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, there is a channel like that within 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, even 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 the gentle slope of the reef 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 agitated by the waves, 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 of 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


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 australia, in 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 coast ; within this bar currents, 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 over the outer edge ; and 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 movements ; and this could hardly have been the case, if 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 mauritius, seems to have met with a structure of this kind : he says " j ' observai que la, ou la mer etale, independamment des rescifs du large, il y a a terre une espece d ' effoncement, ou chemin couvert naturel. on y pourrait mettre du canon, " & c. in another place he adds, " avant de passer le cap, on


une espece 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 eleve de plus de quinze pieds : c ' est une espece de rescif, que la mer a abandonne : il regne au pied * i have described this singular structure in the lond. and edin. phil. mag. oct. 1841. † voyage a 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 coast - land of 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. they rose abruptly 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 in the other at 18°. i suspect 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 suddenly the reefs are much narrower, 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 fr


sea deepens very suddenly the reefs are much narrower, 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, * i may give cuba, 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 coral. 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 from the land. the fragments torn up 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, like barrier - reefs, 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 therefore 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 mu


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 likewise taken. 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 the dimensions and 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


from the description now given, we must conclude, that the dimensions and 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, the reefs lose their fringing character, and 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 these reefs sometimes assume, ( and this circumstance ought not to 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 roussin ' s pilote de bresil, and 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 banks, lying a little beneath the surface, ready to serve 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 islands, which have been worn down by the surf 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 belonging to that class ; — but there are, 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


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 every headland 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 islands, for instance the abrolhos 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 the absence 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, where i detail the principles on which the map ( plate iii. ) is 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 subjects have an important bearing on the theory 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 equator, in which they appear to exist ; 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


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 the lime generally evolved 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, namely the maldiva, chagos, marshall, gilbert, and low 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 america south of the equator, or 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 coasts of 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 volcanic 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


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 the 16th of september 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 the beagle, 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. greater part of hawaii, one of the sandwich 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 with the breaking waves. i have been assured 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. helena, 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 the exception of bermuda, there is not 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


is not 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 matter on the tidal 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 - * it might be concluded, from a 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 - reefs, there are other organic 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 beings ; the decrease of 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, we may infer that changes in the condition of the sea, 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


, we may infer that changes in the condition of the sea, 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 naturalists, which part 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 kinds of coral ( chiefly 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 keeling atoll ), and here the surf 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 appear 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 less flourish in, the 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 situations thought by these naturalists to be most favourable to them. for should the outer and living margin perish, of any one of the many low coral - islands, round


reef undoubtedly is due to those kinds of coral, which cannot exist in the situations thought by these naturalists to be most 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 conquer the mechanical power 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 of the reef of most atolls on the leeward and less exposed side, compared with its state to windward ; and the analogous * ehrenberg, uber die natur und bildung der corallen banke 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. — " la ou les ondes sont agitees, les lytophytes ne peuvent travailler, parce qu ' elles detruraient leurs fragiles edifices, " & c. case of the greater number of breaches on the near sides of those atolls in the maldiva 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 gaimard, where the water is tranquil and the heat intense. the total 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


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. ehrenberg also states, 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 clean, where the coral was growing vigorously. this same 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 means, the inhabitants keep their harbours clear ; and thus the french governor of st. mary ' s, in madagascar, * ehrenberg, uber die natur, & 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 hollows 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 fact to the injurious effects of the 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


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 from the land would only prevent the 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. ruppell ( 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 strata now forming the coast - land, fresh water 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 sediment. their position in front of valleys, although 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


of fine sediment. their position in front of valleys, although 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 group of plants, and similar relations appear to prevail with corals. 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 a short exposure to the air ; whereas, three species of nullipora also live amidst the breakers, but are able to survive uncovered for a part of each tide : at greater depths, a strong madrepora and millepora alcicornis are the commonest kinds ; the former appearing to be confined to this part : beneath the zone of massive corals, minute encrusting corallines and other organic bodies live. 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 beneath the zone of massive corals, there are large beds of seriatopora. there is also a marked difference, according to captain moresby, * between the great branching corals of the red sea, and those on the reefs of the maldiva atolls. these facts, which in themselves are deserving of notice, bear, perhaps, not very remotely on a remarkable circumstance which has been pointed out to me by capt. moresby, * capt. moresby on the northern maldiva atolls, geograph. journ. vol. v. p. 401. namely, that, with very few exceptions, none of the coral - knolls within the lagoons of peros banhos, diego garcia and the great chagos bank, ( all situated in the chagos group, ) rise to the surface of the water ; whereas all those, with equally few exceptions, within solomon and egmont atolls in the same group


, of the chagos group. the lagoon - reefs have reached the surface, some are small, others large, some shallow, others deep, some well enclosed, and others open. capt. moresby informs me that he has seen a french chart of diego garcia made eighty years before his survey, and apparently very accurate ; and from it he infers, that during this interval there has not been the smallest change in the depth on any of the knolls within the lagoon. it is, also, known that during the last fifty - one years, the eastern channel into the lagoon, has neither become narrower, nor decreased in depth ; and as there are numerous small knolls of living coral within it, some change might have been anticipated. moreover, as the whole reef round the lagoon of this atoll has been converted into land — an unparalleled case, i believe, in an atoll of such large size, — and as the strip of land is for considerable spaces more than half a mile wide — also a very unusual circumstance, — we have the best possible evidence, that diego garcia has remained at its present level for a very long period. with this fact, and with the knowledge, that no sensible change has taken place during eighty years in the coral knolls, and considering that every single reef has reached the surface in other atolls, which do not present the smallest appearance of being older that diego garcia and peros banhos, and which are placed under the same external conditions with them, one is led to conclude that these submerged reefs, although covered with luxuriant coral, have no tendency to grow upwards, and that they would remain at their present levels for an almost indefinite period. from the number of these knolls, from their position, size, and form, — many of them being only one or two hundred yards across, with a rounded outline, and precipitous sides, — it is indisputable that they have been formed by the growth of coral ; and this makes the case much more remarkable. in peros banhos and in the great chagos bank, some of these almost columnar masses are 200 feet high, and their summits lie only from two to eight fathoms beneath the surface ; there - fore, a small proportional amount more of growth would cause them to attain the surface, like those numerous knolls, which rise from an equally great depth within the maldiva atolls. we can hardly suppose that time has been wanting for the upward growth


a small proportional amount more of growth would cause them to attain the surface, like those numerous knolls, which rise from an equally great depth within the maldiva atolls. we can hardly suppose that time has been wanting for the upward growth of the coral, whilst in diego garcia, the broad annular strip of land, formed by the continued accumulation of detritus, shows how long this atoll has remained at its present level. we must look to some other cause than the rate of growth ; and i suspect it will be found in the reefs being formed of different species of corals, adapted to live at different depths. the great chagos bank is situated in the centre of the chagos group, and the pitt and speaker banks at its two extreme points. these banks resemble atolls, except in their external rim being about eight fathoms submerged, and in being formed of dead rock, with very little living coral on it : a portion nine miles long of the annular reef of peros banhos atoll is in the same condition. these facts, as will hereafter be shown, render it very probable that the whole group at some former period subsided seven or eight fathoms ; and that the coral perished on the outer margin of those atolls which are now submerged, but that it continued alive, and grew up to the surface on those which are now perfect. if these atolls did subside, and if from the suddenness of the movement or from any other cause, those corals which are better adapted to live at a certain depth than at the surface, once got possession of the knolls, supplanting the former occupants, they would exert little or no tendency to grow upwards. to illustrate this, i may observe, that if the corals of the upper zone on the outer edge of keeling atoll were to perish, it is improbable that those of the lower zone would grow to the surface, and thus become exposed to conditions for which they do not appear to be adapted. the conjecture, that the corals on the submerged knolls within the chagos atolls have analogous habits with those of the lower zone outside keeling atoll, received some support from a remark by captain moresby, namely, that they have a different appearance from those on the reefs in the maldiva atolls, which, as we have seen, all rise to the surface : he compares the kind of difference to that of the vegetation under different climates. i have entered at


they have a different appearance from those on the reefs in the maldiva atolls, which, as we have seen, all rise to the surface : he compares the kind of difference to that of the vegetation under different climates. i have entered at considerable length into this case, although unable to throw much light on it, in order to show that an equal tendency to upward growth ought not to be attributed to all coral - reefs, — to those situated at different depths, — to those forming the ring of an atoll or those on the knolls within a lagoon, — to those in one area and those in another. the inference, therefore, that one reef could not grow up to the surface within a given time, because another, not known to be covered with the same species of corals, and not known to be placed under conditions exactly the same, has not within the same time reached the surface, is unsound. on the rate of growth of coral - reefs. the remark made at the close of the last section, naturally leads to this division of our subject, which has not, i think, hitherto been considered under a right point of view. ehrenberg * has stated, that in the red sea, the corals only coat other rocks in a layer from one to two feet in thickness, or at most to a fathom and a half ; and he disbelieves that, in any case, they form, by their own proper growth, great masses, stratum over stratum. a nearly similar observation has been made by mm. quoy and gaimard, † with respect to the thickness of some upraised beds of coral, which they examined at timor and some other places. ehrenberg ‡ saw certain large massive corals in the red sea, which he imagines to be of such vast antiquity, that they might have been beheld by pharaoh ; and according to mr. lyell § there are certain corals * ehrenberg, as before cited, pp. 39, 46, and 50. † annales des sciences, nat. tom. vi. p. 28. ‡ ehrenberg, ut sup. p. 42. § lyell ' s principles of geology, book iii. ch. xviii. at bermuda, which are known by tradition, to have been living for centuries. to show how slowly coral - reefs grow upwards, captain beechey * has adduced the case of the dolphin reef off tahiti


iii. ch. xviii. at bermuda, which are known by tradition, to have been living for centuries. to show how slowly coral - reefs grow upwards, captain beechey * has adduced the case of the dolphin reef off tahiti, which has remained at the same depth beneath the surface, namely, about two fathoms and a half, for a period of sixty - seven years. there are reefs in the red sea, which certainly do not appear † to have increased in dimensions during the last half century, and from the comparison of old charts with recent surveys, probably not during the last two hundred years. these, and other similar facts, have so strongly impressed many with the belief of the extreme slowness of the growth of corals, that they have even doubted the possibility of islands in the great oceans having been formed by their agency. others again, who have not been overwhelmed by this difficulty, have admitted that it would require thousands, and tens of thousands of years, to form a mass even of inconsiderable thickness : but the subject has not, i believe, been viewed in the proper light. that masses of considerable thickness have been formed by the growth of coral, may be inferred with certainty from the following facts. in the deep lagoons of peros banhos and of the great chagos bank, there are, as already described, small steep - sided knolls covered with living coral. there are similar knolls in the southern maldiva atolls, some of which, as captain moresby assures me, are less than a hundred yards in diameter, and rise to the surface from a depth of between 250 and 300 feet. considering their number, form and position, it would be preposterous to suppose that they are based on pinnacles of any rock, not of coral formation ; or that sediment could have been heaped up into such small and steep isolated cones. as no kind of living coral grows above the height of a few feet, we are compelled to suppose that these knolls have been formed by the successive growth and death of many individuals, — first one being broken off or killed by some accident, and then another, and * beechey ' s voyage to the pacific, ch. viii. † ehrenberg, ut sup. p. 43. one set of species being replaced by another set with different habits, as the reef rose nearer the surface, or as other changes supervened. the spaces between the corals would become


† ehrenberg, ut sup. p. 43. one set of species being replaced by another set with different habits, as the reef rose nearer the surface, or as other changes supervened. the spaces between the corals would become filled up with fragments and sand, and such matter would probably soon be consolidated, for we learn from lieut. nelson, * that at bermuda a process of this kind takes place beneath water, without the aid of evaporation. in reefs, also, of the barrier class, we may feel sure, as i have shown, that masses of great thickness have been formed by the growth of coral : in the case of vanikoro, judging only from the depth of the moat between the land and the reef, the wall of coral - rock must be at least 300 feet in vertical thickness. it is unfortunate that the upraised coral islands in the pacific have not been examined by a geologist. the cliffs of elizabeth island, in the low archipelago, are eighty feet high, and appear from captain beechey ' s description, to consist of a homogeneous coral rock. from the isolated position of this island we may safely infer that it is an upraised atoll, and therefore that it has been formed by masses of coral, grown together : savage island seems, from the description of the younger forster, † to have a similar structure, and its shores are about forty feet high : some of the cook islands also appear ‡ to be similarly composed. capt. belcher, r. n. in a letter, which capt. beaufort showed me at the admiralty, speaking of bow atoll, says, " i have succeeded in boring forty - five feet through coral - sand, when the auger became jammed by the falling in of the surrounding creamy matter. " on one of the maldiva atolls, capt. moresby bored to a depth of twenty - six feet, when his auger also broke : he has had the kindness to give me the matter brought up ; it is perfectly white, and like finely triturated coral - rock. in my description of keeling atoll, i have given some facts, which show that the reef probably has grown out - * geological transactions, vol. v. p. 113. † forster ' s voyage round the world with cook, vol. ii. pp. 163, 167. ‡ williams ' narrative of missionary enterprize, p. 30. wards ; and i found, just within the outer margin, the great mounds of porites


s voyage round the world with cook, vol. ii. pp. 163, 167. ‡ williams ' narrative of missionary enterprize, p. 30. wards ; and i found, just within the outer margin, the great mounds of porites and of millepora, with their summits lately killed, and their sides subsequently thickened by the growth of the coral : a layer, also, of nullipora had already coated the dead surface. as the external slope of the reef, is the same round the whole of this atoll, and round many other atolls, the angle of inclination must result from an adaption between the growing powers of the coral, and the force of the breakers, and their action on the loose sediment. the reef, therefore, could not increase outwards, without a nearly equal addition to every part of the slope, so that the original inclination might be preserved, and this would require a large amount of sediment, all derived from the wear of corals and shells, to be added to the lower part. moreover, at keeling atoll, and probably in many other cases, the different kinds of corals would have to encroach on each other ; thus the nulliporæ cannot increase outwards without encroaching on the porites and millepora complanata, as is now taking place ; nor these latter without encroaching on the strongly branched madrepora, the millepora alcicornis, and some astræas ; nor these again without a foundation being formed for them within the requisite depth, by the accumulation of sediment. how slow, then, must be the ordinary lateral or outward growth of such reefs. but off christmas atoll, where the sea is much more shallow than is usual, we have good reason to believe that, within a period not very remote, the reef has increased considerably in width. the land has the extraordinary breadth of three miles ; it consists of parallel ridges of shells and broken corals, which furnish " an incontestable proof, " as observed by cook, * " that the island has been produced by accessions from the sea, and is in a state of increase. " the land is fronted by a coral - reef, and from the manner in which islets are known to be formed, we may feel confident that the reef was not three miles wide, when the first, or most backward ridge, was * cook ' s third voyage, book iii. ch. x. thrown


the manner in which islets are known to be formed, we may feel confident that the reef was not three miles wide, when the first, or most backward ridge, was * cook ' s third voyage, book iii. ch. x. thrown up ; and, therefore, we must conclude that the reef has grown outwards during the accumulation of the successive ridges. here then, a wall of coral - rock of very considerable breadth has been formed by the outward growth of the living margin, within a period, during which ridges of shells and corals, lying on the bare surface, have not decayed. there can be little doubt, from the account given by capt. beechey, that matilda atoll, in the low archipelago, has been converted in the space of thirty - four years, from being, as described by the crew of a wrecked whaling vessel, a " reef of rocks, " into a lagoon island, fourteen miles in length, with " one of its sides covered nearly the whole way with high trees. " * the islets, also, on keeling atoll, it has been shown, have increased in length, and since the construction of an old chart, several of them have become united into one long islet : but in this case, and in that of matilda atoll, we have no proof, and can only infer as probable, that the reef, that is the foundation of the islets, has increased as well as the islets themselves. after these considerations, i attach little importance, as indicating the ordinary and still less the possible rate of outward growth of coral - reefs, to the fact that certain reefs in the red sea have not increased during a long interval of time ; or to other such cases, as that of ouluthy atoll in the caroline group, where every islet, described a hundred years before by cantova, was found in the same state by lutke, † — without it could be shown that, in these cases, the conditions were favourable to the vigorous and unopposed growth of the corals living in the different zones of depth, and that a proper basis for the extension of the reef was present. the former conditions must depend on many contingencies, and in the deep oceans where coral formations most abound, * beechey ' s voyage to the pacific, ch. vii. and viii. † f. lutke ' s voyage autour du monde. in the group elato, however, it appears that what is now the islet falipi


beechey ' s voyage to the pacific, ch. vii. and viii. † f. lutke ' s voyage autour du monde. in the group elato, however, it appears that what is now the islet falipi, is called in cantova ' s chart, the banc de falipi. it is not stated whether this has been caused by the growth of coral, or by the accumulation of sand. a basis within the requisite depth can rarely be present. nor do i attach any importance to the fact of certain submerged reefs, as those off tahiti or those within diego garcia, not now being nearer the surface than they were many years ago, as an indication of the rate under favourable circumstances of the upward growth of reefs ; after it has been shown, that all the reefs have grown to the surface in some of the chagos atolls, but that in neighbouring atolls, which appear to be of equal antiquity and to be exposed to the same external conditions, every reef remains submerged ; for we are almost driven to attribute this to a difference, not in the rate of growth, but in the habits of the corals in the two cases. in an old - standing reef, the corals, which are so different in kind on different parts of it, are probably all adapted to the stations they occupy, and hold their places, like other organic beings, by a struggle one with another, and with external nature ; hence we may infer that their growth would generally be slow, except under peculiarly favourable circumstances. almost the only natural condition, allowing a quick upward growth of the whole surface of a reef, would be a slow subsidence of the area in which it stood ; — if, for instance, keeling atoll were to subside two or three feet, can we doubt that the protecting margin of live coral, about half an inch in thickness, which surrounds the dead upper surfaces of the mounds of porites, would in this case form a concentric layer over them, and the reef thus increase upwards, instead of, as at present, outwards? the nulliporæ are now encroaching on the porites and millepora, but in this case might we not confidently expect that the latter would, in their turn, encroach on the nulliporæ? after a subsidence of this kind, the sea would gain on the islets, and the great fields of dead but upright corals in the lagoon, would be


, in their turn, encroach on the nulliporæ? after a subsidence of this kind, the sea would gain on the islets, and the great fields of dead but upright corals in the lagoon, would be covered by a sheet of clear water ; and might we not then expect that these reefs would rise to the surface, as they anciently did when the lagoon was less confined by islets, and as they did within a period of ten years in the schooner - channel, cut by the inhabitants. in one of the maldiva atolls, a reef, which within a very few years existed as an islet bearing cocoa - nut trees, was found by lieut. prentice " entirely covered with live coral and madrepore. " the natives believe that the islet was washed away by a change in the currents, but if, instead of this, it had quietly subsided, surely every part of the island which offered a solid foundation, would in a like manner have become coated with living coral. through steps such as these, any thickness of rock, composed of a singular intermixture of various kinds of corals, shells, and calcareous sediment, might be formed ; but without subsidence, the thickness would necessarily be determined by the depth at which the reef - building polypifers can exist. if it be asked, at what rate in years i suppose a reef of coral favourably circumstanced could grow up from a given depth ; i should answer, that we have no precise evidence on this point, and comparatively little concern with it. we see, in innumerable points over wide areas, that the rate has been sufficient, either to bring up the reefs from various depths to the surface, or, as is more probable, to keep them at the surface, during progressive subsidences ; and this is a much more important standard of comparison than any cycle of years. it may, however, be inferred from the following facts, that the rate in years under favourable circumstances would be very far from slow. dr. allan of forres has, in his ms. thesis deposited in the library of the edinburgh university, ( extracts from which i owe to the kindness of dr. malcolmson, ) the following account of some experiments, which he tried during his travels in the years 1830 to 1832 on the east coast of madagascar. " to ascertain the rise and progress of the coral family, and fix the number of species met with at foul point


following account of some experiments, which he tried during his travels in the years 1830 to 1832 on the east coast of madagascar. " to ascertain the rise and progress of the coral family, and fix the number of species met with at foul point ( lat. 17° 40 ' ) twenty species of coral were taken off the reef and planted apart on a sand - bank three feet deep at low water. each portion weighed ten pounds, and was kept in its place by stakes. similar quantities were placed in a clump and secured as the rest. this was done in december 1830. in july following, each detached mass was nearly level with the sea at low water, quite immoveable, and several feet long, stretching, as the parent reef, with the coast current from north to south. the masses accumulated in a clump were found equally increased, but some of the species in such unequal ratios, as to be growing over each other. " the loss of dr. allan ' s magnificent collection by shipwreck, unfortunately prevents its being known to what genera these corals belonged ; but from the numbers experimented on, it is certain that all the more conspicuous kinds must have been included. dr. allan informs me, in a letter, that he believes it was a madrepora, which grew most vigorously. one may be permitted to suspect that the level of the sea might possibly have been somewhat different at the two stated periods ; nevertheless, it is quite evident that the growth of the ten - pound masses, during the six or seven months, at the end of which they were found immoveably fixed * and several feet in length, must have been very great. the fact of the different kinds of coral, when placed in one clump, having increased in extremely unequal ratios, is very interesting, as it shows the manner in which a reef, supporting many species of coral, would probably be affected by a change in the external conditions favouring one kind more than another. the growth of the masses of coral in n. and s. lines parallel to the prevailing currents, whether due to the drifting of sediment or to the simple movement of the water, is, also, a very interesting circumstance. a fact, communicated to me by lieut. wellstead, i. n., in some degree corroborates the result of dr. allan ' s experiments : it is, that in the persian gulf a ship had her copper bottom encrusted in the course of twenty months with a


each other by the most irregular gullies and cavities, into which the lead sinks many feet. outside this irregular border of madrepores, the water deepens gradually to twenty fathoms, which depth generally is found at the distance of from half to three quarters of a mile from the reef. a little further out the depth is thirty fathoms, and thence the bank slopes rapidly into the depths of the ocean. this inclination is very gentle compared with that outside keeling and other atolls, but compared with most coasts it is steep. the water was so clear outside the reef, that i could distinguish every object forming the rugged bottom. in this part, and to a depth of eight fathoms, i sounded repeatedly, and at each cast pounded the bottom with the broad lead, nevertheless the arming invariably came up perfectly clean, but deeply indented. from eight to fifteen fathoms a little calcareous sand was occasionally brought up, but more frequently the arming was simply indented. in all this space the two madrepores above mentioned, and two species of astræa, with rather large * stars, seemed the commonest kinds ; and it must be noticed that twice at the depth of 15 fathoms, the arming was marked with a clean impression of an astræa. besides these lithophytes, some fragments of the millepora alcicornis, which occurs in the same relative position at keeling island, were brought up ; and in the deeper parts there were large beds of a seriatopora, different from s. subulata, but closely allied to it. on the beach within the reef, the rolled fragments consisted chiefly of the corals just mentioned, and of a massive porites, like that at keeling atoll, of a meandrina, pocillopora verrucosa and of numerous fragments of nullipora. from fifteen to twenty fathoms the bottom was, with few exceptions, either formed of sand, or thickly covered with seriatopora : this delicate coral seems to form at these depths extensive beds, unmingled with any other kind. at 20 fathoms, one sounding brought up a fragment of madrepora apparently m. pocillifera, and i believe it is the same species ( for i neglected to bring specimens from both stations ) which mainly forms the upper margin of the reef ; if so, it grows in depths varying from 0 to 20 fat


apparently m. pocillifera, and i believe it is the same species ( for i neglected to bring specimens from both stations ) which mainly forms the upper margin of the reef ; if so, it grows in depths varying from 0 to 20 fathoms. between twenty and thirty - three fathoms i obtained several soundings and they all showed a sandy bottom, with one exception * since the preceding pages were printed off, i have received from mr. lyell a very interesting pamphlet, entitled remarks upon coral - formations, & c., by j. couthouy, boston, united states, 1842. there is a statement ( p. 6 ), on the authority of the rev. j. williams, corroborating the remarks made by ehrenberg and lyell, ( p. 71 of this volume, ) on the antiquity of certain individual corals in the red sea and at bermuda ; namely, that at upolu, one of the navigator islands, " particular clumps of coral are known to the fishermen by name, derived from either some particular configuration or tradition attached to them, and handed down from time immemorial. " with respect to the thickness of masses of coral - rock, it clearly appears, from the descriptions given by mr. couthouy ( pp. 34, 58 ), that mangaia and aurora islands are upraised atolls, composed of coral - rock : the level summit of the former is about 300 feet, and that of aurora island is 200 feet above the sea - level. at 30 fathoms, when the arming came up scooped out, as if by the margin of a large caryophyllia. beyond 33 fathoms i sounded only once ; and from 86 fathoms, at the distance of one mile and a third from the edge of the reef, the arming brought up calcareous sand with a pebble of volcanic rock. the circumstance of the arming having invariably come up quite clean, when sounding within a certain number of fathoms off the reefs of mauritius and keeling atoll ( eight fathoms in the former case, and twelve in the latter ), and of its having always come up ( with one exception ) smoothed and covered with sand, when the depth exceeded 20 fathoms, probably indicates a criterion, by which the limits of the vigorous growth of coral might in all cases be readily ascertained. i do not, however, suppose that if a vast number of


with sand, when the depth exceeded 20 fathoms, probably indicates a criterion, by which the limits of the vigorous growth of coral might in all cases be readily ascertained. i do not, however, suppose that if a vast number of soundings were obtained round these islands, the limit above assigned would be found never to vary, but i conceive the facts are sufficient to show, that the exceptions would be few. the circumstance of a gradual change, in the two cases, from a field of clean coral to a smooth sandy bottom, is far more important in indicating the depth at which the larger kinds of coral flourish, than almost any number of separate observations on the depth, at which certain species have been dredged up. for we can understand the gradation, only as a prolonged struggle against unfavourable conditions. if a person were to find the soil clothed with turf on the banks of a stream of water, but on going to some distance on one side of it, he observed the blades of grass growing thinner and thinner, with intervening patches of sand, until he entered a desert of sand, he would safely conclude, especially if changes of the same kind were noticed in other places, that the presence of the water was absolutely necessary to the formation of a thick bed of turf : so may we conclude, with the same feeling of certainty, that thick beds of coral are formed only at small depths beneath the surface of the sea. i have endeavoured to collect every fact, which might either invalidate or corroborate this conclusion. capt. mores - by, whose opportunities for observation during his survey of the maldiva and chagos archipelagoes have been unrivalled, informs me, that the upper part or zone of the steep - sided reefs, on the inner and outer coasts of the atolls in both groups, invariably consists of coral, and the lower parts of sand. at seven or eight fathoms depth, the bottom is formed, as could be seen through the clear water, of great living masses of coral, which at about ten fathoms generally stand some way apart from each other, with patches of white sand between them, and at a little greater depth these patches become united into a smooth steep slope, without any coral. capt. moresby, also, informs me in support of his statement, that he found only decayed coral on the padua bank ( northern part of the laccadive group ) which has an average depth between 25 and 35 fat


any coral. capt. moresby, also, informs me in support of his statement, that he found only decayed coral on the padua bank ( northern part of the laccadive group ) which has an average depth between 25 and 35 fathoms, but that on some other banks in the same group with only ten or twelve fathoms water on them, ( for instance, the tillacapeni bank ), the coral was living. with regard to the coral - reefs in the red sea, ehrenberg has the following passage. " the living corals do not descend there into great depths. on the edges of islets and near reefs, where the depth was small, very many lived ; but we found no more even at six fathoms. the pearl - fishers at yemen and massaua asserted that there was no coral near the pearl - banks at nine fathoms depth, but only sand. we were not able to institute any more special researches. " * i am, however, assured both by captain moresby and lieut. wellstead, that in the more northern parts of the red sea, there are extensive beds of living coral at a depth of 25 fathoms, in which the anchors of their vessels were frequently entangled. captain moresby attributes the less depth, 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 uber die natur, & c. p. 50. of mauritius where i found the 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, and considering the manner in 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 have the power to keep free from sediment. mm. quo


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 have the power to keep free from sediment. 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 has given me a 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 fathoms off juan fernandez ( lat. 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 those corals and corallines can live, which there is no reason to suppose ever materially aid in the construction of a reef. | name of zoophyte. | | depth in fathoms | | country


tropics, at which those corals and corallines can live, which there is no reason to suppose ever materially aid in the construction of a reef. | name of zoophyte. | | depth in fathoms | | country and s. latitude | | authority. | | sertularia | | 40 | | cape horn 66° | | [ where none is given, the observation is my own. ] | |,, a minute scarlet encrusting species, found living | | 190 | | keeling at. 12° | |,, an allied, small stony subgeneric form | | 48 | | s. cruz riv. 50° | | a coral allied to vincularia, with eight rows of cells | | 40 | | cape horn | | tubulipora, near to t. patina | | ditto | | ditto | | do. do. | | 94 | | east chiloe 43° | | cellepora, several species, and allied sub - generic forms | | 40 | | cape horn | | ditto | | 40 and 57 | | chonos arch. 45° | | ditto | | 48 | | s. cruz 50° | | eschara | | 30 | | tierra del fuego 53° | | ditto | | 48 | | s. cruz r. 50° | | ditto | | 100 | | c. good hope 34° | | quoy and gaimard, ann. scien. nat., t. vi. p. 284. | | millepora, a strong coral with cylindrical branches, of a pink colour, about two inches high, resembling in the form of its orifices m. aspera of lamarck | | 94 and 30 | | east chiloe 43° tierra del fuego 53° | | coralium | | 120 | | barbary 33° n. | | peyssonel in paper read to royal society may, 1752. | | gorgonia ( or an allied form ) | | 160 | | abrolhos on the coast of brazil 18° | | capt. beechey informed me of this fact in a letter. | 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


the west indies it was noticed, that between the depth of 10 and 200 fathoms, the sounding - lead very generally came up, coated with the dead joints 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 quantities of fragments 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 platform for the growth of the stronger kinds. 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 accumulation of small organic bodies into 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 del fuego would in 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 effects of storms and earthquakes on atolls — recent changes in their state — the origin of barrier - reefs and of atolls — their relative forms — the step - formed ledges and walls round the shores of some lagoons — the ring - formed reefs of the maldiva atolls — the submerged condition of parts or of the whole of some annular reefs — the disseverment of large atolls — the union of atolls by linear reefs — the great chagos


ring - formed reefs of the maldiva atolls — the submerged condition of parts or of the whole of some annular reefs — the disseverment of large atolls — the union of atolls by linear reefs — the great chagos bank — objections from the area and amount of subsidence required by the theory, considered — the probable composition of the lower parts of atolls. the naturalists who have visited the pacific, seem to have had their attention 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, the scarcely less wonderful encircling barrier - reefs. the theory most generally received on the formation of atolls, is that they are based on submarine craters : but where 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 craters of immense size crowded 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 reef like 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 reef rising from a submarine basis, 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,


, in a reef rising from a submarine basis, would first reach the surface and consequently form a ring. but on this view it must be 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 edges of a 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 steep sides and being situated in a deep sea, a reef not to be distinguished from an atoll, might be formed : i believe some such exist in the west indies. but a difficulty of the same kind with that affecting the crater theory, renders, as we shall presently see, this view inapplicable to the greater number of atolls. no theory worthy of notice has been advanced to account for those barrier - reefs, which encircle islands of moderate * 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 to the shore. the origin of the third class 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? let us see whether an important deduction will not follow 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


follow 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 waves and winds. i do not make this latter statement vaguely ; i have carefully sought for descriptions of every island in the intertropical seas ; and my task has been in some degree abridged by a map of the pacific, corrected in 1834 by 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 ) by being 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 on the next page are grounded. i have ascertained, and chiefly from the writings of cook, kotzebue, bellinghausen, duperrey, beechey, and lutke, regarding the pacific ; and from moresby * 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 laccadive archipelagoes. these officers particularly refer to the lowness 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 line ( the plan i have always adopted ) joining the external atolls of that part of the low archipelago in which the islands are numerous, the figure will be a pointed ellipse ( reaching from hood to lazaref island ), of which the longer axis is 840 geographical miles, and the shorter 420 miles : in this space, * none of the innumerable islets, united into great rings, rise above the stated level. the gilbert group is very narrow, and 300 miles in length. in a prolonged line from this group, at the distance of


in this space, * none of the innumerable islets, united into great rings, rise above the stated level. the gilbert group is very narrow, and 300 miles in length. in a prolonged line from this group, at the distance of 240 miles, is the marshall archipelago, the figure of which is an irregular square, one end being broader than the other ; its length is 520 miles, with an average width of 240 : these two groups together are 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 archipelago 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 islands. lastly, in the indian ocean, the archipelago of the maldivas is 470 miles in length, and 60 in breadth ; that of the laccadives is 150 by 100 miles : as there is a low island between these two groups, they may be 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 beagle, whilst sailing by, is 114 feet from the summit of the trees to the water ' s edge. this island appeared to resemble the other atolls of the group. 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


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 am, then, fully 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 must originally have been present beneath each atoll at that limited depth, which is indispensable for the first growth of the reef - building polypifers. a conjecture will perhaps be hazarded, that the requisite bases might have been afforded by the accumulation of great banks of sediment, which owing to the action of superficial currents, ( aided possibly by the undulatory movement of the sea ) did not quite reach the surface, — as actually appears to have been the case in some parts of the west indian sea. but in the form and disposition of the groups of atolls, there is nothing to countenance this notion ; and the assumption without any proof, that a number of immense piles of sediment have been heaped on the floor of the great pacific and indian oceans, in their central parts far remote from land, and where the dark blue colour of the limpid water bespeaks its purity, cannot for one moment be admitted. the many widely - scattered atolls must, therefore, rest on rocky bases. but we cannot believe that the broad summit of a mountain lies buried at the depth of a few fathoms beneath every atoll, and nevertheless throughout the immense areas above - named, with not one point of rock projecting above the level of the sea ; for we may judge with some accuracy of mountains beneath the sea, by those on the land ; and where can we find a single chain several hundred miles in length and of considerable breadth, much less several such chains, with their many broad summits attaining the same height, within from 120 to 180 feet? if the data be thought insufficient, on which i have grounded my belief, respecting the depth at which the reef - building polypifers can exist, and it be assumed that they can flourish at a depth of even 100 fat


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 weight of the above argument is but little diminished, for it is almost equally improbable, that as many submarine mountains, as there are low islands in the several great and widely - separated areas above - specified, should all rise within 600 feet of the surface of the sea and not one above it, as that they should be of the same height within the smaller limit of one or two hundred feet. so highly improbable is this supposition, that we are compelled to believe, that the bases of the many atolls did never at any one period all lie submerged within the depth of a few fathoms beneath the surface, but that they were brought into the requisite position or level, some at one period and some at another, through movements in the earth ' s crust. but this could not have been effected by elevation, for the belief that points so numerous and so widely - separated were successively uplifted to a certain level, but that not one point was raised above that level, is quite as improbable as the former supposition, and indeed differs little from it. it will probably occur to those who have read ehrenberg ' s account of the reefs of the red sea, that many points in these great areas 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, will show that this 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 were not uplifted into 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 surface, as island


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 surface, as island after island slowly disappeared. thus areas of immense extent in the central and most profound parts of the great oceans, might become interspersed with coral - islets, none of which would rise to a greater height than that attained by detritus heaped up by the sea, and nevertheless they might all have been formed by corals, which absolutely required for their growth a solid foundation within a few fathoms of the surface. it would be out of place here to do more than allude to the many facts, showing that the supposition of a gradual subsidence over large areas is by no means improbable. we have the clearest proof that a movement of this kind is possible, in the upright trees buried under strata many thousand * the additional difficulty on the crater hypothesis before alluded to, will now be evident ; for on this view the volcanic action must be supposed to have formed within the areas specified a vast number of craters, all rising within a few fathoms of the surface, and not one above it. the supposition that the craters were at different times upraised above the surface, and were there abraded by the surf and subsequently coated by corals, is subject to nearly the same objections with those given 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, all lying at the 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 submarine bank : it likewise applies to the 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 lagoon, which belong to other genera,


instinctive tendencies of the polypifers. according to this latter view, the corals on the outer margin of the reef instinctively expose themselves to the surf in order to afford protection to corals living in the lagoon, which belong to other genera, and to other families! feet in thickness ; we have also every reason for believing that there are now large areas gradually sinking, in the same manner as others are rising. and when we consider how many parts of the surface of the globe have been elevated within recent geological periods, we must admit that there have been subsidences on a corresponding scale, for otherwise the whole globe would have swollen. it is very remarkable that mr. lyell, * even in the first edition of his principles of geology, inferred that the amount of subsidence in the pacific must have exceeded that of elevation, from the area of land being very small relatively to the agents there tending to form it, namely, the growth of coral and volcanic action. but it will be asked, are there any direct proofs of a subsiding movement in those areas, in which subsidence will explain a phenomenon otherwise inexplicable? this, however, can hardly be expected, for it must ever be most difficult, excepting in countries long civilized, to detect a movement, the tendency of which is to conceal the part 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 new islands : but on the theory of a gradual subsidence, all that would be apparent would be, the water sometimes encroaching slowly on the land, and the land again recovering by the 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 place at the periods when the sea rose above its usual limits, during a gale of more than ordinary strength ; and the effects of the two causes would be hardly distinguishable. in kotzebue ' s voyage there are accounts of islands, both in the caroline and marshall archipelagoes, which have been * principles of geology, sixth edition, vol. iii. p. 386. † williams ' s narrative of missionary enterprise, p. 31. partly


voyage there are accounts of islands, both in the caroline and marshall archipelagoes, which have been * principles of geology, sixth edition, vol. iii. p. 386. † williams ' s narrative of missionary enterprise, 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, also, 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 completely devastated by a hurricane, and not less than 300 lives lost : " in this instance it was evident, even to the natives, that the hurricane alone was not sufficient to account for the violent agitation of the ocean. " ‡ that considerable changes have taken place recently in some of the atolls in the low archipelago, appears certain from the case already given of matilda island : with respect to whitsunday and gloucester islands in this same group, we must either attribute great inaccuracy to their discoverer, the famous 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 described by wallis as " about four miles long, and three wide, " now it is only one mile and a half long. the appearance of gloucester island, in 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, surrounding a lagoon ten fathoms deep ; on its surface there were a few worn patches of conglomerate coral - rock, of about the size of hovels ; and these * kotzebue ' s first voyage, vol. iii. p. 168. † m. desmoulins in comptes rendus, 1840, p. 837.


, 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. moresby considered, as being, without doubt, the last remnants of islets ; so that here an atoll has been converted into an atoll - formed reef. the inhabitants of the maldiva archipelago, as long ago as 1605, declared, " that the high tides and violent currents were always diminishing the number of the islands : " * and i have already shown, on the authority of capt. moresby, that the work of destruction is still in progress ; but that on the other hand the first formation of some islets is known to the present inhabitants. in such cases, it would be exceedingly difficult to detect a gradual subsidence of the foundation, on which these mutable structures rest. some of the archipelagoes of low coral - islands are subject to earthquakes : capt. moresby informs me that they are frequent, though not very strong, in the chagos group, which occupies a very central position in the indian ocean, and is far from any land not of coral formation. one of the islands in this group was formerly covered by a bed of mould, which, after an earthquake, disappeared, and was believed by the resident 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 atoll, admiral lutke, as he had the kindness to inform me, observed several straight fissures about a foot in width, running for some hundred yards obliquely across the whole width of the reef. fissures indicate a stretching of the earth ' s crust, and, therefore, probably changes in its level ; but these coral - islands, which have been shaken and fissured, certainly have not been elevated, and, therefore, probably they have subsided. in the chapter on keeling atoll, i attempted to show by direct evidence, that *


level ; but these coral - islands, which have been shaken and fissured, certainly have not been elevated, and, therefore, probably they have subsided. in the chapter on keeling atoll, i attempted to show by direct evidence, that * 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 movement of subsidence, during the earthquakes lately felt there. the facts stand thus ; — there are many large tracts of ocean, without any high land, interspersed with reefs and islets, formed by the growth of those kinds of corals, which cannot live at great depths ; and the existence of these reefs and low islets, in such numbers and at such distant points, is quite inexplicable, excepting on the theory, that the bases on which the reefs first became attached, slowly and successively sank beneath the level of the sea, whilst the corals continued to grow upwards. no positive facts are opposed to this view, and some general considerations render it probable. there is evidence of change in form, whether or not from subsidence, on some of these coral - islands ; and there is evidence of subterranean disturbances beneath them. will then the theory, to which we have thus been led, solve the curious problem, — what has given to each class of reef its peculiar form? let us in imagination place within one of the subsiding areas, an island surrounded by a " fringing reef, " — that kind, which alone offers no difficulty in 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 encircled 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


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, as the island sinks down, 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 becoming 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 lagoon - channel. this 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 the delicately 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 many feet as there have 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