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of the mounds of Porites with their summits apparently lately killed, and their sides only three or four inches lower down thickened by a fresh layer of living coral. But there is a difficulty on this supposition which I must not pass over. If the whole, or a large part of the 'flat,' had been formed by the outward gro... |
of the margin, each successive margin would naturally have been coated by the Nulliporæ, and so much of the surface would have been of equal height with the existing zone of living Nulliporæ: this is not the case, as may be seen in the wood-cut. It is, however, evident from the abraded state of the 'flat,' with its ori... |
filled up, that its surface has been much modified; and it is possible that the hinder portions of the zone of Nulliporæ, perishing as the reef grows outwards, might be worn down by the surf. If this has not taken place, the reef can in no part have increased outwards in breadth since its formation, or at least since t... |
Nulliporæ formed the convex mound on its margin: for the zone thus formed, and which stands between two and three feet above the other parts of the reef, is nowhere much above twenty yards in width. Thus far we have considered facts, which indicate, with more or less probability, the increase of the atoll in its differ... |
others having an opposite tendency. On the S.E. side, Lieut. Sulivan, to whose kindness I am indebted for many interesting observations, found the conglomerate projecting on the reef nearly fifty yards in front of the beach: we may infer from what we see in all others parts of the atoll, that the conglomerate was not o... |
formed the base of an islet, the front and upper part of which has since been swept away. The degree to which the conglomerate, round nearly the whole atoll, has been scooped, broken up and the fragments cast on the beach, is certainly very surprising, even on the view that it is the office of occasional gales to pile ... |
fragments, and of the daily tides to wear them away. On the western side, also, of the atoll, where I have described a bed of sand and fragments with trees growing out of it, in front of an old beach, it struck both Lieut. Sulivan and myself, from the manner in which the trees were being washed down, that the |
surf had lately recommenced an attack on this line of coast. Appearances indicating a slight encroachment of the water on the land, are plainer within the lagoon: I noticed in several places, both on its windward and leeward shores, old cocoa-nut trees falling with their roots undermined, and the rotten stumps of other... |
us the cocoa-nut could not now grow. Capt. FitzRoy pointed out to me, near the settlement, the foundation posts of a shed, now washed by every tide, but which the inhabitants stated, had seven years before stood above high water-mark. In the calm waters of the lagoon, directly connected with a great, and therefore stab... |
that a change in the currents, sufficiently great to cause the water to eat into the land on all sides, should have taken place within a limited period. From these considerations I inferred, that probably the atoll had lately subsided to a small amount; and this inference was strengthened by the circumstance, that in 1... |
the island had been shaken by a severe earthquake, and by two slighter ones during the ten previous years. If, during these subterranean disturbances, the atoll did subside, the downward movement must have been very small, as we must conclude from the fields of dead coral still lipping the surface of the lagoon, and fr... |
shore not having yet regained the line of their former action. The subsidence must, also, have been preceded by a long period of rest, during which the islets extended to their present size, and the living margin of the reef grew either upwards, or as I believe outwards, to its present distance from the beach. Whether ... |
or not, the above facts are worthy of attention, as showing how severe a struggle is in progress on these low coral-formations between the two nicely balanced powers of land and water. With respect to the future state of Keeling atoll, if left undisturbed, we can see that the islets may still extend in length; but as t... |
the surf until broken by rolling over a wide space, their increase in breadth must depend on the increasing breadth of the reef; and this must be limited by the steepness of the submarine flanks, which can be added to only by sediment derived from the wear and tear of the coral. From the rapid growth of the coral in |
the channel cut for the schooner, and from the several agents at work in producing fine sediment, it might be thought that the lagoon would necessarily become quickly filled up. Some of this sediment, however, is transported into the open sea, as appears from the soundings off the mouth of the lagoon, instead of being ... |
moreover, of sediment, checks the growth of coral reefs, so that these two agencies cannot act together with full effect in filling it up. We know so little of the habits of the many different species of corals, which form the lagoon-reefs, that we have no more reason for supposing that their whole surface would grow u... |
the coral did in the schooner-channel, than for supposing that the whole surface of a peat-moss would increase as quickly as parts are known to do in holes, where the peat has been cut away. These agencies, nevertheless, tend to fill up the lagoon; but in proportion as it becomes shallower, so must the polypifers be su... |
agencies, such as impure water and loss of food. For instance, Mr. Liesk informed me, that some years before our visit unusually heavy rain killed nearly all the fish in the lagoon, and probably the same cause would likewise injure the corals. The reefs also, it must be remembered, cannot possibly rise above the level ... |
in the reef—Ledge-formed shores round certain lagoons—Conversion of lagoons into land. I WILL here give a sketch of the general form and structure of many atolls and atoll-formed reefs which occur in the Pacific and Indian oceans, comparing them with Keeling atoll. The Maldiva atolls and the Great Chagos Bank differ in... |
them, besides occasional references, a third section of this chapter. Keeling atoll may be considered as of moderate dimensions and of regular form. Of the thirty-two islands surveyed by Capt. Beechey in the Low Archipelago, the longest was found to be thirty miles, and the shortest less than a mile; but Vliegen atoll,... |
group, appears to be sixty miles long and twenty broad. Most of the atolls in this group are of an elongated form; thus Bow Island is thirty miles in length, and on an average only six in width (See Fig. 4, Plate I.), and Clermont Tonnere has nearly the same proportions. In the Marshall Archipelago (the Ralick and Rada... |
of Kotzebue) several of the atolls are more than thirty miles in length, and Rimsky Korsacoff is fifty-four long, and twenty wide at the broadest part of its irregular outline. Most of the atolls in the Maldiva Archipelago are of great size, one of them (which, however, bears a double name) measured in a medial and sli... |
no less than eighty-eight geographical miles long, its greatest width being under twenty, and its least only nine-and-a-half miles. Some atolls have spurs projecting from them; and in the Marshall group there are atolls united together by linear reefs, for instance Menchioff Island, (See Fig. 3, Plate II.) which is six... |
together. In far the greater number of cases an atoll consists of a simple elongated ring, with its outline moderately regular. The average width of the annular reef may be taken at about a quarter of a mile. Capt. Beechey* says that in the atolls of the Low Archipelago it exceeded in no instance half a mile. The descr... |
of the structure and proportional dimensions of the reef and islets of Keeling atoll, appears to apply perfectly to nearly all the atolls in the Pacific and Indian oceans. The islets are first formed some way back either on the projecting points of the reef, especially if its form be angular, or on the sides of the mai... |
the lagoon—that is in both cases, on points where the breakers can act during gales of wind in somewhat different * Beechey's Voyage to the Pacific and Beering's Straits, chap. viii. directions, so that the matter thrown up from one side may accumulate against that before thrown up from another. In Lutkè's chart of the... |
instances of the former case; and the occurrence of islets, as if placed for beacons, on the points where there is a gateway or breach through the reef, has been noticed by several authors. There are some atoll-formed reefs, rising to the surface of the sea and partly dry at low water, on which from such cause islets h... |
been formed; and there are others, on which they have been formed, but have subsequently been worn away. In atolls of small dimensions the islets frequently become united into a single horse-shoe or ring-formed strip; but Diego Garcia, although an atoll of considerable size, being thirteen miles and a half in length, h... |
northern end, by a belt of land, on an average a third of a mile in width. To show how small the total area of the annular reef and the land is in islands of this class, I may quote a remark from the voyage of Lutkè, namely, that if the forty-three rings, or atolls, in the Caroline Archipelago, were |
put one within another, and over a steeple in the centre of St. Petersburg, the whole would not cover that city and its suburbs. The form of the bottom off Keeling atoll, which gradually slopes to about twenty fathoms at the distance of between one and two hundred yards from the edge of the reef, and then plunges at an |
angle of 45° into unfathomable depths, is exactly the same* with that of the sections of the atolls in the Low Archipelago given by Capt. Beechey. The nature, however, of the bottom seems to differ, for this officer† informs * The form of the bottom round the Marshall atolls in the Northern Pacific is probably similar:... |
ii. p. 16.) says, "We had at a small distance from the reef, forty fathoms depth, which increased a little further so much that we could find no bottom." † I must be permitted to express my obligation to Captain Beechey, for the very kind manner in which he has given me information on several points, and to own the |
great assistance I have derived from his excellent published work. me that all the soundings, even the deepest, were on coral, but he does not know whether dead or alive. The slope round Christmas atoll, (Lat. 1° 4' N., 157° 45' W.) described by Cook,* is considerably less; at about half a mile from the edge of the ree... |
average depth was about fourteen fathoms on a fine sandy bottom, and at a mile, only between twenty and forty fathoms. It has no doubt been owing to this gentle slope, that the strip of land surrounding its lagoon, has increased in one part to the extraordinary width of three miles: it is formed of successive ridges of... |
and corals, like those on the beach. I know of no other instance of such width in the reef of an atoll; but Mr. F. D. Bennett informs me that the inclination of the bottom round Caroline atoll in the Pacific, is like that off Christmas island, very gentle. Off the Maldiva and Chagos atolls, the inclination is much more |
abrupt; thus at Heawandoo Pholo, Lieut. Powell† found 50 and 60 fathoms close to the edge of the reef, and at 300 yards distance there was no bottom with a 300 yard line. Capt. Moresby informs me, that at 100 fathoms from the mouth of the lagoon of Diego Garcia, he found no bottom with 150 fathoms: this is the |
more remarkable, as the slope is generally less abrupt in front of channels through a reef, owing to the accumulation of sediment. At Egmont Island, also, at 150 fathoms from the reef, soundings were struck with 150 fathoms. Lastly, at Cardoo atoll, only sixty yards from the reef, no bottom was obtained, as I am inform... |
a line of two hundred fathoms! The currents run with great force round these atolls, and where they are strongest, the inclination appears to be most abrupt. I am informed by the same authority, that wherever soundings were obtained off these islands, the bottom was invariably sandy: nor was there any reason to suspect... |
vol. ii. chap. 10. † This fact is taken from a MS. account of these groups lent me by Capt. Moresby. See also Capt. Moresby's paper on the Maldiva atolls in the Geographical Journal, vol. v. p. 401. of submarine cliffs, as there was at Keeling Island.* Here then occurs a difficulty;—can sand accumulate on a slope, whic... |
cases, appears to exceed fifty-five degrees? It must be observed, that I speak of slopes where soundings were obtained, and not of such cases, as that of Cardoo, where the nature of the bottom is unknown, and where its inclination must be nearly vertical. M. Élie de Beaumont† has argued, and there is no higher authorit... |
the inclination at which snow slides down in avalanches, that a bed of sand or mud cannot be formed at a greater angle than thirty degrees. Considering the number of soundings on sand, obtained round the Maldiva and Chagos atolls, which appear to indicate a greater angle, and the extreme abruptness of the sand-banks in... |
be mentioned in the Appendix, I must conclude that the adhesive property of wet sand counteracts its gravity, in a much greater ratio than has been allowed for by M. Élie de Beaumont. From the facility with which calcareous sand becomes agglutinated, it is not necessary to suppose that the bed of loose sand is thick. C... |
that the submarine slope is much less at the extremities of the more elongated atolls in the Low Archipelago, than at their sides; in speaking of Ducie's Island he says‡ the buttress, as it may be called, which "has the most powerful enemy (the S.W. swell) to oppose, is carried out much further, and with less abruptnes... |
In some cases, the less inclination of a certain part of the external slope, for instance of the * Off some of the islands in the Low Archipelago the bottom appears to descend by ledges. Off Elizabeth Island, which, however, consists of raised coral, Capt. Beechey (p. 45, quarto ed.) describes three ledges: the first h... |
the beach to a distance of about fifty yards; the second extended two hundred yards with twenty-five fathoms on it, and then ended abruptly, like the first; and immediately beyond this there was no bottom with two hundred fathoms. † Mémoires pour servir à une description Géolog. de France, tome iv. p. 216. ‡ Beechey's ... |
northern extremities of the two Keeling atolls, is caused by a prevailing current which there accumulates a bed of sand. Where the water is perfectly tranquil, as within a lagoon, the reefs generally grow up perpendicularly, and sometimes even overhang their bases: on the other hand, on the leeward side of Mauritius, w... |
invariably so, the reef is very gently inclined. Hence it appears that the exterior angle varies much; nevertheless, in the close similarity in form between the sections of Keeling atoll and of the atolls in the Low Archipelago, in the general steepness of the reefs of the Maldiva and Chagos atolls, and in the perpendi... |
water always tranquil, we may discern the effects of uniform laws; but from the complex action of the surf and currents, on the growing powers of the coral and on the deposition of sediment, we can by no means follow out all the results. Where islets have been formed on the reef, that part which I have sometimes called... |
'flat,' and which is partly dry at low water, appears similar in every atoll. In the Marshall group in the N. Pacific, it may be inferred from Chamisso's description, that the reef, where islets have not been formed on it, slopes gently from the external margin to the shores of the lagoon: Flinders states that the Aust... |
similar inclination inwards, and I have no doubt it is of general occurrence, although, according to Ehrenberg, the reefs of the Red Sea offer an exception. Chamisso observes that "the red colour of the reef (at the Marshall atolls) under the breakers is caused by a Nullipora, which covers the stone wherever the waves ... |
a stalactitical form,"—a description perfectly applicable to the margin of Keeling atoll.* Although Chamisso * Kotzebue's First Voyage, vol. iii. p. 142. Near Porto Praya, in the Cape de Verd Islands, some basaltic rocks, lashed by no inconsiderable surf, were completely enveloped with a layer of Nullipora. The entire ... |
the layer, however, was of no greater thickness than paper. Another does not state that the masses of Nulliporæ form points or a mound, higher than the flat, yet I believe that this is the case; for Kotzebue,* in another part, speaks of the rocks on the edge of the reef "as visible for about two feet at low water," |
and these rocks we may feel quite certain are not formed of true coral.† Whether a smooth convex mound of Nulliporæ, like that which appears as if artificially constructed to protect the margin of Keeling Island, is of frequent occurrence round atolls, I know not; but we shall presently meet with it, under precisely th... |
edge of the 'barrier reefs' which encircle the Society Islands. There appears to be scarcely a feature in the structure of Keeling reef, which is not of common, if not of universal occurrence, in other atolls. Thus Chamisso describes‡ a layer of coarse conglomerate, outside the islets round the Marshall atolls, which "... |
away." From drawings, with appended remarks, of Diego Garcia in the Chagos group and of several of the Maldiva atolls, shown me by Captain Moresby§, it is evident that their outer coasts are subject to the same round of decay and renovation as those of Keeling atoll. From the description of the atolls in the Low Archip... |
kind, in the form of projecting knobs, grew in the same situation. These Nulliporæ are closely related to those described on the coral-reefs, but I believe are of different species. * Kotzebue's First Voyage, vol. ii. p. 16. Lieut. Nelson in his excellent memoir in the Geological Transactions, (vol. ii p. 105.) alludes... |
and infers that they consist of Serpulæ, which compose incrusting masses on the reefs of Bermudas, as they likewise do on a sandstone bar off the coast of Brazil, (which I have described in London Phil. Journ. Oct. 1841). These masses of Serpulæ hold the same position, relatively to the action of the sea, with the Null... |
in the Indian and Pacific Oceans. † Capt. Moresby, in his valuable paper "on the Northern Atolls of Maldivas," (Geographical Journal, vol. v.), says that the edges of the reefs there stand above water at low spring tides. ‡ Kotzebue's First Voyage, vol. iii p. 144. § See also Moresby on the Northern atolls of the Maldi... |
v. p. 400. Beechey's Voyage, it is not apparent that any conglomerate coral-rock was there observed. The lagoon in Keeling atoll is shallow: in the atolls of the Low Archipelago the depth varies from 20 to 38 fathoms, and in the Marshal Group, according to Chamisso, from 30 to 35: in the Caroline atolls it is only a li... |
Within the Maldiva atolls there are large spaces with 45 fathoms, and some soundings are laid down of 49 fathoms. The greater part of the bottom in most lagoons, is formed of sediment; large spaces have exactly the same depth, or the depth varies so insensibly, that it is evident that no other means, excepting aqueous ... |
the surface so equally. In the Maldiva atolls this is very conspicuous, and likewise in some of the Caroline and Marshall Islands. In the former large spaces consist of sand and soft clay; and Kotzebue speaks of clay having been found within one of the Marshall atolls. No doubt this clay is calcareous mud, similar to t... |
and to that at Bermuda already referred to, as undistinguishable from disintegrated chalk, and which Lieut. Nelson says is called there pipe-clay.* Where the waves act with unequal force on the two sides of an atoll, the islets appear to be first formed, and are generally of greater continuity on the more exposed shore... |
to leeward, are in most parts of the Pacific liable to be occasionally swept entirely away by gales, equalling hurricanes in violence, which blow in an opposite direction to the ordinary trade-wind. The absence of the * I may here observe that on the coast of Brazil, where there is much coral, the soundings near the la... |
Admiral Roussin, in the Pilote du Brésil, as siliceous sand, mingled with much finely comminuted particles of shells and coral. Further in the offing, for a space of 1300 miles along the coast, from the Abrolhos islands to Maranham, the bottom in many places is composed of "tuf blanc, mêlé ou formé de madrépores broyés... |
analogous to that which occurs within the above-mentioned lagoons; it is sometimes, according to Roussin, firm, and he compares it to mortar. islets on the leeward side of atolls, or when present their lesser dimensions compared with those to windward, is a comparatively unimportant fact; but in several instances the r... |
outline, does not rise to the surface by several fathoms. This is the case with the southern side of Peros Banhos (Plate I. fig. 9) in the Chagos group, with Mourileu atoll*, in the Caroline Archipelago, and with the barrier reef (Plate I., fig. 8) of the Gambier Islands. I allude to the latter reef, although belonging... |
because Captain Beechey was first led by it to observe the peculiarity in the question. At Peros Banhos the submerged part is nine miles in length, and lies at an average depth of about 5 fathoms; its surface is nearly level, and consists of hard stone, with a thin covering of loose sand. There is scarcely any living c... |
it, even on the outer margin, as I have been particularly assured by Capt. Moresby: it is in fact a wall of dead coral-rock, having the same width and transverse section with the reef in its ordinary state, of which it is a continuous portion. The living and perfect parts terminate abruptly, and abut on the submerged p... |
same manner as on the sides of an ordinary passage through the reef. The reef to leeward in other cases is nearly or quite obliterated, and one side of the lagoon is left open; for instance at Oulleay (Caroline Archipelago), where a crescent-formed reef is fronted by an irregular bank, on which the other half of the an... |
once stood. At Namonouïto, in the same Archipelago, both these modifications of the reef concur; it consists of a great flat bank, with from 20 to 25 fathoms water on it; for a length of more than 40 miles on its southern side it is open and without any reef, whilst on the other sides it is bounded by a |
reef, in parts rising to the surface and perfectly characterized, in parts lying some fathoms submerged. In the Chagos group there are annular reefs, entirely submerged, * Frederic Lutkè's Voyage autour du Monde, vol. ii. p. 291. See also his account of Namonouïto, at pp. 97 and 105, and the chart of Oulleay in the Atl... |
structure as the submerged and defined portions just described. The Speaker's Bank offers an excellent example of this structure; its central expanse, which is about 22 fathoms deep, is 24 miles across; the external rim is of the usual width of annular reefs, and is well-defined; it lies between 6 and 8 fathoms beneath... |
depth there are scattered knolls in the lagoon. Capt. Moresby believes the rim consists of dead rock, thinly covered with sand, and he is certain this is the case with the external rim of the Great Chagos Bank, which is also essentially a submerged atoll. In both these cases, as in the submerged portion of the reef at ... |
Capt. Moresby feels sure that the quantity of living coral, even on the outer edge overhanging the deep-sea water, is quite insignificant. Lastly, in several parts of the Pacific and Indian Oceans there are banks, lying at greater depths than in the cases just mentioned, of the same form and size with the neighbouring ... |
wholly obliterated. It appears from the survey of Freycinet, that there are banks of this kind in the Caroline Archipelago, and, as is reported, in the Low Archipelago. When we discuss the origin of the different classes of coral formations, we shall see that the submerged state of the whole of some atoll-formed reefs,... |
but not invariably on the leeward side, and the existence of more deeply submerged banks now possessing little or no signs of their original atoll-like structure, are probably the effects of a uniform cause,—namely, the death of the coral, during the subsidence of the area, in which the atolls or banks are situated. Th... |
the Maldiva atolls, more than two or three channels, and generally only one leading into the lagoon, of sufficient depth for a ship to enter. In small atolls, there is usually not even one. Where there is deep water, for instance above twenty fathoms, in the middle of the lagoon, the channels through the reef are seldo... |
the centre,—it may be said that the rim only of the saucer-shaped hollow forming the lagoon is notched. Mr. Lyell* has observed that the growth of the coral would tend to obstruct all the channels through a reef, except those kept open by discharging the water, which during high tide and the greater part of each ebb is... |
its circumference. Several facts indicate that a considerable quantity of sediment is likewise discharged through these channels; and Captain Moresby informs me that he has observed, during the change of the monsoon, the sea discoloured to a distance off the entrances into the Maldiva and Chagos atolls. This, probably,... |
effectually than a mere current of water. In the many small atolls without any channel, these causes have not prevented the entire ring attaining the surface. The channels, like the submerged and effaced parts of the reef, very generally though not invariably occur on the leeward side of the atoll, or on that side, acc... |
in the same direction with the prevalent wind, is not fully exposed to it. Passages between the islets on the reef, through which boats can pass at high-water, must not be confounded with ship-channels, by which the annular reef itself is breached. The passages between the islets occur, of course, on the windward as we... |
but they are more frequent and broader to leeward, owing to the lesser dimensions of the islets on that side. At Keeling atoll the shores of the lagoon shelve gradually, where the bottom is of sediment, and irregularly or abruptly where there are coral reefs; but this is by no means the universal structure in other ato... |
general terms of the lagoons in the Marshall atolls, says the lead generally sinks "from a depth of two or three fathoms to twenty or twenty-four, and you may pursue a line in which on one side of the boat you may see the bottom, and on the other the azure blue deep water." The shores of the * Principles |
of Geology, vol. iii. p. 289. † Beechey's Voyage, 4to. ed. vol. i. p. 189. ‡ Kotzebue's First Voyage, vol. iii. p. 142. lagoon-like channel within the barrier-reef at Vanikoro have a similar structure. Captain Beechey has described a modification of this structure (and he believes it is not uncommon) in two atolls in t... |
shores of the lagoon descend by a few, broad, slightly inclined, ledges or steps: thus at Matilda atoll,* the great exterior reef, the surface of which is gently inclined towards and beneath the surface of the lagoon, ends abruptly in a little cliff three fathoms deep; at its foot, a ledge forty yards wide extends, she... |
surface-reef, and terminated by a second little cliff five fathoms deep; beyond this, the bottom of the lagoon slopes to twenty fathoms, which is the average depth of its centre. These ledges seem to be formed of coral rock; and Captain Beechey says that the lead often descended several fathoms through holes in them. I... |
reefs or knolls in the lagoon come to the surface at low-water; in other cases of rarer occurrence, all lie at nearly the same depth beneath it, but most frequently they are quite irregular,—some with perpendicular, some with sloping sides,—some rising to the surface, and others lying at all intermediate depths from th... |
the union of such reefs could produce even one uniformly sloping ledge, and much less two or three, one beneath the other, and each terminated by an abrupt wall. At Matilda Island, which offers the best example of the step-like structure, Captain Beechey observes that the coral knolls within the lagoon are quite irregu... |
see that the theory which accounts for the ordinary form of atolls, apparently includes this occasional peculiarity in their structure. In the midst of a group of atolls, there sometimes occur small, flat, very low islands of coral formation, which probably once included a lagoon, since filled up with sediment and cora... |
160. At Whitsunday Island the bottom of the lagoon slopes gradually towards the centre, and then deepens suddenly, the edge of the bank being nearly perpendicular. This bank is formed of coral and dead shells. reefs. Captain Beechey entertains no doubt that this has been the case with the two small islands, which alone... |
the Low Archipelago, did not contain lagoons. Romanzoff Island (in lat. 15° S.) is described by Chamisso* as formed by a dam of madreporitic rock inclosing a flat space, thinly covered with trees, into which the sea on the leeward side occasionally breaks. North Keeling atoll appears to be in a rather less forward stag... |
consists of a horse-shoe shaped strip of land surrounding a muddy flat, one mile in its longest axis, which is covered by the sea only at high water. When describing South Keeling atoll, I endeavoured to show how slow the final process of filling up a lagoon must be; nevertheless, as all causes do tend to produce this ... |
is very remarkable that not one instance, as I believe, is known of a moderately-sized lagoon being filled up even to the low water-line at spring-tides, much less of such a one being converted into land. It is, likewise, in some degree remarkable, how few atolls, except small ones, are surrounded by a single linear st... |
the union of separate islets. We cannot suppose that the many atolls in the Pacific and Indian oceans all have had a late origin, and yet should they remain at their present level, subjected only to the action of the sea and to the growing powers of the coral, during as many centuries as must have elapsed since any of |
the earlier tertiary epochs, it cannot, I think, be doubted that their lagoons and the islets on their reef, would present a totally different appearance from what they now do. This consideration leads to the suspicion that some renovating agency (namely subsidence) comes into play at intervals, and perpetuates their o... |
condition and extraordinary structure. ALTHOUGH occasional references have been made to the Maldiva atolls, and to the banks in the Chagos group, some points of their structure deserve further consideration. My description is derived from an examination of the admirable charts lately published from the survey of Captai... |
has communicated to me in the kindest manner. The Maldiva Archipelago is 470 miles in length, with an average breadth of about 50 miles. The form and dimensions of the atolls, and their singular position in a double line, may be seen, but not well, in the greatly reduced chart (Fig. 6.) in Plate II. The dimensions of t... |
atoll in the group, (called by the double name of Milla-dou-Madou and Tilla-dou-Matte) has already been given; it is 88 miles in a medial and slightly curved line, and is less than 20 miles in its broadest part. Suadiva, also, is a noble atoll, being 44 miles across in one direction, and 34 in another, and the great in... |
of water has a depth of between 250 and 300 feet. The smaller atolls in this group differ in no respect from ordinary ones; but the larger ones are remarkable from being breached by numerous deep-water channels leading into the lagoon; for instance, there are 42 channels, through which a ship could enter the lagoon of ... |
southern large atolls, the separate portions of reef between these channels have the ordinary structure, and are linear; but in the other atolls, especially the more northern ones, these portions are ring-formed, like miniature atolls. Other ring-formed reefs rise out of the lagoons, in the place of those irregular one... |
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