which forms great irregularly rounded masses ( like those of an astræa, but larger ) from four to eight feet broad, and little less in thickness. these mounds are separated from each other by narrow crooked channels, about six feet deep, most of which intersect the line of reef at right angles. on the furthest mound, which i was able to reach by the aid of a leaping - pole, and over which the sea broke with some violence, although the day was quite calm and the tide low, the polypifers in the uppermost cells were all dead, but between three and four inches lower down on its side they were living, and formed a projecting border round the upper and dead surface. the coral being thus checked in its upward growth, extends laterally, and hence most of the masses, especially those a little further inwards, had broad flat dead summits. on the other hand i could see, during the recoil of the breakers, that a few yards further seaward, the whole convex surface of the porites was alive : so that the point where we were standing was almost on the exact upward and shoreward limit of existence of those corals which form the outer margin of the reef. we shall presently see that there are other organic productions, fitted to bear a somewhat longer exposure to the air and sun. next, but much inferior in importance to the porites, is the millepora complanata. * it grows in thick vertical plates, intersecting each other at various angles, and forms an exceedingly strong honeycombed mass, which generally affects a circular form, the marginal plates alone being alive. between these plates and in the protected crevices on the reef, a multitude of branching zoophytes and other productions flourish, but the porites and millepora alone seem able to resist the fury of the breakers on its upper and outer edge : at the depth of a few fathoms other kinds of stony corals live. mr. liesk, who was intimately acquainted with every part of this reef, and likewise with that of north keeling atoll, assured me that these corals invariably compose the outer margin. the lagoon is inhabited by quite a distinct set of corals, generally brittle and thinly branched ; but a porites, apparently of the same species with that on the outside, is found there, although it does not seem to thrive, and certainly does not attain the thousandth part in bulk of the masses opposed to the breakers. the woodcut shews the


living specimen of this madrepore, i conclude that it is confined to a zone outside, and beneath the surface, where it must be very abundant. fragments of the millepora alcicornis and of an astræa were also numerous ; the former is found, but not in proportionate numbers, in the hollows on the reef ; but the astræa i did not see living. hence we may infer, that these are the kinds of coral which form the rugged sloping surface, ( represented in the wood cut by an uneven line ) round and beneath the external margin. between 12 and 20 fathoms the arming came up an equal number of times smoothed with sand, and indented with coral : an anchor and lead were lost at the respective depths of 13 and 16 fathoms. out of twenty - five soundings, taken at a greater depth than 20 fathoms, every one shewed that the bottom was covered with sand ; whereas at a less depth than 12 fathoms, every sounding shewed that it was exceedingly rugged, and free from all extraneous particles. two soundings were obtained at the depth of 360 fathoms, and several between 200 and 300 fathoms. the sand brought up from these depths consisted of finely triturated fragments of stony zoophytes, but not, as far as i could distinguish, of a particle of any lamelliform genus : fragments of shells were rare. at a distance of 2200 yards from the breakers, captain fitzroy found no bottom with a line 7200 feet in length ; hence the submarine slope of this coral formation is steeper than that of any volcanic cone. off the mouth of the lagoon, and likewise off the northern point of the atoll, where the currents act violently, the inclination, owing to the accumulation of sediment, is less. as the arming of the lead from all the greater depths showed a smooth sandy bottom, i at first concluded that the whole consisted of a vast conical pile of calcareous sand, but the sudden increase of depth at some points, and the circumstance of the line having been cut, as if rubbed, when between 500 and 600 fathoms were out, indicate the probable existence of submarine cliffs. on the margin of the reef, close within the line where the upper surface of the porites and of the millepora is dead, three species of nullipora flourish. one grows in thin sheets, like a lichen on old trees ; the second in stony


the reef, close within the line where the upper surface of the porites and of the millepora is dead, three species of nullipora flourish. one grows in thin sheets, like a lichen on old trees ; the second in stony knobs, as thick as a man ' s finger, radiating from a common centre ; and the third, which is less common, in a moss - like reticulation of thin, but perfectly rigid branches. * the three species occur either separately or mingled together ; and they form by their successive growth a layer two or three feet in thickness, which in some cases is hard, but where formed of the lichen - like kind, readily yields an impression to the hammer : the surface is of a reddish colour. these nulliporæ, although able to exist above the limit of true corals, seem to require to be bathed during the greater part of each tide by breaking water, for they are not found in any abundance in the protected hollows on the back part of the reef, where they might be immersed either during the whole or an equal proportional time of each tide. it is remarkable that organic productions of such extreme simplicity, for the nulliporæ undoubtedly belong to one of the lowest classes of the vegetable kingdom, should be limited to a zone so peculiarly circumstanced. hence the layer composed by their growth, merely fringes the reef for a * this last species is of a beautiful bright peach blossom colour. its branches are about as thick as crow - quills ; they are slightly flattened and knobbed at the extremities. the extremities only are alive and brightly coloured. the two other species are of a dirty purplish white. the second species is extremely hard ; its short knob - like branches are cylindrical, and do not grow thicker at their extremities. space of about 20 yards in width, either under the form of separate mammillated projections, where the outer masses of coral are separate, or more commonly, where the corals are united into a solid margin, as a continuous smooth convex mound, ( b in wood - cut, ) like an artificial breakwater. both the mound and mammillated projections stand about three feet higher than any other part of the reef, by which term i do not include the islets, formed by the accumulation of rolled fragments. we shall hereafter see that other coral reefs are protected by a similar thick growth of nulliporæ on the outer margin, the part


reef, by which term i do not include the islets, formed by the accumulation of rolled fragments. we shall hereafter see that other coral reefs are protected by a similar thick growth of nulliporæ on the outer margin, the part most exposed to the breakers, and this must effectually aid in preserving it from being worn down. the wood - cut represents a section across one of the islets on the reef, but if all that part which is above the level of c were removed, the section would be that of the simple reef, as it occurs where no islet has been formed. it is this reef which essentially forms the atoll. it is a ring, enclosing the lagoon on all sides except at the northern end, where there are two open spaces, through one of which ships can enter. the reef varies in width from 250 to 500 yards ; its surface is level, or very slightly inclined towards the lagoon, and at high - tide the sea breaks entirely over it : the water at low tide thrown by the breakers on the reef, is carried by the many narrow and shoal gullies or channels on its surface, into the lagoon : a return stream sets out of the lagoon through the main entrance. the most frequent coral in the hollows on the reef is pocillopora verrucosa, which grows in short sinuous plates, or branches, and when alive is of a beautiful pale lake - red : a madrepora, closely allied or identical with m. pocillifera is also common. as soon as an islet is formed, and the waves are prevented breaking entirely over the reef, the channels and hollows in it become filled up with cemented fragments, and its surface is converted into a hard smooth floor, ( c of wood - cut ), like an artificial one of freestone. this flat surface varies in width from 100 to 200, or even 300 yards, and is strewed with a few large fragments of coral torn up during gales : it is uncovered only at low water. i could with difficulty, and only by the aid of a chisel, procure chips of rock from its surface, and therefore could not ascertain how much of it is formed by the aggregation of detritus, and how much by the outward growth of mounds of corals, similar to those now living on the margin. nothing can be more singular than the appearance at low - tide of this ' flat ' of naked stone, especially where it


detritus, and how much by the outward growth of mounds of corals, similar to those now living on the margin. nothing can be more singular than the appearance at low - tide of this ' flat ' of naked stone, especially where it is externally bounded by the smooth convex mound of nulliporæ, appearing like a breakwater built to resist the waves, which are constantly throwing over it sheets of foaming water. the characteristic appearance of this ' flat ' is shown in the foregoing wood - cut of whitsunday atoll. the islets on the reef are first formed between 200 and 300 yards from its outer edge, through the accumulation of a pile of fragments, thrown together by some unusually strong gale. their ordinary width is under a quarter of a mile, and their length varies from a few yards to several miles. those on the s. e. and windward side of the atoll, increase solely by the addition of fragments on their outer side ; hence the loose blocks of coral, of which their surface is composed, as well as the shells mingled with them, almost exclusively consist of those kinds which live on the outer coast. the highest part of the islets ( excepting hillocks of blown sand, some of which are 30 feet high ), is close to the outer beach ( e of the wood - cut ) and averages from six to ten feet above ordinary high - water mark. from the outer beach the surface slopes gently to the shores of the lagoon, which no doubt has been caused by the breakers, the further they have rolled over the reef, having had less power to throw up fragments. the little waves of the lagoon heap up sand and fragments of thinly - branched corals on the inner side of the islets on the leeward side of the atoll ; and these islets are broader than those to windward, some being even 800 yards in width ; but the land thus added is very low. the fragments be - neath the surface are cemented into a solid mass, which is exposed as a ledge ( d of the wood - cut ), projecting some yards in front of the outer shore, and from two to four feet high. this ledge is just reached by the waves at ordinary high - water : it extends in front of all the islets, and everywhere has a water - worn and scooped appearance. the fragments of coral which are occasionally cast on the ' flat ' are during gales of unusual violence swept together on the beach, where the waves each day at high - water


the islets, and everywhere has a water - worn and scooped appearance. the fragments of coral which are occasionally cast on the ' flat ' are during gales of unusual violence swept together on the beach, where the waves each day at high - water tend to remove and gradually wear them down ; but the lower fragments having become firmly cemented together by the percolation of calcareous matter, resist the daily tides longer, and hence project as a ledge. the cemented mass is generally of a white colour, but in some few parts reddish from ferruginous matter : it is very hard, and is sonorous under the hammer : it is obscurely divided by seams, dipping at a small angle seaward : it consists of fragments of the corals which grow on the outer margin, some quite and others partially rounded, some small and others between two and three feet across ; and of masses of previously - formed conglomerate, torn up, rounded, and re - cemented : or it consists of a calcareous sandstone, entirely composed of rounded particles, generally almost blended together, of shells, corals, the spines of echini, and other such organic bodies ; — rocks, of this latter kind, occur on many shores, where there are no coral - reefs. the structure of the coral in the conglomerate has generally been much obscured by the infiltration of spathose calcareous matter ; and i collected a very interesting series, beginning with fragments of unaltered coral, and ending with others, where it was impossible to discover with the naked eye any trace of organic structure. in some specimens i was unable, even with the aid of a lens, and by wetting them, to distinguish the boundaries of the altered coral and spathose limestone. many even of the blocks of coral lying loose on the beach, had their central parts altered and infiltrated. the lagoon alone remains to be described ; it is much shallower than that of most atolls of considerable size. the southern part is almost filled up with banks of mud and fields of coral, both dead and alive ; but there are considerable spaces, between three and four fathoms, and smaller basins from eight to ten fathoms deep. probably about half its area consists of sediment, and half of coral - reefs. the corals composing these reefs have a very different aspect from those on the outside : they are very numerous in kind, and most of them are thinly branched. meandrina, however, lives in the lagoon,


half of coral - reefs. the corals composing these reefs have a very different aspect from those on the outside : they are very numerous in kind, and most of them are thinly branched. meandrina, however, lives in the lagoon, and great rounded masses of this coral are numerous, lying quite or almost loose on the bottom. the other commonest kinds consist of three closely allied species of true madrepora in thin branches ; of seriatapora subulata ; two species of porites * with cylindrical branches, one of which forms circular clumps, with the exterior branches only alive ; and lastly, a coral something like an explanaria, but with stars on both surfaces, growing in thin, brittle, stony, foliaceous expansions, especially in the deeper basins of the lagoon. the reefs on which these corals grow are very irregular in form, are full of cavities, and have not a solid flat surface of dead rock, like that surrounding the lagoon ; nor can they be nearly so hard, for the inhabitants made with crowbars a channel of considerable length through these reefs, in which a schooner, built on the s. e. islet was floated out. it is a very interesting circumstance, pointed out to us by mr. liesk, that this channel, although made less than ten years before our visit, was then, as we saw, almost choked up with living coral, so that fresh excavations would be absolutely necessary to allow another vessel to pass through it. the sediment from the deepest parts in the lagoon, when wet, appeared chalky, but when dry, like very fine * this porites has somewhat the habit of p. clavaria, but the branches are not knobbed at their ends. when alive it is of a yellow colour, but after having been washed in fresh water and placed to dry, a jet - black slimy substance exuded from the entire surface, so that the specimen now appears as if it had been dipped in ink. sand. large soft banks of similar, but even finer grained mud, occur on the s. e. shore of the lagoon, affording a thick growth of a fucus, on which turtle feed : this mud, although discoloured by vegetable matter, appears from its entire solution in acids to be purely calcareous. i have seen in the museum of the geological society, a similar but more remarkable substance, brought by lieut. nelson from the reefs of bermuda, which, when


vegetable matter, appears from its entire solution in acids to be purely calcareous. i have seen in the museum of the geological society, a similar but more remarkable substance, brought by lieut. nelson from the reefs of bermuda, which, when shewn to several experienced geologists, was mistaken by them for true chalk. on the outside of the reef much sediment must be formed by the action of the surf on the rolled fragments of coral ; but, in the calm waters of the lagoon, this can take place only in a small degree. there are, however, other and unexpected agents at work here : large shoals of two species of scarus, one inhabiting the surf outside the reef and the other the lagoon, subsist entirely, as i was assured by mr. liesk the intelligent resident before referred to, by browsing on the living polypifers. i opened several of these fish, which are very numerous and of considerable size, and i found their intestines distended by small pieces of coral, and finely ground calcareous matter. this must daily pass from them as the finest sediment ; much also must be produced by the infinitely numerous vermiform and molluscous animals, which make cavities in almost every block of coral. dr. j. allan of forres, who has enjoyed the best means of observation, informs me in a letter, that the holuthuriæ ( a family of radiata ), subsist on living coral ; and the singular structure of bone within the anterior extremity of their bodies, certainly appears well adapted for this purpose. the number of the species of holuthuria, and of the individuals which swarm on every part of these coral - reefs, is extraordinarily great ; and many ship - loads are annually freighted, as is well known, for china with the trepang, which is a species of this genus. the amount of coral yearly consumed, and ground down into the finest mud, by these several creatures, and probably by many other kinds, must be immense. these facts are, however, of more im - portance in another point of view, as showing us that there are living checks to the growth of coral - reefs, and that the almost universal law of ' consume and be consumed, ' holds good even with the polypifers forming those massive bulwarks, which are able to withstand the force of the open ocean. considering that keeling atoll, like other coral formations,


almost universal law of ' consume and be consumed, ' holds good even with the polypifers forming those massive bulwarks, which are able to withstand the force of the open ocean. considering that keeling atoll, like other coral formations, has been entirely formed by the growth of organic beings, and the accumulation of their detritus, one is naturally led to enquire, how long it has continued, and how long it is likely to continue, in its present state. mr. liesk informed me that he had seen an old chart in which the present long island on the s. e. side was divided by several channels into as many islets ; and he assures me that the channels can still be distinguished by the smaller size of the trees on them. on several islets, also, i observed that only young cocoa - nut trees were growing on the extremities ; and that older and taller trees rose in regular succession behind them ; which shows that these islets have very lately increased in length. in the upper and south - eastern part of the lagoon, i was much surprised by finding an irregular field of at least a mile square of branching corals, still upright, but entirely dead. they consisted of the species already mentioned ; they were of a brown colour, and so rotten, that in trying to stand on them, i sank half way up the leg, as it through decayed brushwood. the tops of the branches were barely covered by water at the time of lowest tide. several facts having led me to disbelieve in any elevation of the whole atoll, i was at first unable to imagine what cause could have killed so large a field of coral. upon reflection, however, it appeared to me that the closing up of the above mentioned channels would be a sufficient cause ; for before this, a strong breeze by forcing water through them into the head of the lagoon, would tend to raise its level. but now this cannot happen, and the inhabitants observe that the tide rises to a less height, during a high s. e. wind, at the head than at the mouth of the lagoon. the corals, which, under the former condition of things, had attained the utmost possible limit of upward growth, would thus occasionally be exposed for a short time to the sun, and be killed. besides the increase of dry land, indicated by the foregoing facts, the exterior solid reef appears to have grown outwards. on the western side of the atoll, the ' flat ' lying


for a short time to the sun, and be killed. besides the increase of dry land, indicated by the foregoing facts, the exterior solid reef appears to have grown outwards. on the western side of the atoll, the ' flat ' lying between the margin of the reef and the beach, is very wide : and in front of the regular beach with its conglomerate basis, there is, in most parts, a bed of sand and loose fragments with trees growing out of it, which apparently is not reached even by the spray at high water. it is evident some change has taken place since the waves formed the inner beach : that they formerly beat against it with violence was evident, from a remarkably thick and water - worn point of conglomerate at one spot, now protected by vegetation and a bank of sand ; that they beat against it in the same peculiar manner in which the swell from windward now obliquely curls round the margin of the reef, was evident from the conglomerate having been worn into a point projecting from the beach in a similarly oblique manner. this retreat in the line of action of the breakers might result, either from the surface of the reef in front of the islets having been submerged at one time, and afterward having grown upwards, or from the mounds of coral on the margin having continued to grow outwards. that an outward growth of this part is in process, can hardly be doubted from the fact already mentioned of the mounds of porites with their summits apparently lately killed, and their sides only three or four inches lower down thickened by a fresh layer of living coral. but there is a difficulty on this supposition which i must not pass over. if the whole, or a large part of the ' flat, ' had been formed by the outward growth of the margin, each successive margin would naturally have been coated by the nulliporæ, and so much of the surface would have been of equal height with the existing zone of living nulliporæ : this is not the case, as may be seen in the wood - cut. it is, however, evident from the abraded state of the ' flat, ' with its original inequalities filled up, that its surface has been much modified ; and it is possible that the hinder portions of the zone of nulliporæ, perishing as the reef grows outwards, might be worn down by the surf. if this has not taken place, the reef can in no part have increased outwards in breadth since its formation, or at


the zone of nulliporæ, perishing as the reef grows outwards, might be worn down by the surf. if this has not taken place, the reef can in no part have increased outwards in breadth since its formation, or at least since the nulliporæ formed the convex mound on its margin : for the zone thus formed, and which stands between two and three feet above the other parts of the reef, is nowhere much above twenty yards in width. thus far we have considered facts, which indicate, with more or less probability, the increase of the atoll in its different parts : there are others having an opposite tendency. on the s. e. side, lieut. sulivan, to whose kindness i am indebted for many interesting observations, found the conglomerate projecting on the reef nearly fifty yards in front of the beach : we may infer from what we see in all others parts of the atoll, that the conglomerate was not originally so much exposed but formed the base of an islet, the front and upper part of which has since been swept away. the degree to which the conglomerate, round nearly the whole atoll, has been scooped, broken up and the fragments cast on the beach, is certainly very surprising, even on the view that it is the office of occasional gales to pile up fragments, and of the daily tides to wear them away. on the western side, also, of the atoll, where i have described a bed of sand and fragments with trees growing out of it, in front of an old beach, it struck both lieut. sulivan and myself, from the manner in which the trees were being washed down, that the surf had lately recommenced an attack on this line of coast. appearances indicating a slight encroachment of the water on the land, are plainer within the lagoon : i noticed in several places, both on its windward and leeward shores, old cocoa - nut trees falling with their roots undermined, and the rotten stumps of others on the beach, where the inhabitants assured us the cocoa - nut could not now grow. capt. fitzroy pointed out to me, near the settlement, the foundation posts of a shed, now washed by every tide, but which the inhabitants stated, had seven years before stood above high water - mark. in the calm waters of the lagoon, directly connected with a great, and therefore stable ocean, it seems very improbable that a change in the currents, sufficiently great to cause the


, had seven years before stood above high water - mark. in the calm waters of the lagoon, directly connected with a great, and therefore stable ocean, it seems very improbable that a change in the currents, sufficiently great to cause the water to eat into the land on all sides, should have taken place within a limited period. from these considerations i inferred, that probably the atoll had lately subsided to a small amount ; and this inference was strengthened by the circumstance, that in 1834, two years before our visit, the island had been shaken by a severe earthquake, and by two slighter ones during the ten previous years. if, during these subterranean disturbances, the atoll did subside, the downward movement must have been very small, as we must conclude from the fields of dead coral still lipping the surface of the lagoon, and from the breakers on the western shore not having yet regained the line of their former action. the subsidence must, also, have been preceded by a long period of rest, during which the islets extended to their present size, and the living margin of the reef grew either upwards, or as i believe outwards, to its present distance from the beach. whether this view be correct or not, the above facts are worthy of attention, as showing how severe a struggle is in progress on these low coral - formations between the two nicely balanced powers of land and water. with respect to the future state of keeling atoll, if left undisturbed, we can see that the islets may still extend in length ; but as they cannot resist the surf until broken by rolling over a wide space, their increase in breadth must depend on the increasing breadth of the reef ; and this must be limited by the steepness of the submarine flanks, which can be added to only by sediment derived from the wear and tear of the coral. from the rapid growth of the coral in the channel cut for the schooner, and from the several agents at work in producing fine sediment, it might be thought that the lagoon would necessarily become quickly filled up. some of this sediment, however, is transported into the open sea, as appears from the soundings off the mouth of the lagoon, instead of being deposited within it. the deposition, moreover, of sediment, checks the growth of coral reefs, so that these two agencies cannot act together with full effect in filling it up. we know so little of the habits of the many different species of corals, which form the lagoon - reefs, that we have no


checks the growth of coral reefs, so that these two agencies cannot act together with full effect in filling it up. we know so little of the habits of the many different species of corals, which form the lagoon - reefs, that we have no more reason for supposing that their whole surface would grow up as quickly as the coral did in the schooner - channel, than for supposing that the whole surface of a peat - moss would increase as quickly as parts are known to do in holes, where the peat has been cut away. these agencies, nevertheless, tend to fill up the lagoon ; but in proportion as it becomes shallower, so must the polypifers be subject to many injurious agencies, such as impure water and loss of food. for instance, mr. liesk informed me, that some years before our visit unusually heavy rain killed nearly all the fish in the lagoon, and probably the same cause would likewise injure the corals. the reefs also, it must be remembered, cannot possibly rise above the level of the lowest spring - tide, so that the final conversion of the lagoon into land must be due to the accumulation of sediment ; and in the midst of the clear water of the ocean, and with no surrounding high land, this process must be exceedingly slow. general form and size of atolls, their reefs and islets — external slope — zone of nulliporæ — conglomerate — depth of lagoons — sediment — reefs submerged wholly or in part — breaches in the reef — ledge - formed shores round certain lagoons — conversion of lagoons into land. i will here give a sketch of the general form and structure of many atolls and atoll - formed reefs which occur in the pacific and indian oceans, comparing them with keeling atoll. the maldiva atolls and the great chagos bank differ in so many respects, that i shall devote to them, besides occasional references, a third section of this chapter. keeling atoll may be considered as of moderate dimensions and of regular form. of the thirty - two islands surveyed by capt. beechey in the low archipelago, the longest was found to be thirty miles, and the shortest less than a mile ; but vliegen atoll, situated in another part of the same group, appears to be sixty miles long and twenty broad. most of the atolls in this group are of an elongated form ; thus bow island is thirty miles in length, and on an average only six in width ( see fig. 4,


same group, appears to be sixty miles long and twenty broad. most of the atolls in this group are of an elongated form ; thus bow island is thirty miles in length, and on an average only six in width ( see fig. 4, plate i. ), and clermont tonnere has nearly the same proportions. in the marshall archipelago ( the ralick and radack group of kotzebue ) several of the atolls are more than thirty miles in length, and rimsky korsacoff is fifty - four long, and twenty wide at the broadest part of its irregular outline. most of the atolls in the maldiva archipelago are of great size, one of them ( which, however, bears a double name ) measured in a medial and slightly curved line, is no less than eighty - eight geographical miles long, its greatest width being under twenty, and its least only nine - and - a - half miles. some atolls have spurs projecting from them ; and in the marshall group there are atolls united together by linear reefs, for instance menchioff island, ( see fig. 3, plate ii. ) which is sixty miles in length, and consists of three loops tied together. in far the greater number of cases an atoll consists of a simple elongated ring, with its outline moderately regular. the average width of the annular reef may be taken at about a quarter of a mile. capt. beechey * says that in the atolls of the low archipelago it exceeded in no instance half a mile. the description given of the structure and proportional dimensions of the reef and islets of keeling atoll, appears to apply perfectly to nearly all the atolls in the pacific and indian oceans. the islets are first formed some way back either on the projecting points of the reef, especially if its form be angular, or on the sides of the main entrances into the lagoon — that is in both cases, on points where the breakers can act during gales of wind in somewhat different * beechey ' s voyage to the pacific and beering ' s straits, chap. viii. directions, so that the matter thrown up from one side may accumulate against that before thrown up from another. in lutke ' s chart of the caroline atolls, we see many instances of the former case ; and the occurrence of islets, as if placed for beacons, on the points where there is a gateway or breach through the reef, has been noticed by several authors. there are some atoll


excellent published work. me that all the soundings, even the deepest, were on coral, but he does not know whether dead or alive. the slope round christmas atoll, ( lat. 1° 4 ' n., 157° 45 ' w. ) described by cook, * is considerably less ; at about half a mile from the edge of the reef, the average depth was about fourteen fathoms on a fine sandy bottom, and at a mile, only between twenty and forty fathoms. it has no doubt been owing to this gentle slope, that the strip of land surrounding its lagoon, has increased in one part to the extraordinary width of three miles : it is formed of successive ridges of broken shells and corals, like those on the beach. i know of no other instance of such width in the reef of an atoll ; but mr. f. d. bennett informs me that the inclination of the bottom round caroline atoll in the pacific, is like that off christmas island, very gentle. off the maldiva and chagos atolls, the inclination is much more abrupt ; thus at heawandoo pholo, lieut. powell † found 50 and 60 fathoms close to the edge of the reef, and at 300 yards distance there was no bottom with a 300 yard line. capt. moresby informs me, that at 100 fathoms from the mouth of the lagoon of diego garcia, he found no bottom with 150 fathoms : this is the more remarkable, as the slope is generally less abrupt in front of channels through a reef, owing to the accumulation of sediment. at egmont island, also, at 150 fathoms from the reef, soundings were struck with 150 fathoms. lastly, at cardoo atoll, only sixty yards from the reef, no bottom was obtained, as i am informed by capt. moresby, with a line of two hundred fathoms! the currents run with great force round these atolls, and where they are strongest, the inclination appears to be most abrupt. i am informed by the same authority, that wherever soundings were obtained off these islands, the bottom was invariably sandy : nor was there any reason to suspect the existence * cook ' s third voyage, vol. ii. chap. 10. † this fact is taken from a ms. account of these groups lent me by capt. moresby. see also capt. moresby ' s paper on the maldiva atolls in the geographical journal, vol. v


chap. 10. † this fact is taken from a ms. account of these groups lent me by capt. moresby. see also capt. moresby ' s paper on the maldiva atolls in the geographical journal, vol. v. p. 401. of submarine cliffs, as there was at keeling island. * here then occurs a difficulty ; — can sand accumulate on a slope, which, in some cases, appears to exceed fifty - five degrees? it must be observed, that i speak of slopes where soundings were obtained, and not of such cases, as that of cardoo, where the nature of the bottom is unknown, and where its inclination must be nearly vertical. m. elie de beaumont † has argued, and there is no higher authority on this subject, from the inclination at which snow slides down in avalanches, that a bed of sand or mud cannot be formed at a greater angle than thirty degrees. considering the number of soundings on sand, obtained round the maldiva and chagos atolls, which appear to indicate a greater angle, and the extreme abruptness of the sand - banks in the west indies as will be mentioned in the appendix, i must conclude that the adhesive property of wet sand counteracts its gravity, in a much greater ratio than has been allowed for by m. elie de beaumont. from the facility with which calcareous sand becomes agglutinated, it is not necessary to suppose that the bed of loose sand is thick. capt. beechey has observed, that the submarine slope is much less at the extremities of the more elongated atolls in the low archipelago, than at their sides ; in speaking of ducie ' s island he says ‡ the buttress, as it may be called, which " has the most powerful enemy ( the s. w. swell ) to oppose, is carried out much further, and with less abruptness, than the other. " in some cases, the less inclination of a certain part of the external slope, for instance of the * off some of the islands in the low archipelago the bottom appears to descend by ledges. off elizabeth island, which, however, consists of raised coral, capt. beechey ( p. 45, quarto ed. ) describes three ledges : the first had an easy slope from the beach to a distance of about fifty yards ; the second extended two hundred yards with twenty - five fathoms on it, and then ended abruptly, like the


by a nullipora, which covers the stone wherever the waves beat ; and, under favourable circumstances, assumes a stalactitical form, " — a description perfectly applicable to the margin of keeling atoll. * although chamisso * kotzebue ' s first voyage, vol. iii. p. 142. near porto praya, in the cape de verd islands, some basaltic rocks, lashed by no inconsiderable surf, were completely enveloped with a layer of nullipora. the entire surface over many square inches, was coloured of a peach - blossom red ; the layer, however, was of no greater thickness than paper. another does not state that the masses of nulliporæ form points or a mound, higher than the flat, yet i believe that this is the case ; for kotzebue, * in another part, speaks of the rocks on the edge of the reef " as visible for about two feet at low water, " and these rocks we may feel quite certain are not formed of true coral. † whether a smooth convex mound of nulliporæ, like that which appears as if artificially constructed to protect the margin of keeling island, is of frequent occurrence round atolls, i know not ; but we shall presently meet with it, under precisely the same form, on the outer edge of the ' barrier reefs ' which encircle the society islands. there appears to be scarcely a feature in the structure of keeling reef, which is not of common, if not of universal occurrence, in other atolls. thus chamisso describes ‡ a layer of coarse conglomerate, outside the islets round the marshall atolls, which " appears on its upper surface uneven and eaten away. " from drawings, with appended remarks, of diego garcia in the chagos group and of several of the maldiva atolls, shown me by captain moresby §, it is evident that their outer coasts are subject to the same round of decay and renovation as those of keeling atoll. from the description of the atolls in the low archipelago, given in capt. kind, in the form of projecting knobs, grew in the same situation. these nulliporæ are closely related to those described on the coral - reefs, but i believe are of different species. * kotzebue ' s first voyage, vol. ii. p. 16. lieut. nelson in his excellent memoir in the geological transactions, ( vol. ii p. 105. ) all


, but i believe are of different species. * kotzebue ' s first voyage, vol. ii. p. 16. lieut. nelson in his excellent memoir in the geological transactions, ( vol. ii p. 105. ) alludes to the rocky points mentioned by kotzebue, and infers that they consist of serpulæ, which compose incrusting masses on the reefs of bermudas, as they likewise do on a sandstone bar off the coast of brazil, ( which i have described in london phil. journ. oct. 1841 ). these masses of serpulæ hold the same position, relatively to the action of the sea, with the nulliporæ on the coral - reefs in the indian and pacific oceans. † capt. moresby, in his valuable paper " on the northern atolls of maldivas, " ( geographical journal, vol. v. ), says that the edges of the reefs there stand above water at low spring tides. ‡ kotzebue ' s first voyage, vol. iii p. 144. § see also moresby on the northern atolls of the maldivas, geographical journal, vol. v. p. 400. beechey ' s voyage, it is not apparent that any conglomerate coral - rock was there observed. the lagoon in keeling atoll is shallow : in the atolls of the low archipelago the depth varies from 20 to 38 fathoms, and in the marshal group, according to chamisso, from 30 to 35 : in the caroline atolls it is only a little less. within the maldiva atolls there are large spaces with 45 fathoms, and some soundings are laid down of 49 fathoms. the greater part of the bottom in most lagoons, is formed of sediment ; large spaces have exactly the same depth, or the depth varies so insensibly, that it is evident that no other means, excepting aqueous deposition, could have levelled the surface so equally. in the maldiva atolls this is very conspicuous, and likewise in some of the caroline and marshall islands. in the former large spaces consist of sand and soft clay ; and kotzebue speaks of clay having been found within one of the marshall atolls. no doubt this clay is calcareous mud, similar to that at keeling island, and to that at bermuda already referred to, as undistinguishable from disintegrated chalk, and which lieu


the marshall atolls. no doubt this clay is calcareous mud, similar to that at keeling island, and to that at bermuda already referred to, as undistinguishable from disintegrated chalk, and which lieut. nelson says is called there pipe - clay. * where the waves act with unequal force on the two sides of an atoll, the islets appear to be first formed, and are generally of greater continuity on the more exposed shore. the islets, also, which are placed to leeward, are in most parts of the pacific liable to be occasionally swept entirely away by gales, equalling hurricanes in violence, which blow in an opposite direction to the ordinary trade - wind. the absence of the * i may here observe that on the coast of brazil, where there is much coral, the soundings near the land are described by admiral roussin, in the pilote du bresil, as siliceous sand, mingled with much finely comminuted particles of shells and coral. further in the offing, for a space of 1300 miles along the coast, from the abrolhos islands to maranham, the bottom in many places is composed of " tuf blanc, mele ou forme de madrepores broyes. " this white substance, probably is analogous to that which occurs within the above - mentioned lagoons ; it is sometimes, according to roussin, firm, and he compares it to mortar. islets on the leeward side of atolls, or when present their lesser dimensions compared with those to windward, is a comparatively unimportant fact ; but in several instances the reef itself on the leeward side, retaining its usual defined outline, does not rise to the surface by several fathoms. this is the case with the southern side of peros banhos ( plate i. fig. 9 ) in the chagos group, with mourileu atoll *, in the caroline archipelago, and with the barrier reef ( plate i., fig. 8 ) of the gambier islands. i allude to the latter reef, although belonging to another class, because captain beechey was first led by it to observe the peculiarity in the question. at peros banhos the submerged part is nine miles in length, and lies at an average depth of about 5 fathoms ; its surface is nearly level, and consists of hard stone, with a thin covering of loose sand. there is scarcely any


peros banhos the submerged part is nine miles in length, and lies at an average depth of about 5 fathoms ; its surface is nearly level, and consists of hard stone, with a thin covering of loose sand. there is scarcely any living coral on it, even on the outer margin, as i have been particularly assured by capt. moresby : it is in fact a wall of dead coral - rock, having the same width and transverse section with the reef in its ordinary state, of which it is a continuous portion. the living and perfect parts terminate abruptly, and abut on the submerged portions, in the same manner as on the sides of an ordinary passage through the reef. the reef to leeward in other cases is nearly or quite obliterated, and one side of the lagoon is left open ; for instance at oulleay ( caroline archipelago ), where a crescent - formed reef is fronted by an irregular bank, on which the other half of the annular reef probably once stood. at namonouito, in the same archipelago, both these modifications of the reef concur ; it consists of a great flat bank, with from 20 to 25 fathoms water on it ; for a length of more than 40 miles on its southern side it is open and without any reef, whilst on the other sides it is bounded by a reef, in parts rising to the surface and perfectly characterized, in parts lying some fathoms submerged. in the chagos group there are annular reefs, entirely submerged, * frederic lutke ' s voyage autour du monde, vol. ii. p. 291. see also his account of namonouito, at pp. 97 and 105, and the chart of oulleay in the atlas. which have the same structure as the submerged and defined portions just described. the speaker ' s bank offers an excellent example of this structure ; its central expanse, which is about 22 fathoms deep, is 24 miles across ; the external rim is of the usual width of annular reefs, and is well - defined ; it lies between 6 and 8 fathoms beneath the surface, and at the same depth there are scattered knolls in the lagoon. capt. moresby believes the rim consists of dead rock, thinly covered with sand, and he is certain this is the case with the external rim of the great chagos bank, which is also essentially a submerged atoll. in both these cases, as in the submerged portion of the reef at peros


, thinly covered with sand, and he is certain this is the case with the external rim of the great chagos bank, which is also essentially a submerged atoll. in both these cases, as in the submerged portion of the reef at peros banhos, capt. moresby feels sure that the quantity of living coral, even on the outer edge overhanging the deep - sea water, is quite insignificant. lastly, in several parts of the pacific and indian oceans there are banks, lying at greater depths than in the cases just mentioned, of the same form and size with the neighbouring atolls, but with their atoll - like structure wholly obliterated. it appears from the survey of freycinet, that there are banks of this kind in the caroline archipelago, and, as is reported, in the low archipelago. when we discuss the origin of the different classes of coral formations, we shall see that the submerged state of the whole of some atoll - formed reefs, and of portions of others, generally but not invariably on the leeward side, and the existence of more deeply submerged banks now possessing little or no signs of their original atoll - like structure, are probably the effects of a uniform cause, — namely, the death of the coral, during the subsidence of the area, in which the atolls or banks are situated. there is seldom, with the exception of the maldiva atolls, more than two or three channels, and generally only one leading into the lagoon, of sufficient depth for a ship to enter. in small atolls, there is usually not even one. where there is deep water, for instance above twenty fathoms, in the middle of the lagoon, the channels through the reef are seldom as deep as the centre, — it may be said that the rim only of the saucer - shaped hollow forming the lagoon is notched. mr. lyell * has observed that the growth of the coral would tend to obstruct all the channels through a reef, except those kept open by discharging the water, which during high tide and the greater part of each ebb is thrown over its circumference. several facts indicate that a considerable quantity of sediment is likewise discharged through these channels ; and captain moresby informs me that he has observed, during the change of the monsoon, the sea discoloured to a distance off the entrances into the maldiva and chagos atolls. this, probably, would check the growth of the coral in them, far


me that he has observed, during the change of the monsoon, the sea discoloured to a distance off the entrances into the maldiva and chagos atolls. this, probably, would check the growth of the coral in them, far more effectually than a mere current of water. in the many small atolls without any channel, these causes have not prevented the entire ring attaining the surface. the channels, like the submerged and effaced parts of the reef, very generally though not invariably occur on the leeward side of the atoll, or on that side, according to beechey, † which, from running in the same direction with the prevalent wind, is not fully exposed to it. passages between the islets on the reef, through which boats can pass at high - water, must not be confounded with ship - channels, by which the annular reef itself is breached. the passages between the islets occur, of course, on the windward as well as on the leeward side ; but they are more frequent and broader to leeward, owing to the lesser dimensions of the islets on that side. at keeling atoll the shores of the lagoon shelve gradually, where the bottom is of sediment, and irregularly or abruptly where there are coral reefs ; but this is by no means the universal structure in other atolls. chamisso, ‡ speaking in general terms of the lagoons in the marshall atolls, says the lead generally sinks " from a depth of two or three fathoms to twenty or twenty - four, and you may pursue a line in which on one side of the boat you may see the bottom, and on the other the azure blue deep water. " the shores of the * principles of geology, vol. iii. p. 289. † beechey ' s voyage, 4to. ed. vol. i. p. 189. ‡ kotzebue ' s first voyage, vol. iii. p. 142. lagoon - like channel within the barrier - reef at vanikoro have a similar structure. captain beechey has described a modification of this structure ( and he believes it is not uncommon ) in two atolls in the low archipelago, in which the shores of the lagoon descend by a few, broad, slightly inclined, ledges or steps : thus at matilda atoll, * the great exterior reef, the surface of which is gently inclined towards and beneath the surface of the lagoon, ends abruptly in a little cliff three fathoms deep ; at


broad, slightly inclined, ledges or steps : thus at matilda atoll, * the great exterior reef, the surface of which is gently inclined towards and beneath the surface of the lagoon, ends abruptly in a little cliff three fathoms deep ; at its foot, a ledge forty yards wide extends, shelving gently inwards like the surface - reef, and terminated by a second little cliff five fathoms deep ; beyond this, the bottom of the lagoon slopes to twenty fathoms, which is the average depth of its centre. these ledges seem to be formed of coral rock ; and captain beechey says that the lead often descended several fathoms through holes in them. in some atolls, all the coral reefs or knolls in the lagoon come to the surface at low - water ; in other cases of rarer occurrence, all lie at nearly the same depth beneath it, but most frequently they are quite irregular, — some with perpendicular, some with sloping sides, — some rising to the surface, and others lying at all intermediate depths from the bottom upwards. i cannot, therefore, suppose that the union of such reefs could produce even one uniformly sloping ledge, and much less two or three, one beneath the other, and each terminated by an abrupt wall. at matilda island, which offers the best example of the step - like structure, captain beechey observes that the coral knolls within the lagoon are quite irregular in their height. we shall hereafter see that the theory which accounts for the ordinary form of atolls, apparently includes this occasional peculiarity in their structure. in the midst of a group of atolls, there sometimes occur small, flat, very low islands of coral formation, which probably once included a lagoon, since filled up with sediment and coral - * beechey ' s voyage, 4to. ed. vol. i. p. 160. at whitsunday island the bottom of the lagoon slopes gradually towards the centre, and then deepens suddenly, the edge of the bank being nearly perpendicular. this bank is formed of coral and dead shells. reefs. captain beechey entertains no doubt that this has been the case with the two small islands, which alone of thirty - one surveyed by him in the low archipelago, did not contain lagoons. romanzoff island ( in lat. 15° s. ) is described by chamisso * as formed by a dam of madreporitic rock inclosing a flat space, thinly covered with trees, into which


lagoons. romanzoff island ( in lat. 15° s. ) is described by chamisso * as formed by a dam of madreporitic rock inclosing a flat space, thinly covered with trees, into which the sea on the leeward side occasionally breaks. north keeling atoll appears to be in a rather less forward stage of conversion into land : it consists of a horse - shoe shaped strip of land surrounding a muddy flat, one mile in its longest axis, which is covered by the sea only at high water. when describing south keeling atoll, i endeavoured to show how slow the final process of filling up a lagoon must be ; nevertheless, as all causes do tend to produce this effect, it is very remarkable that not one instance, as i believe, is known of a moderately - sized lagoon being filled up even to the low water - line at spring - tides, much less of such a one being converted into land. it is, likewise, in some degree remarkable, how few atolls, except small ones, are surrounded by a single linear strip of land, formed by the union of separate islets. we cannot suppose that the many atolls in the pacific and indian oceans all have had a late origin, and yet should they remain at their present level, subjected only to the action of the sea and to the growing powers of the coral, during as many centuries as must have elapsed since any of the earlier tertiary epochs, it cannot, i think, be doubted that their lagoons and the islets on their reef, would present a totally different appearance from what they now do. this consideration leads to the suspicion that some renovating agency ( namely subsidence ) comes into play at intervals, and perpetuates their original structure. * kotzebue ' s first voyage, vol. iii. p. 221. maldiva archipelago — ring - formed reefs marginal and central — great depth in the lagoons of the s. atolls — reefs in the lagoons all rising to the surface — position of islets, and breaches in the reefs with respect to the prevalent winds and action of the waves — destruction of islets — connection in the position and submarine foundation of distinct atolls — the apparent disseverment of large atolls — the great chagos bank — its submerged condition and extraordinary structure. although occasional references have been made to the maldiva atolls, and to the banks in the chagos group, some points of their


disseverment of large atolls — the great chagos bank — its submerged condition and extraordinary structure. although occasional references have been made to the maldiva atolls, and to the banks in the chagos group, some points of their structure deserve further consideration. my description is derived from an examination of the admirable charts lately published from the survey of captain moresby and lieut. powell, and more especially from information which captain moresby has communicated to me in the kindest manner. the maldiva archipelago is 470 miles in length, with an average breadth of about 50 miles. the form and dimensions of the atolls, and their singular position in a double line, may be seen, but not well, in the greatly reduced chart ( fig. 6. ) in plate ii. the dimensions of the longest atoll in the group, ( called by the double name of milla - dou - madou and tilla - dou - matte ) has already been given ; it is 88 miles in a medial and slightly curved line, and is less than 20 miles in its broadest part. suadiva, also, is a noble atoll, being 44 miles across in one direction, and 34 in another, and the great included expanse of water has a depth of between 250 and 300 feet. the smaller atolls in this group differ in no respect from ordinary ones ; but the larger ones are remarkable from being breached by numerous deep - water channels leading into the lagoon ; for instance, there are 42 channels, through which a ship could enter the lagoon of suadiva. in the three southern large atolls, the separate portions of reef between these channels have the ordinary structure, and are linear ; but in the other atolls, especially the more northern ones, these portions are ring - formed, like miniature atolls. other ring - formed reefs rise out of the lagoons, in the place of those irregular ones which ordinarily occur there. in the reduction of the chart of mahlos mahdoo ( plate ii. fig. 4 ), it was not found easy to define the islets and the little lagoons within each reef, so that the ring - formed structure is very imperfectly shown : in the large published charts of tilla - dou - matte, the appearance of these rings, from standing further apart from each other, is very remarkable. the rings on the margin are generally elongated ; many of them are three, and some even five miles, in diameter ; those


- dou - matte, the appearance of these rings, from standing further apart from each other, is very remarkable. the rings on the margin are generally elongated ; many of them are three, and some even five miles, in diameter ; those within the lagoon are usually smaller, few being more than two miles across, and the greater number rather less than one. the depth of the little lagoon within these small annular reefs is generally from five to seven fathoms, but occasionally more ; and in ari atoll many of the central ones are twelve, and some even more than twelve fathoms deep. these rings rise abruptly from the platform or bank, on which they are placed ; their outer margin is invariably bordered by living coral, * within which there is a flat surface of coral - rock ; on this flat, sand and fragments have in many cases accumulated and been converted into islets, clothed with vegetation. i can, in fact, point out no essential difference between these little ring - formed reefs ( which, however, are larger, and contain deeper lagoons than many true atolls that stand in the open sea ), and the most perfectly characterized atolls, excepting that the ring - formed reefs are based on a shallow foundation, instead of on the floor of the open sea, and that instead of being scattered irregularly, they are grouped closely together on one large platform, with the marginal rings arranged in a rudely - formed circle. the perfect series which can be traced from portions of simple linear reef, to others including long linear lagoons, * capt. moresby informs me that millepora complanata, is one of the commonest kinds on the outer margin, as it is at keeling atoll. and from these again to oval or almost circular rings, renders it probable that the latter are merely modifications of the linear or normal state. it is conformable with this view, that the ring - formed reefs on the margin, even where most perfect and standing furthest apart, generally have their longest axes directed in the line which the reef would have held, if the atoll had been bounded by an ordinary wall. we may also infer that the central ring - formed reefs are modifications of those irregular ones, which are found in the lagoons of all common atolls. it appears from the charts on a large scale, that the ring - like structure is contingent on the marginal channels or breaches being wide ; and, consequently, on the whole interior of the atoll being freely exposed to the waters


all common atolls. it appears from the charts on a large scale, that the ring - like structure is contingent on the marginal channels or breaches being wide ; and, consequently, on the whole interior of the atoll being freely exposed to the waters of the open sea. when the channels are narrow or few in number, although the lagoon be of great size and depth ( as in suadiva ), there are no ring - formed reefs ; where the channels are somewhat broader, the marginal portions of reef, and especially those close to the larger channels, are ring - formed, but the central ones are not so ; where they are broadest, almost every reef throughout the atoll is more or less perfectly ring - formed. although their presence is thus contingent on the openness of the marginal channels, the theory of their formation, as we shall hereafter see, is included in that of the parent atolls, of which they form the separate portions. the lagoons of all the atolls in the southern part of the archipelago are from ten to twenty fathoms deeper than those in the northern part. this is well exemplified in the case of addoo, the southernmost atoll in the group, for although only 9 miles in its longest diameter ; it has a depth of 39 fathoms, whereas all the other small atolls have comparatively shallow lagoons ; i can assign no adequate cause for this difference in depth. in the central and deepest part of the lagoons, the bottom consists, as i am informed by capt. moresby, of stiff clay ( probably a calcareous mud ) ; nearer the border it consists of sand, and in the channels through the reef, of hard sand - banks, sandstone, conglo - merate rubble, and a little live coral. close outside the reef and the line joining its detached portions ( where intersected by many channels ), the bottom is sandy, and it slopes abruptly into unfathomable depths. in most lagoons the depth is considerably greater in the centre than in the channels ; but in tilla - dou - matte, where the marginal ring - formed reefs stand far apart, the same depth is carried across the entire atoll, from the deep - water line on one side to that on the other. i cannot refrain from once again remarking on the singularity of these atolls, — a great sandy and generally concave disk rises abruptly from the unfathomable ocean, with its central expanse studded and its border symmetrically fringed with oval


cannot refrain from once again remarking on the singularity of these atolls, — a great sandy and generally concave disk rises abruptly from the unfathomable ocean, with its central expanse studded and its border symmetrically fringed with oval basons of coral - rock, just lipping the surface of the sea, sometimes clothed with vegetation, and each containing a little lake of clear water! in the southern maldiva atolls, of which there are nine large ones, all the small reefs within the lagoons come to the surface, and are dry at low water spring - tides ; hence in navigating them, there is no danger from submarine banks. this circumstance is very remarkable, as within some atolls, for instance those of the neighbouring chagos group, not a single reef comes to the surface, and in most other cases a few only do, and the rest lie at all intermediate depths from the bottom upwards. when treating of the growth of coral i shall again refer to this subject. although in the neighbourhood of the maldiva archipelago the winds, during the monsoons, blow during nearly an equal time from opposite quarters, and although, as i am informed by capt. moresby, the westerly winds are the strongest, yet the islets are almost all placed on the eastern side of the northern atolls, and on the south - eastern side of the southern atolls. that the formation of the islets is due to detritus thrown up from the outside, as in the ordinary manner, and not from the interior of the lagoons, may, i think, be safely inferred from several considerations, which it is hardly worth while to detail. as the easterly winds are not the strongest, their action probably is aided by some prevailing swell or current. in groups of atolls, exposed to a trade wind, the ship - channels into the lagoons are almost invariably situated on the leeward or less exposed side of the reef, and the reef itself is sometimes either wanting there, or is submerged. a strictly analogous, but different, fact, may be observed at the maldiva atolls — namely, that where two atolls stand in front of each other, the breaches in the reef are the most numerous on their near, and therefore less exposed, sides. thus on the near sides of ari and the two nillandoo atolls, which face s. male, phaleedoo, and moloque atolls, there are seventy - three deep - water


, and therefore less exposed, sides. thus on the near sides of ari and the two nillandoo atolls, which face s. male, phaleedoo, and moloque atolls, there are seventy - three deep - water channels, and only twenty - five on their outer sides ; on the near side of the three latter - named atolls there are fifty - six openings, and only thirty - seven on their outsides. it is scarcely possible to attribute this difference to any other cause than the somewhat different action of the sea on the two sides, which would ensue from the protection afforded by the two rows of atolls to each other. i may here remark that in most cases, the conditions favourable to the greater accumulation of fragments on the reef and to its more perfect continuity, on one side of the atoll than on the other, have concurred, but this has not been the case with the maldivas ; for we have seen that the islets are placed on the eastern or south eastern sides, whilst the breaches in the reef occur indifferently on any side, where protected by an opposite atoll. the reef being more continuous on the outer and more exposed sides of those atolls which stand near each other, accords with the fact, that the reef of the southern atolls is more continuous than that of the northern ones ; for the former, as i am informed by capt. moresby, are more constantly exposed than the northern atolls, to a heavy surf. the date of the first formation of some of the islets in this archipelago is known to the inhabitants ; on the other hand, several islets, and even some of those which are believed to be very old, are now fast wearing away. the work of destruction has, in some instances, been completed in ten years. capt. moresby found on one water - washed reef the marks of wells and graves, which were excavated when it supported an islet. in south nillandoo atoll, the natives say that three of the islets were formerly larger : in north nillandoo there is one now being washed away ; and in this latter atoll lieut. prentice found a reef, about six hundred yards in diameter, which the natives positively affirmed was lately an island covered with cocoa - nut trees. it is now only partially dry at low water spring tides, and is, ( in lieut. prentice ' s words ) " entirely covered with live coral and madrepore. " in the northern part, also


covered with cocoa - nut trees. it is now only partially dry at low water spring tides, and is, ( in lieut. prentice ' s words ) " entirely covered with live coral and madrepore. " in the northern part, also, of the maldiva archipelago and in the chagos group, it is known that some of the islets are disappearing. the natives attribute these effects to variations in the currents of the sea. for my own part i cannot avoid suspecting, that there must be some further cause, which gives rise to such a cycle of change in the action of the currents of the great and open ocean. several of the atolls in this archipelago are so related to each other in form and position, that at the first glance one is led to suspect that they have originated in the disseverment of a single one. male consists of three perfectly characterized atolls, of which the shape and relative position are such, that a line drawn closely round all three, gives a symmetrical figure : to see this clearly, a larger chart is required than that of the archipelago in plate ii. : the channel separating the two northern male atolls is only little more than a mile wide, and no bottom was found in it with 100 fathoms. powell ' s island is situated at the distance of two miles and a half off the northern end of mahlos mahdoo, ( see fig. 4. plate ii. ), at the exact point where the two sides of the latter, if prolonged, would meet : no bottom, however, was found in the channel with 200 fathoms : in the wider channel between horsburgh atoll and the southern end of mahlos mahdoo, no bottom was found with 250 fathoms. in these and similar cases, the relation consists only in the form and position of the atolls. but in the channel between the two nillandoo atolls, although three miles and a quarter wide, soundings were struck at the depth of 200 fathoms : the channel between ross and ari atolls is four miles wide, and only 150 fathoms deep. here then we have, besides the relation of form, a submarine connection. the fact of soundings having been obtained between two separate and perfectly characterized atolls is in itself interesting, as it has never, i believe, been effected in any of the many other groups of atolls in the pacific and indian seas. in continuing to trace the connection of adjoining atolls, if


and perfectly characterized atolls is in itself interesting, as it has never, i believe, been effected in any of the many other groups of atolls in the pacific and indian seas. in continuing to trace the connection of adjoining atolls, if a hasty glance be taken at the chart ( fig. 4. plate ii. ) of mahlos mahdoo and the line of unfathomable water be followed, no one will hesitate to consider it as one atoll. but a second look will shew that it is divided by a bifurcating channel, of which the northern arm is about one mile and three quarters in width, with an average depth of 125 fathoms, and the southern one three quarters of a mile wide, and rather less deep. these channels resemble in the slope of their sides and general form, those which separate atolls in every respect distinct ; and the northern arm is wider than that dividing two of the male atolls. the ring - formed reefs on the sides of this bifurcating channel are elongated, so that the northern and southern portions of mahlos mahdoo may claim, as far as their external outline is concerned, to be considered as distinct and perfect atolls. but the intermediate portion, lying in the fork of the channel, is bordered by reefs less perfect than those, which surround any other atoll in the group of equally small dimensions. mahlos mahdoo, therefore, is in every respect in so intermediate a condition, that it may be considered either as a single atoll nearly dissevered into three portions, or as three atolls almost perfect and intimately connected. this is an instance of a very early stage of the apparent disseverment of an atoll, but a still earlier one in many respects is exhibited at tilla - dou - matte. in one part of this atoll, the ring - formed reefs stand so far apart from each other, that the inhabitants have given different names to the northern and southern halves : nearly all the rings, moreover, are so perfect and stand so separate, and the space from which they rise is so level and unlike a true lagoon, that we can easily imagine the conversion of this one great atoll, not into two or three portions, but into a whole group of miniature atolls. a perfect series, such as we have here traced, impresses the mind with an idea of actual change ; and it will hereafter be seen, that the theory of subsidence, with the upward growth of the coral


miniature atolls. a perfect series, such as we have here traced, impresses the mind with an idea of actual change ; and it will hereafter be seen, that the theory of subsidence, with the upward growth of the coral, modified by accidents of probable occurrence, will account for the occasional disseverment of large atolls. the great chagos bank alone remains to be described. in the chagos group there are some ordinary atolls, some annular reefs rising to the surface but without any islets on them, and some atoll - formed banks, either quite submerged, or nearly so. of the latter, the great chagos bank is much the largest, and differs in its structure from the others : a plan of it is given in plate ii., fig. 1, in which, for the sake of clearness, i have had the parts under ten fathoms deep finely shaded : an east and west vertical section is given in fig. 2, in which the vertical scale has been necessarily exaggerated. its longest axis is ninety nautical miles, and another line drawn at right angles to the first, across the broadest part, is seventy. the central part consists of a level muddy flat, between forty and fifty fathoms deep, which is surrounded on all sides, with the exception of some breaches, by the steep edges of a set of banks, rudely arranged in a circle. these banks consist of sand, with a very little live coral ; they vary in breadth from five to twelve miles, and on an average lie about sixteen fathoms beneath the surface ; they are bordered by the steep edges of a third narrow and upper bank, which forms the rim to the whole. this rim is about a mile in width, and with the exception of two or three spots where islets have been formed, is submerged between five and ten fathoms. it consists of smooth hard rock, covered with a thin layer of sand, but with scarcely any live coral ; it is steep on both sides, and outwards slopes abruptly into unfathomable depths. at the distance of less than half a mile from one part, no bottom was found with 190 fathoms ; and off another point, at a somewhat greater distance, there was none with 210 fathoms. small steep - sided banks or knolls, covered with luxuriantly - growing coral, rise from the interior expanse to the same level with the external rim, which, as we have seen, is formed only


none with 210 fathoms. small steep - sided banks or knolls, covered with luxuriantly - growing coral, rise from the interior expanse to the same level with the external rim, which, as we have seen, is formed only of dead rock. it is impossible to look at the plan ( fig. 1. plate ii. ), although reduced to so small a scale, without at once perceiving that the great chagos bank is, in the words of capt. moresby, * " nothing more than a half - drowned atoll. " but of what great dimensions, and of how extraordinary an internal structure! we shall hereafter have to consider both the cause of its submerged condition, a state common to other banks in the group, and the origin of the singular submarine terraces, which bound the central expanse : these, i think, it can be shown, have resulted from a cause analogous to that, which has produced the bifurcating channel across mahlos mahdoo. * this officer has had the kindness to lend me an excellent ms. account of the chagos islands ; from this paper, from the published charts, and from verbal information communicated to me by capt. moresby, the above account of the great chagos bank is taken. closely resemble in general form and structure atoll - reefs — width and depth of the lagoon - channels — breaches through the reef in front of valleys, and generally on the leeward side — checks to the filling up of the lagoon - channels — size and constitution of the encircled islands — number of islands within the same reef — barrier - reefs of new caledonia and australia — position of the reef relative to the slope of the adjoining land — probable great thickness of barrier - reefs. the term ' barrier ' has been generally applied to that vast reef which fronts the n. e. shore of australia, and by most voyagers likewise to that on the western coast of new caledonia. at one time i thought it convenient thus to restrict the term, but as these reefs are similar in structure, and in position relatively to the land, to those, which, like a wall with a deep moat within, encircle many smaller islands, i have classed them together. the reef, also, on the west coast of new caledonia, circling round the extremities of the island, is an intermediate form between a small encircling reef and the australian barrier, which stretches for a thousand miles in nearly a straight line. the geographer balbi has in


new caledonia, circling round the extremities of the island, is an intermediate form between a small encircling reef and the australian barrier, which stretches for a thousand miles in nearly a straight line. the geographer balbi has in effect described those barrier - reefs, which encircle moderately sized islands, by calling them atolls with high land rising from within their central expanse. the general resemblance between the reefs of the barrier and atoll classes may be seen in the small, but accurately reduced charts on plate i., * and this resemblance can be * the authorities from which these charts have been reduced, together with some remarks on them, are given in a separately appended page, descriptive of the plates. further shown to extend to every part of their structure. beginning with the outside of the reef ; many scattered soundings off gambier, oualan, and some other encircled islands, show that close to the breakers there exists a narrow shelving margin, beyond which the ocean becomes suddenly unfathomable : but off the west coast of new caledonia, capt. kent * found no bottom with 150 fathoms, at two ships ' length from the reef ; so that the slope here must be nearly as precipitous as off the maldiva atolls. i can give little information regarding the kinds of corals which live on the outer margin. when i visited the reef at tahiti, although it was low water, the surf was too violent for me to see the living masses ; but, according to what i heard from some intelligent native chiefs, they resemble in their rounded and branchless forms, those on the margin of keeling atoll. the extreme verge of the reef, which was visible between the breaking waves at low water, consisted of a rounded, convex, artificial - like breakwater, entirely coated with nulliporæ, and absolutely similar to that which i have described at keeling atoll. from what i heard when at tahiti, and from the writings of the revs. w. ellis and j. williams, i conclude that this peculiar structure is common to most of the encircled islands of the society archipelago. the reef within this mound or breakwater, has an extremely irregular surface, even more so than between the islets on the reef of keeling atoll, with which alone ( as there are no islets on the reef of tahiti ) it can properly be compared. at tahiti, the reef is very irregular in width ; but round many other encircled islands, for


the reef of keeling atoll, with which alone ( as there are no islets on the reef of tahiti ) it can properly be compared. at tahiti, the reef is very irregular in width ; but round many other encircled islands, for instance vanikoro or gambier islands, ( figs. 1. and 8. plate i. ) it is quite as regular, and of the same average width, as in true atolls. most barrier - reefs on the inner side slope irregularly into the lagoon - channel, ( as the space of deep water separating the reef from the included land may be called, ) but at vanikoro the reef slopes only for a short distance, and then terminates abruptly in a submarine wall, forty feet high, — a structure * dalrymple, hydrog. mem. vol. iii. absolutely similar to that described by chamisso in the marshall atolls. in the society archipelago, ellis * states, that the reefs generally lie at the distance of from one to one and a half miles, and, occasionally, even at more than three miles, from the shore. the central mountains are generally bordered by a fringe of flat, and often marshy, alluvial land, from one to four miles in width. this fringe consists of coral - sand and detritus thrown up from the lagoon - channel, and of soil washed down from the hills ; it is an encroachment on the channel, analogous to that low and inner part of the islets in many atolls, which is formed by the accumulation of matter from the lagoon. at hogoleu ( fig. 2. plate i. ) in the caroline archipelago, † the reef on the south side is no less than 20 miles ; on the east side, five ; and on the north side, 14 miles from the encircled high islands. the lagoon - channels may be compared in every respect with true lagoons. in some cases they are open, with a level bottom of fine sand ; in others they are choked up with reefs of delicately branched corals, which have the same general character as those within keeling atoll. these internal reefs either stand separately, or more commonly skirt the shores of the included high islands. the depth of the lagoon - channel round the society islands varies from two or three, to thirty fathoms ; in cook ' s ‡ chart of ulietea, however, there is one sounding laid down of 48 fathoms : at vanikoro there are several of


channel round the society islands varies from two or three, to thirty fathoms ; in cook ' s ‡ chart of ulietea, however, there is one sounding laid down of 48 fathoms : at vanikoro there are several of 54 and one of 56½ fathoms ( english ), a depth which even exceeds by a little that of the interior of the great maldiva atolls. some barrier - reefs have very few islets on them ; whilst others are surmounted by numerous ones ; and those round part of bolabola ( plate i. fig. 5. ) form a single linear strip. the * consult, on this and other points, the polynesian researches by the rev. w. ellis, an admirable work, full of curious information. † see hydrographical mem. and the atlas of the voyage of the astrolabe, by capt. dumont d ' urville, p. 428. ‡ see the chart in vol. i. of hawkesworth ' s 4to ed. of cook ' s first voyage. islets first appear either on the angles of the reef, or on the sides of the breaches through it, and are generally most numerous on the windward side. the reef to leeward retaining its usual width, sometimes lies submerged several fathoms beneath the surface ; i have already mentioned gambier island as an instance of this structure. submerged reefs, having a less defined outline, dead, and covered with sand, have been observed ( see appendix ) off some parts of huaheine and tahiti. the reef is more frequently breached to leeward than to windward ; thus i find in krusenstern ' s memoir on the pacific, that there are passages through the encircling reef on the leeward side of each of the seven society islands, which possess ship - harbours ; but that there are openings to windward through the reef of only three of them. the breaches in the reef are seldom as deep as the interior lagoon - like channel ; they generally occur in front of the main valleys, a circumstance which can be accounted for, as will be seen in the fourth chapter, without much difficulty. the breaches being situated in front of the valleys, which descend indifferently on all sides, explains their more frequent occurrence through the windward side of barrier - reefs than through the windward side of atolls, — for in atolls there is no included land to influence the position of the breaches.


descend indifferently on all sides, explains their more frequent occurrence through the windward side of barrier - reefs than through the windward side of atolls, — for in atolls there is no included land to influence the position of the breaches. it is remarkable, that the lagoon - channels round mountainous islands have not in every instance been long ago filled up with coral and sediment ; but it is more easily accounted for than appears at first sight. in cases like that of hogoleu and the gambier islands, where a few small peaks rise out of a great lagoon, the conditions scarcely differ from those of an atoll ; and i have already shown, at some length, that the filling up of a true lagoon must be an extremely slow process. where the channel is narrow, the agency, which on unprotected coasts is most productive of sediment, namely the force of the breakers, is here entirely excluded, and the reef being breached in the front of the main valleys, much of the finer mud from the rivers must be transported into the open sea. as a current is formed by the water thrown over the edge of atoll - formed reefs, which carries sediment with it through the deepwater breaches, the same thing probably takes place in barrier - reefs, and this would greatly aid in preventing the lagoon - channel from being filled up. the low alluvial border, however, at the foot of the encircled mountains, shows that the work of filling up is in progress ; and at maurua ( plate i. fig. 6. ) in the society group, it has been almost effected, so that there remains only one harbour for small craft. if we look at a set of charts of barrier - reefs, and leave out in imagination the encircled land, we shall find that besides the many points already noticed of resemblance, or rather of identity in structure with atolls, there is a close general agreement in form, average dimensions, and grouping. encircling barrier - reefs, like atolls, are generally elongated, with an irregularly rounded, though sometimes angular outline. there are atolls of all sizes, from less than two miles in diameter to sixty miles, ( excluding tilla - dou - matte, as it consists of a number of almost independent atoll - formed reefs ) ; and there are encircling barrier - reefs from three miles and a half to forty - six miles in diameter, — turtle island being an instance of the former, and hogoleu of the latter. at


independent atoll - formed reefs ) ; and there are encircling barrier - reefs from three miles and a half to forty - six miles in diameter, — turtle island being an instance of the former, and hogoleu of the latter. at tahiti the encircled island is 36 miles in its longest axis, whilst at maurua it is only a little more than two miles. it will be shown, in the last chapter in this volume, that there is the strictest resemblance in the grouping of atolls and of common islands, and consequently there must be the same resemblance in the grouping of atolls and of encircling barrier - reefs. the islands lying within reefs of this class, are of very various heights. tahiti * is 7, 000 feet ; maurua about 800 ; aituaki 360, and manouai only 50. the geological nature * the height of tahiti is given from capt. beechey ; maurua from mr. f. d. bennett ( geograph. journ. vol. viii. p. 220 ) ; aitutaki from measurements made on board the beagle ; and manouai, or harvey island, from an estimate by the rev. j. williams. the two latter islands, however, are not in some respects well characterized examples of the encircled class. of the included land varies ; in most cases it is of ancient volcanic origin, owing apparently to the fact that islands of this nature are most frequent within all great seas : some, however, are of madreporitic limestone, and others of primary formation, of which latter kind new caledonia offers the best example. the central land consists either of one island, or of several ; thus in the society group, eimeo stands by itself ; while taha and raiatea, ( fig. 3. plate i. ) both moderately large islands, of nearly equal size, are included in one reef. within the reef of the gambier group there are four large and some smaller islands ( fig. 8. plate i. ) ; within that of hogoleu ( fig. 2. plate i. ) nearly a dozen small islands are scattered over the expanse of one vast lagoon. after the details now given, it may be asserted, that there is not one point of essential difference between encircling barrier - reefs and atolls ; — the latter enclose a simple sheet of water, the former encircle an expanse with one or more islands rising from it. i


that there is not one point of essential difference between encircling barrier - reefs and atolls ; — the latter enclose a simple sheet of water, the former encircle an expanse with one or more islands rising from it. i was much struck with this fact, when viewing, from the heights of tahiti, the distant island of eimeo standing within smooth water, and encircled by a ring of snow - white breakers. remove the central land, and an annular reef like that of an atoll in an early stage of its formation is left ; remove it from bolabola, and there remains a circle of linear coral - islets, crowned with tall cocoa - nut trees, like one of the many atolls scattered over the pacific and indian oceans. the barrier - reefs of australia and of new caledonia, deserve a separate notice from their great dimensions. the reef on the west coast of new caledonia ( fig. 5. plate ii. ) is 400 miles in length ; and for a length of many leagues it seldom approaches within eight miles of the shore ; and near the southern end of the island, the space between the reef and the land is sixteen miles in width. the australian barrier extends, with a few interruptions, for nearly a thousand miles : its average distance from the land is between twenty and thirty miles, and in some parts from fifty to seventy. the great arm of the sea thus included, is from ten to twenty - five fathoms deep, with a sandy bottom ; but towards the southern end, where the reef is further from the shore, the depth gradually increases to forty, and in some parts to more than sixty fathoms. flinders * has described the surface of this reef as consisting of a hard white agglomerate of different kinds of coral, with rough projecting points. the outer edge is the highest part ; it is traversed by narrow gullies, and at rare intervals is breached by ship - channels. the sea close outside is profoundly deep ; but, in front of the main breaches, soundings can sometimes be obtained. some low islets have been formed on the reef. there is one important point in the structure of barrier - reefs which must here be considered. the accompanying diagrams represent north and south vertical sections, taken through the highest points of vanikoro, gambier, and maurua islands, and through their encircling reefs. the scale both in the horizontal and vertical direction is the same, namely, a quarter of an inch to


taken through the highest points of vanikoro, gambier, and maurua islands, and through their encircling reefs. the scale both in the horizontal and vertical direction is the same, namely, a quarter of an inch to a nautical mile. the height and width of these islands is known ; and i have attempted to represent the form of the land from the shading of the hills in the large published charts. it has long been remarked, even from the time of dampier, that a considerable degree of relation subsists between the inclination of that part of the land which is beneath water and that above it : hence the dotted line in the three sections, probably, does not widely differ in inclination from the actual submarine prolongation of the land. if we now look at the outer edge of the reef ( a a ), and bear in mind that the plummet on the right hand represents a depth of 1200 feet, we must conclude that the vertical thickness of these barrier coral reefs is very great. * flinders ' voyage to terra australis, vol. ii. p. 88. 1 — vanikoro, from the atlas of the voyage of the astrolabe, by d. d ' urville. 2 — gambier island, from beechey. 3 — maurua, from the atlas of the voyage of the coquille, by duperrey. the horizontal line is the level of the sea, from which on the right hand a plummet descends, representing a depth of 200 fathoms, or 1200 feet. the vertical shading shows the section of the land, and the horizontal shading that of the encircling barrier - reef ; from the smallness of the scale, the lagoon - channel could not be represented. a a — outer edge of the coral - reefs, where the sea breaks. b b — the shore of the encircled islands. i must observe, that if the sections had been taken in any other direction across these islands, or across other encircled islands, * the result would have been the same. in the succeeding chapter it will be shown that reef - building polypifers cannot flourish at great depths, — for instance, it is highly improbable that they could exist at a quarter of the depth represented by the plummet on the right hand of the woodcut. here then is a great apparent difficulty — how were the basal parts of these barrier - reefs formed. it will, perhaps, occur to some, that the actual


quarter of the depth represented by the plummet on the right hand of the woodcut. here then is a great apparent difficulty — how were the basal parts of these barrier - reefs formed. it will, perhaps, occur to some, that the actual reefs formed of coral * in the fifth chapter an east and west section across the island of bolabola and its barrier - reefs is given, for the sake of illustrating another point. the unbroken line in it ( woodcut no. 5 ) is the section referred to. the scale is. 57 of an inch to a mile ; it is taken from the atlas of the voyage of the coquille by duperrey. the depth of the lagoon - channel is exaggerated. are not of great thickness, but that before their first growth, the coasts of these encircled islands were deeply eaten into, and a broad but shallow submarine ledge thus left, on the edge of which the coral grew ; but if this had been the case, the shore would have been invariably bounded by lofty cliffs, and not have sloped down to the lagoon - channel, as it does in many instances. on this view, * moreover, the cause of the reef springing up at such a great distance from the land, leaving a deep and broad moat within, remains altogether unexplained. a supposition of the same nature, and appearing at first more probable, is, that the reefs sprung up from banks of sediment, which had accumulated round the shore previously to the growth of the coral ; but the extension of a bank to the same distance round an unbroken coast, and in front of those deep arms of the sea, ( as in raiatea, see plate ii. fig. 3. ) which penetrate nearly to the heart of some encircled islands, is exceedingly improbable. and why, again, should the reef spring up, in some cases steep on both sides like a wall, at a distance of two, three, or more miles from the shore, leaving a channel often between 200 and 300 feet deep, and rising, from a depth which we have reason to believe is destructive to the growth of coral? an admission of this nature cannot possibly be made. the existence, also, of the deep channel, utterly precludes the idea of the reef having grown outwards, on a foundation slowly formed on its outside, by the accumulation of sediment and coral detritus. nor, again, can it be asserted, that the reef - building corals will not grow


; * and aitutaki consists of calcareous rock. we must, therefore, reject these several explanations, and conclude that the vertical thickness of barrier - reefs, from their outer edges to the foundation on which they rest ( from aa in the section to the dotted lines ) is really great : but in this, there is no difficulty, for it is not necessary to suppose that the coral has sprung up from an immense depth, as will be evident when the theory of the upward growth of coral reefs, during the slow subsidence of their foundations, is discussed. * i have been informed that this is the case by dr. allan of forres, who has visited this group. fringing or shore reefs. reefs of mauritius — shallow channel within the reef — its slow filling up — currents of water formed within it — upraised reefs — narrow fringing - reefs in deep seas — reefs on the coast of e. africa and of brazil — fringing reefs in very shallow seas, round banks of sediment and on worn - down islands — fringing reefs affected by currents of the sea — coral coating bottom of the sea, but not forming reefs. fringing reefs, or, as they have been called by some voyagers, shore reefs, whether skirting an island or part of a continent, might at first be thought to differ little, except in generally being of less breadth, from barrier - reefs. as far as the superficies of the actual reef is concerned this is the case ; but the absence of an interior deep - water channel, and the close relation in their horizontal extension with the probable slope beneath the sea of the adjoining land, present essential points of difference. the reefs which fringe the island of mauritius offer a good example of this class. they extend round its whole circumference, with the exception of two or three parts, * where the coast is almost precipitous, and where, if as is probable the bottom of the sea has a similar inclination, the coral would have no foundation on which to become attached. a similar fact may sometimes be observed even in reefs of the barrier class, which follow much less closely the * this fact is stated on the authority of the officier du roi, in his extremely interesting " voyage a l ' isle de france, " undertaken in 1768. according to capt. carmichael ( hooker ' s bot. misc. vol. ii. p. 316. ) on one part of the coast there is a space for 16 miles without a reef.


' isle de france, " undertaken in 1768. according to capt. carmichael ( hooker ' s bot. misc. vol. ii. p. 316. ) on one part of the coast there is a space for 16 miles without a reef. outline of the adjoining land ; as, for instance, on the s. e. and precipitous side of tahiti, where the encircling reef is interrupted. on the western side of the mauritius, which was the only part i visited, the reef generally lies at the distance of about half a mile from the shore ; but in some parts it is distant from one to two, and even three miles. but even in this last case, as the coast - land is gently inclined from the foot of the mountains to the sea - beach, and as the soundings outside the reef indicate an equally gentle slope beneath the water, there is no reason for supposing that the basis of the reef, formed by the prolongation of the strata of the island, lies at a greater depth than that, at which the polypifers could begin constructing the reef. some allowance, however, must be made for the outward extension of the corals on a foundation of sand and detritus, formed from their own wear, which would give to the reef a somewhat greater vertical thickness, than would otherwise be possible. the outer edge of the reef on the western or leeward side of the island, is tolerably well defined, and is a little higher than any other part. it chiefly consists of large strongly branched corals, of the genus madrepora, which also form a sloping bed some way out to sea : the kinds of coral growing in this part will be described in the ensuing chapter. between the outer margin and the beach, there is a flat space with a sandy bottom and a few tufts of living coral ; in some parts it is so shallow, that people, by avoiding the deeper holes and gullies, can wade across it at low water ; in other parts it is deeper, seldom however exceeding ten or twelve feet, so that it offers a safe coasting channel for boats. on the eastern and windward side of the island, which is exposed to a heavy surf, the reef described to me as having a hard smooth surface, very slightly inclined inwards, just covered at low - water, and traversed by gullies ; it appears to be quite similar in structure to the reefs of the barrier and atoll classes. the reef of mauritius