is exaggerated. islands (or more, according to the number of the hills) included within one annular reef. Let the island continue subsiding, and the coral-reef will continue growing up on its own foundation, whilst the water gains inch by inch on the land, until the last and highest pinnacle is covered, ... |
section of this atoll is shown in the woodcut by the dotted lines;—a ship is anchored in its lagoon, but islets are not supposed yet to have been formed on the reef. The depth of the lagoon and the width and slope of the reef, will depend on the circumstances just referred to under barrier-reefs. Any further subsidence... |
no change in the atoll, except perhaps a diminution in its size, from the reef not growing vertically upwards; but should the currents of the sea act violently on it, and should the corals perish on part or on the whole of its margin, changes would result during subsidence which will be presently noticed. I may here ob... |
bank either of rock or of hardened sediment, level with the surface of the sea, and fringed with living coral, would (if not so small as to allow the central space to be quickly filled up with detritus) by subsidence be converted immediately into an atoll, without passing, as in the case of a reef fringing the shore of... |
island, through the intermediate form of a barrier-reef. If such a bank lay a few fathoms submerged, the simple growth of the coral (as remarked in the third chapter) without the aid of subsidence, would produce a structure scarcely to be distinguished from a true atoll; for in all cases the corals on the outer margin ... |
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