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front the shores of the lofty island? We shall hereafter see, that there is one, and I believe only one, solution of this difficulty. One other supposition to account for the position of encircling barrier-reefs remains, but it is almost too preposterous to be mentioned;—namely, that they rest on enormous submarine cra... |
and form of the islands in the Society group are considered, together with the fact that all are thus encircled, such a notion will be rejected by almost every one. New Caledonia, moreover, besides its size, is composed of primitive formations, as are some of the Comoro Islands;* and Aitutaki consists of calcareous roc... |
explanations, and conclude that the vertical thickness of barrier-reefs, from their outer edges to the foundation on which they rest (from AA in the section to the dotted lines) is really great: but in this, there is no difficulty, for it is not necessary to suppose that the coral has sprung up from an immense depth, a... |
in deep seas—Reefs on the coast of E. Africa and of Brazil—Fringing reefs in very shallow seas, round banks of sediment and on worn-down islands—Fringing reefs affected by currents of the sea—Coral coating bottom of the sea, but not forming reefs. FRINGING reefs, or, as they have been called by some voyagers, shore ree... |
of a continent, might at first be thought to differ little, except in generally being of less breadth, from barrier-reefs. As far as the superficies of the actual reef is concerned this is the case; but the absence of an interior deep-water channel, and the close relation in their horizontal extension with the probable... |
adjoining land, present essential points of difference. The reefs which fringe the island of Mauritius offer a good example of this class. They extend round its whole circumference, with the exception of two or three parts,* where the coast is almost precipitous, and where, if as is probable the bottom of the sea has a... |
have no foundation on which to become attached. A similar fact may sometimes be observed even in reefs of the barrier class, which follow much less closely the * This fact is stated on the authority of the Officier du Roi, in his extremely interesting "Voyage à l'isle de France," undertaken in 1768. According to Capt. ... |
vol. ii. p. 316.) on one part of the coast there is a space for 16 miles without a reef. outline of the adjoining land; as, for instance, on the S.E. and precipitous side of Tahiti, where the encircling reef is interrupted. On the western side of the Mauritius, which was the only part I visited, the reef generally lies |
at the distance of about half a mile from the shore; but in some parts it is distant from one to two, and even three miles. But even in this last case, as the coast-land is gently inclined from the foot of the mountains to the sea-beach, and as the soundings outside the reef indicate an equally gentle slope beneath |
the water, there is no reason for supposing that the basis of the reef, formed by the prolongation of the strata of the island, lies at a greater depth than that, at which the polypifers could begin constructing the reef. Some allowance, however, must be made for the outward extension of the corals on a foundation of s... |
formed from their own wear, which would give to the reef a somewhat greater vertical thickness, than would otherwise be possible. The outer edge of the reef on the western or leeward side of the island, is tolerably well defined, and is a little higher than any other part. It chiefly consists of large strongly branched... |
Madrepora, which also form a sloping bed some way out to sea: the kinds of coral growing in this part will be described in the ensuing chapter. Between the outer margin and the beach, there is a flat space with a sandy bottom and a few tufts of living coral; in some parts it is so shallow, that people, by |
avoiding the deeper holes and gullies, can wade across it at low water; in other parts it is deeper, seldom however exceeding ten or twelve feet, so that it offers a safe coasting channel for boats. On the eastern and windward side of the island, which is exposed to a heavy surf, the reef described to me as having a |
hard smooth surface, very slightly inclined inwards, just covered at low-water, and traversed by gullies; it appears to be quite similar in structure to the reefs of the barrier and atoll classes. The reef of Mauritius, in front of every river and streamlet, is breached by a straight passage: at Grand Port, however, th... |
a barrier-reef: it extends parallel to the shore for four miles, and has an average depth of 10 or 12 fathoms; its presence may probably be accounted for by two rivers which enter at each end of the channel, and bend towards each other. The fact of reefs of the fringing class being always breached in front of streams, ... |
of those which are dry during the greater part of the year, will be explained, when the conditions unfavourable to the growth of coral are considered. Low coral-islets, like those on barrier-reefs and atolls, are seldom formed on reefs of this class, owing apparently in some cases to their narrowness, and in others to ... |
outside not yielding many fragments to the breakers. On the windward side, however, of the Mauritius, two or three small islets have been formed. It appears, as will be shewn in the ensuing chapter, that the action of the surf is favourable to the vigorous growth of the stronger corals, and that sand or sediment, if ag... |
is injurious to them. Hence it is probable that a reef on a shelving shore, like that of Mauritius, would at first grow up, not attached to the actual beach, but at some little distance from it; and the corals on the outer margin would be the most vigorous. A shallow channel would thus be formed within the reef, and |
as the breakers are prevented acting on the shores of the island, and as they do not ordinarily tear up many fragments from the outside, and as every streamlet has its bed prolonged in a straight line through the reef, this channel could be filled up only very slowly with sediment. But a beach of sand and of fragments ... |
the smaller kinds of coral seems, in the case of Mauritius, to be slowly encroaching on the shallow channel. On many shelving and sandy coasts, the breakers tend to form a bar of sand a little way from the beach, with a slight increase of depth within it: for instance, Capt. Grey* states that the west coast of Australi... |
lat. 24°, is fronted by a sand bar, about 200 yards in width, on which there is only two feet of water; but within * Capt. Grey's Journal of Two Expeditions, vol. i. p. 369. it the depth increases to two fathoms. Similar bars, more or less perfect, occur on other coasts. In these cases I suspect that the shallow |
channel (which no doubt during storms is occasionally obliterated) is scooped out by the flowing away of the water thrown beyond the line, on which the waves break with the greatest force. At Pernambuco a bar of hard sandstone,* which has the same external form and height as a coral-reef, extends nearly parallel to the... |
apparently caused by the water thrown over it during the greater part of each tide, run strongly, and are wearing away its inner wall. From these facts it can hardly be doubted, that within most fringing reefs, especially within those lying some distance from the land, a return stream must carry away the water thrown o... |
the current thus produced, would tend to prevent the channel being filled up with sediment, and might even deepen it under certain circumstances. To this latter belief I am led, by finding that channels are almost universally present within the fringing reefs of those islands, which have undergone recent elevatory move... |
the conversion of the very shallow channel into land had not been counteracted to a certain extent. A fringing reef, if elevated in a perfect condition above the level of the sea, ought to present the singular appearance of a broad dry moat within a low mound. The author† of an interesting pedestrian tour round the Mau... |
met with a structure of this kind: he says "j'observai que là, où la mer étale, indépendamment des rescifs du large, il y a à terre une espèce d'effoncement, ou chemin couvert naturel. On y pourrait mettre du canon," &c. In another place he adds, "Avant de passer le Cap, on remarque un gros banc de corail elévé de plus |
de quinze pieds: c'est une espèce de rescif, que la mer a abandonné: il regne au pied * I have described this singular structure in the Lond. and Edin. Phil. Mag. Oct. 1841. † Voyage à l'Isle de France, par un Officier du Roi, Part. i. pp. 192, 200. une longue flaque d'eau, dont on pourrait faire un bassin pour |
de petits vaisseaux." But the margin of the reef, although the highest and most perfect part, from being most exposed to the surf, would generally during a slow rise of the land be either partially or entirely worn down to that level, at which corals could renew their growth on its upper edge. On some parts of the coas... |
Mauritius there are little hillocks of coral-rock, which are either the last remnants of a continuous reef, or of low islets formed on it. I observed two such hillocks between Tamarin Bay and the Great Black River, they were nearly 20 feet high, about 200 yards from the present beach, and about 30 feet above its level.... |
from a smooth surface, strewed with worn fragments of coral. They consisted in their lower part of hard calcareous sandstone, and in their upper of great blocks of several species of Astræa and Madrepora, loosely aggregated: they were divided into irregular beds, dipping seaward, in one hillock at an angle of 8°, and i... |
that the superficial parts of the reefs, which have been upraised together with the islands they fringe, have generally been much more modified by the wearing action of the sea, than those of Mauritius. Many islands* are fringed by reefs quite similar to those of Mauritius: but on coasts where the sea deepens very sudd... |
and their limited extension seems evidently to depend on the high inclination of the submarine slope;—a relation which, as we have seen, does not exist in reefs of the barrier class. The fringing-reefs on steep coasts are frequently not more than from 50 to 100 yards in width: they have a nearly smooth, hard, surface, ... |
as another instance; Mr. Taylor (Loudon's Mag. of Nat. Hist. vol. ix. p. 449.) has described a reef several miles in length between Gibara and Vjaro, which extends parallel to the shore at the distance of between half and the third part of a mile, and encloses a space of shallow water, with a sandy bottom and tufts of ... |
Outside the edge of the reef, which is formed of great branching corals, the depth is six and seven fathoms. This coast has been upheaved at no very distant geological period. scarcely uncovered at low-water, and without any interior shoal channel, like that within those fringing reefs, which lie at a greater distance ... |
during gales from the outer margin, are thrown over the reef on the shores of the island. I may give as instances, Wateeo, where the reef is described by Cook as being a hundred yards wide; and Mauti and Elizabeth* Islands, where it is only 50 yards in width: the sea round these islands is very deep. Fringing-reefs, li... |
both surround islands, and front the shores of continents. In the charts of the eastern coast of Africa, by Capt. Owen, many extensive fringing reefs are laid down;—thus, for a space of nearly forty miles, from lat. 1° 15' to 1° 45' S., a reef fringes the shore at an average distance of rather more than one mile, and t... |
at a greater distance than is usual in reefs of this class; but as the coast-land is not lofty, and as the bottom shoals very gradually (the depth being only from 8 to 14 fathoms at a mile and a half outside the reef) its extension thus far from the land offers no difficulty. The external margin of this reef |
is described, as formed of projecting points, within which there is a space, from six to twelve feet deep, with patches of living coral on it. At Mukdeesha (lat. 2° 1' N.) "the port is formed," it is said,† "by a long reef extending eastward four or five miles, within which there is a narrow channel, with ten to twelve |
feet water at low spring tides:" it lies at the distance of a quarter of a mile from the shore. Again, in the plan of Mombas (lat. 4° S.) a reef extends for thirty-six miles, at the distance of from half a mile to one mile and a quarter from the shore; within it, there is a channel navigable "for |
canoes and small craft," between six and fifteen feet deep: outside the reef the depth is about 30 fathoms at the distance of nearly half * Mauti is described by Lord Byron in the voyage of H.M.S. Blonde, and Elizabeth Island by Capt. Beechey. † Owen's Africa, vol. i. p. 357, from which work the foregoing facts are lik... |
a mile. Part of this reef is very symmetrical, and has a uniform breadth of 200 yards. The coast of Brazil is in many parts fringed by reefs. Of these, some are not of coral formation; for instance those near Bahia, and in front of Pernambuco; but a few miles south of this latter city, the reef follows* so closely |
every turn of the shore, that I can hardly doubt it is of coral: it runs at the distance of three quarters of a mile from the land, and within it the depth is from ten to fifteen feet. I was assured by an intelligent pilot, that at Ports Frances and Maceio, the outer part of the reef consists of |
living coral, and the inner of a white stone, full of large irregular cavities, communicating with the sea. The bottom of the sea off the coast of Brazil shoals gradually to between 30 and 40 fathoms, at the distance of between nine and ten leagues from the land. From the description now given, we must conclude, that t... |
structure of fringing reefs depend entirely on the greater or less inclination of the submarine slope, conjoined with the fact, that reef-building polypifers can exist only at limited depths. It follows from this, that where the sea is very shallow, as in the Persian Gulf and in parts of the East Indian Archipelago, th... |
appear as separate and irregularly scattered patches, often of considerable area. From the more vigorous growth of the coral on the outside, and from the conditions being less favourable in several respects within, such reefs are generally higher and more perfect in their marginal than in their central parts: hence the... |
be overlooked,) the appearance of atolls; but they differ from atolls in their central expanse being much less deep, in their form being less defined, and in being based on a shallow foundation. But when in a deep sea reefs fringe banks of sediment, which have accumulated beneath the surface, round either * See Baron R... |
accompanying hydrographical memoir. islands or submerged rocks, they are distinguished with difficulty on the one hand from encircling barrier-reefs, and on the other from atolls. In the West Indies there are reefs, which I should probably have arranged under both these classes, had not the existence of large and level... |
as the basis for the attachment of coral, been occasionally brought into view by the entire or partial absence of reefs on them,—and had not the formation of such banks, through the accumulation of sediment now in progress, been sufficiently evident. Fringing reefs sometimes coat, and thus protect the foundations of is... |
to the level of the sea. According to Ehrenberg, this has been extensively the case with the islands in the Red Sea, which formerly ranged parallel to the shores of the mainland, with deep water within them: hence the reefs now coating their bases, are situated relatively to the land like barrier-reefs, although not be... |
as I believe, in the Red Sea some true barrier-reefs. The reefs of this sea and of the West Indies will be described in the Appendix. In some cases, fringing reefs appear to be considerably modified in outline by the course of the prevailing currents; Dr. J. Allan informs me that on the east coast of Madagascar almost ... |
and low point of sand, has a coral-reef extending from it in a S.W. and N.E. line, parallel to the currents on that shore. I should think the influence of the currents chiefly consisted in causing an extension, in a certain direction, of a proper foundation for the attachment of the coral. Round many intertropical isla... |
on the coast of Brazil surveyed by Capt. FitzRoy, and, as I am informed by Mr. Cuming, round the Philippines, the bottom of the sea is entirely coated by irregular masses of coral, which although often of large size, do not reach the surface and form proper reefs. This must be owing either to insufficient growth, or to... |
of those kinds of corals which can withstand the breaking of the waves. The three classes, atoll-formed, barrier, and fringing reefs, together with the modifications just described of the latter, include all the most remarkable coral-formations anywhere existing. At the commencement of the last chapter in the volume, w... |
coloured, the exceptional cases will be enumerated. ON THE GROWTH OF CORAL REEFS. IN this chapter I will give all the facts, which I have collected, relating to the distribution of coral-reefs,—to the conditions favourable to their increase,—to the rate of their growth,—and to the depth at which they are formed. These ... |
of the origin of the different classes of coral-reefs. On the distribution of Coral-Reefs, and on the Conditions favourable to their Increase. WITH regard to the limits of latitude, over which coral-reefs extend, I have nothing new to add. The Bermuda Islands, in 32° 15' N. is the point furthest removed from the equato... |
and it has been suggested, that their extension so far northward in this instance is owing to the warmth of the Gulf stream. In the Pacific, the Loo Choo islands, in lat. 27° N. have reefs on their shores, and there is an atoll in 28° 30', situated N.W. of the Sandwich Archipelago. In the Red Sea there are coral-reefs |
in lat. 30°. In the southern hemisphere coral-reefs do not extend so far from the equatorial sea. In the Southern Pacific there are only a few reefs beyond the line of the tropic, but Houtmans Abrolhos on the western shores of Australia in lat. 29° S., are of coral-formation. The proximity of volcanic land, owing to th... |
from it, has been thought to be favourable to the increase of coral-reefs. There is, however, not much foundation for this view; for no where are coral-reefs more extensive than on the shores of New Caledonia, and of north-eastern Australia, which consist of primary formations; and in the largest groups of atolls, name... |
Archipelagoes, there is no volcanic or other kind of rock, excepting that formed of coral. The entire absence of coral reefs in certain large areas within the tropical seas, is a remarkable fact. Thus no coral-reefs were observed, during the surveying voyages of the Beagle and her Tender on the west coast of South Amer... |
round the Galapagos Islands. It appears, also, that there are none* north of the Equator; Mr. Lloyd, who surveyed the isthmus of Panama, remarked to me, that although he had seen corals living in the Bay of Panama, yet he had never observed any reefs formed by them. I at first attributed this absence of reefs on the co... |
Peru and of the Galapagos Islands†, to the coldness of the currents from the south, but the Gulf of Panama is one of the hottest pelagic districts in the world.‡ In the central parts of the Pacific there are islands, entirely free from reefs; in some few of these cases I have thought, that this was owing to recent volc... |
action; but the existence of reefs round the * I have been informed that this is the case, by Lieut. Ryder, R.N. and others who have had ample opportunities for observation. † The mean temperature of the surface sea from observations made by the direction of Capt. FitzRoy on the shores of the Galapagos Islands, between... |
and the 20th of October, 1835, was 68° Fahr. The lowest temperature observed was 58°.5 at the S.W. end of Albemarle Island; and on the west coast of this island, it was several times 62° and 63°. The mean temperature of the sea in the Low Archipelago of atolls, and near Tahiti, from similar observations made on board t... |
was (although further from the equator) 77°.5, the lowest any day being 76°.5. Therefore we have here a difference of 9°.5 in mean temperature, and 18° in extremes; a difference doubtless quite sufficient to affect the distribution of organic beings in the two areas. ‡ Humboldt's Personal Narrative, vol. vii. p. 434. g... |
Islands, shows that recent volcanic action does not necessarily prevent their growth. In the last chapter I stated that the bottom of the sea round some islands is thickly coated with living corals, which nevertheless do not form reefs, either from insufficient growth, or from the species not being adapted to contend w... |
by several people, that there are no coral-reefs on the west coast of Africa,* or round the islands in the Gulf of Guinea. This perhaps may be attributed, in part, to the sediment brought down by the many rivers debouching on that coast, and to the extensive mud-banks, which line great part of it. But the islands of St... |
Ascension, the Cape Verdes, St. Paul's, and Fernando Noronha, are, also, entirely without reefs, although they lie far out at sea, are composed of the same ancient volcanic rocks, and have the same general form, with those islands in the Pacific, the shores of which are surrounded by gigantic walls of coral-rock. With ... |
a single coral reef in the central expanse of the Atlantic ocean. It will, perhaps, be suggested, that the quantity of carbonate of lime in different parts of the sea, may regulate the presence of reefs. But this cannot be the case, for at Ascension, the waves charged to excess precipitate a thick layer of calcareous m... |
rocks; and at St. Jago, in the Cape Verdes, carbonate of lime, not only is abundant on the shores, but it forms the chief part of some upraised post-tertiary strata. The apparently capricious distribution, therefore, of coral-reefs, cannot be explained by any of these obvious causes: but, as the study of the terres- * ... |
paper by Capt. Owen (Geograph. Jour. vol. ii. p. 89.) that the reefs off Cape St. Anne and the Sherboro' Islands were of coral, although the author states that they are not purely corralline. But I have been assured by Lieut. Holland, R.N. that these reefs are not of coral, or at least they do not at all resemble those |
in the West Indies. trial and better known half of the world, must convince every one that no station capable of supporting life is lost,—nay more, that there is a struggle for each station, between the different orders of nature,—we may conclude that in those parts of the intertropical sea, in which there are no coral... |
bodies, supplying the place of the reef-building polypifers. It has been shown in the chapter on Keeling atoll that there are some species of large fish, and the whole tribe of Holothuriæ which prey on the tenderer parts of the corals. On the other hand, the polypifers in their turn must prey on some other organic bein... |
which from any cause, would cause a proportionate destruction of the living coral. The relations, therefore, which determine the formation of reefs on any shore, by the vigorous growth of the efficient kinds of coral, must be very complex, and with our imperfect knowledge quite inexplicable. From these considerations, ... |
not obvious to our senses, might destroy all the coral-reefs in one area, and cause them to appear in another: thus, the Pacific or Indian ocean might become as barren of coral-reefs as the Atlantic now is, without our being able to assign any adequate cause for such a change. It has been a question with some naturalis... |
of a reef is most favourable to the growth of coral. The great mounds of living Porites and of Millepora round Keeling atoll occur exclusively on the extreme verge of the reef, which is washed by a constant succession of breakers; and living coral no where else forms solid masses. At the Marshall islands the larger kin... |
species of Astræa, a genus closely allied to Porites) "which form rocks measuring several fathoms in thickness," prefer, according to Chamisso*, the most violent surf. I have stated that the outer margin of the Maldiva atolls consists of living corals (some of which, if not all, are of the same species with those at Ke... |
is so tremendous, that even large ships have * Kotzebue's First Voyage, (Eng. Transl.), vol. iii. pp. 142, 143, 331. been thrown, by a single heave of the sea, high and dry on the reef, all on board thus escaping with their lives. Ehrenberg* remarks, that in the Red Sea the strongest corals live on the outer reefs, and... |
to love the surf; he adds, that the more branched kinds abound a little way within, but that even these in still more protected places, become smaller. Many other facts having a similar tendency might be adduced.† It has, however, been doubted by MM. Quoy and Gaimard, whether any kind of coral can even withstand, much ... |
breakers of an open sea:‡ they affirm that the saxigenous lithophytes flourish only where the water is tranquil, and the heat intense. This statement has passed from one geological work to another; nevertheless, the protection of the whole reef undoubtedly is due to those kinds of coral, which cannot exist in the situa... |
favourable to them. For should the outer and living margin perish, of any one of the many low coral-islands, round which a line of great breakers is incessantly foaming, the whole, it is scarcely possible to doubt, would be washed away and destroyed, in less than half a century. But the vital energies of the corals con... |
of the waves; and the large fragments of reef torn up by every storm, are replaced by the slow but steady growth of the innumerable polypifers, which form the living zone on its outer edge. From these facts, it is certain, that the strongest and most massive corals flourish, where most exposed. The less perfect state o... |
most atolls on the leeward and less exposed side, compared with its state to windward; and the analogous * Ehrenberg, über die Nätur und Bildung der Corallen Bänke im rothen Meere, p. 49. † In the West Indies, as I am informed by Capt. Bird Allen, R.N., it is the common belief of those, who are best acquainted with the |
reefs, that the coral flourishes most, where freely exposed to the swell of the open sea. ‡ Annales des Sciences Naturelles, tom. vi. pp. 276, 278.—"Là où les ondes sont agitées, les Lytophytés ne peuvent travailler, parce qu'elles détruraient leurs fragiles édifices," &c. case of the greater number of breaches on the ... |
Archipelago, which afford some protection to each other, are obviously explained by this circumstance. If the question had been, under what conditions the greater number of species of coral, not regarding their bulk and strength, were developed, I should answer,—probably in the situations described by MM. Quoy and Gaim... |
number of species of coral in the circumtropical seas must be very great: in the Red Sea alone, 120 kinds, according to Ehrenberg,* have been observed. The same author has observed that the recoil of the sea from a steep shore is injurious to the growth of coral, although waves breaking over a bank are not so. Ehrenber... |
that where there is much sediment, placed so as to be liable to be moved by the waves, there is little or no coral; and a collection of living specimens placed by him on a sandy shore died in the course of a few days.† An experiment, however, will presently be related, in which some large masses of living coral |
increased rapidly in size, after having been secured by stakes on a sand-bank. That loose sediment should be injurious to the living polypifers, appears, at first sight, probable; and accordingly, in sounding off Keeling atoll, and (as will hereafter be shown) off Mauritius, the arming of the lead invariably came up cl... |
circumstance has probably given rise to a strange belief, which, according to Capt. Owen,‡ is general amongst the inhabitants of the Maldiva atolls, namely, that corals have roots, and therefore that if merely broken down to the surface, they grow up again; but if rooted out, they are permanently destroyed. By this mea... |
thus the French Governor of St. Mary's, in Madagascar, * Ehrenberg, über die Nätur, &c. &c. p. 46. † Ibid. p. 49. ‡ Capt. Owen on the Geography of the Maldiva Islands, Geograph. Journal, vol. ii. p. 88. "cleared out and made a beautiful little port at that place." For it is probable that sand would accumulate in the ho... |
formed by tearing out the corals, but not on the broken and projecting stumps, and therefore, in the former case, the fresh growth of the coral might be thus prevented. In the last chapter I remarked, that fringing reefs are almost universally breached, where streams enter the sea.* Most authors have attributed this fa... |
fresh water, even where it enters the sea only in small quantity, and during a part of the year. No doubt brackish water would prevent or retard the growth of coral; but I believe that the mud and sand, which is deposited, even by rivulets when flooded, is a much more efficient check. The reef, on each side of the |
channel leading into Port Louis at Mauritius, ends abruptly in a wall, at the foot of which I sounded, and found a bed of thick mud. This steepness of the sides appears to be a general character in such breaches: Cook,† speaking of one at Raiatea, says: "like all the rest, it is very steep on both sides." Now, if |
it were the fresh water mingling with the salt, which prevented the growth of coral, the reef certainly would not terminate abruptly; but as the polypifers nearest the impure stream would grow less vigorously than those farther off, so would the reef gradually thin away. On the other hand, the sediment brought down fro... |
growth of the coral in the line of its deposition, but would not check it on the side, so that the reefs might increase till they overhung the bed of the channel. The breaches are much fewer in number, and front only the larger valleys in reefs of the encircling barrier class. They probably are kept open in the * |
Lieut. Wellstead and others have remarked that this is the case in the Red Sea: Dr. Rüppell (Reise. in Abyss. Band. i. p. 142) says that there are pear-shaped harbours in the upraised coral-coast, into which periodical streams enter. From this circumstance, I presume, we must infer that before the upheaval of the strat... |
and sediment entered the sea at these points; and the coral being thus prevented growing, the pear-shaped harbours were produced. † Cook's First Voyage, vol. ii. p. 271.—(Hawkesworth's Edit.) same manner, as those into the lagoon of an atoll, namely, by the force of the currents and the drifting outwards of fine sedime... |
often separated from the land by deep-water lagoon-channels, which it might be thought would entirely remove the injurious effects both of the fresh water and the sediment, will receive a simple explanation when we discuss the origin of barrier-reefs. In the vegetable kingdom every different station has its peculiar gr... |
We have already described the great difference between the corals within the lagoon of an atoll and those on its outer margin. The corals, also, on the margin of Keeling Island 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... |
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,... |
bodies live. If we compare the external margin of the reef at Keeling atoll with that on the leeward side of Mauritius, which are very differently circumstanced, we shall find a corresponding difference in the appearance of the corals. At the latter place, the genus Madrepora is preponderant over every other kind; and ... |
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