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the following figure have been altered from French feet to English fathoms; height of highest point of the island 4026 feet. Fig. 6.—MAURUA, in the Society Archipelago; from the survey by Capt. Duperrey in the Coquille: scale ¼ of an inch to a mile: height of land about 800 feet. Fig. 7.—POUYNIPÈTE, or SENIAVINE, in th... |
survey by Admiral Lutké: scale ¼ of an inch to a mile. Fig. 8.—GAMBIER ISLANDS, in the southern part of the Low Archipelago; from the survey by Capt. Beechey; scale ¼ of an inch to a mile; height of highest island, 1246 feet; the islands are surrounded by extensive and irregular reefs; the reef on the southern side is ... |
Fig. 9.—PEROS BANHOS ATOLL, (or lagoon-island), in the Chagos group in the Indian Ocean; from the survey by Capt. Moresby and Lieut. Powell; scale ¼ of an inch to a mile; not nearly all the small sub- merged reefs in the lagoon are represented; the annular reef on the southern side is submerged. Fig. 10.—KEELING, or CO... |
in the Indian Ocean; from the survey by Capt. FitzRoy; scale ¼ of an inch to a mile; the lagoon south of the dotted line is very shallow, and is left almost bare at low water; the part north of the line is choked up with irregular reefs. The annular reef on the N.W. side is broken, and blends into |
a shoal sand-bank, on which the sea breaks. Fig. 1.—GREAT CHAGOS BANK, in the Indian Ocean; taken from the survey by Capt. Moresby and Lieut. Powell; scale 1/20 of an inch to a mile (same scale as Hogoleu, in Plate I.); the parts which are shaded, with the exception of two or three islets on the western and northern si... |
do not rise to the surface, but are submerged from 4 to 10 fathoms; the banks bounded by the dotted lines lie from 15 to 20 fathoms beneath the surface, and are formed of sand; the central space is of mud, and from 30 to 50 fathoms deep. Fig. 2.—A vertical section, on the same scale, in an E. and |
W. line across the Great Chagos Bank, given for the sake of exhibiting more clearly its structure. Fig. 3.—MENCHICOFF ATOLL (or lagoon-island), in the Marshall Archipelago, northern Pacific Ocean; from Krusenstern's atlas of the Pacific; originally surveyed by Capt. Hagemeister; scale 1/20 of an inch to a mile; the dep... |
with Horsburgh atoll, in the Maldiva archipelago; from the survey by Capt. Moresby and Lieut. Powell; scale 1/20 of an inch to a mile; the white spaces in the middle of the separate small reefs, both on the margin and in the middle part, are meant to represent little lagoons; but it was found not possible to distinguis... |
from the small islets, which have been formed on these same small reefs; many of the smaller reefs could not be introduced; the nautical mark () over the figures 250 and 200 between Mahlos Mahdoo and Horsburgh atoll and Powell's island, signifies that soundings were not obtained at these depths. Fig. 5.—NEW CALEDONIA, ... |
from Krusenstern's atlas, compiled from several surveys; I have slightly altered the northern point of the reef, in accordance with the atlas of the voyage of the Astrolabe. In Krusenstern's atlas, the reef is represented by a single line with crosses; I have for the sake of uniformity added an interior line; scale 1/6... |
Fig. 6.—MALDIVA ARCHIPELAGO, in the Indian Ocean; from the survey by Capt. Moresby and Lieut. Powell; scale 1/60 of an inch to a mile. The principles, on which this map is coloured, are explained in the beginning of Chapter VI.; and the authorities for colouring each particular spot are detailed in the Appendix. The na... |
59 ON THE DISTRIBUTION AND GROWTH OF CORAL REEFS. SECTION I.—ON THE DISTRIBUTION OF CORAL REEFS, AND ON THE CONDITIONS FAVOURABLE TO THEIR INCREASE . page 60 to 71 SECTION II.—ON THE RATE OF GROWTH OF CORAL-REEFS . page 71 to 79 SECTION III.—ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE …. page 80 to 87 THEO... |
my own observation and the works of others, the principal kinds of coral reefs, more especially those occurring in the open ocean, and to explain the origin of their peculiar forms. I do not here treat of the polypifers, which construct these vast works, except so far as relates to their distribution, and to the condit... |
growth. Without any distinct intention to classify coral-reefs, most voyagers have spoken of them under the following heads: 'lagoon-islands,' or 'atolls,' 'barrier,' or 'encircling reefs' and 'fringing,' or 'shore reefs.' The lagoon-islands have received much the most attention; and it is not surprising, for every one... |
of coral-rock, often many leagues in diameter, here and there surmounted by a low verdant island with dazzling white shores, bathed on the outside by the foaming breakers of the ocean, and on the inside surrounding a calm expanse of water, which, from reflection, is of a bright but pale green colour. The naturalist wil... |
after having examined the soft and almost gelatinous bodies of these apparently insignificant creatures, and when he knows that the solid reef increases only on the outer edge, which day and night is lashed by the breakers of an ocean never at rest. Well did François Pyrard de Laval, in the year 1605, exclaim, "C'est u... |
de ces atollons, enuironné d'un grand banc de pierre tout autour, n'y ayant point d'artifice humain." The accompanying sketch of Whitsunday Island, in the S. Pacific, taken from Capt. Beechey's admirable Voyage, although excellent of its kind, gives but a faint idea of the singular aspect of one of these lagoon-islands... |
circle has been converted into land, which is a comparatively rare circumstance. As the reef of a lagoon-island generally supports many separate small islands, the word 'island,' applied to the whole, is often the cause of confusion; hence I have invariably used in this volume the term 'atoll,' which is the name given ... |
by their inhabitants in the Indian Ocean, and is synonymous with 'lagoon-island.' Barrier reefs, when encircling small islands, have been comparatively little noticed by voyagers; but they well deserve attention. In their structure they are little less marvellous than atolls, and they give a singular and most picturesq... |
sketch, taken from the voyage of the Coquille, the reef is seen from within, from one of the high peaks of the island of Bolabola.* Here, as in Whitsunday island, the whole of that part of the reef which is * I have taken the liberty of simplifying the foreground, and leaving out a mountainous island in the far distanc... |
visible is converted into land. This is a circumstance of rare occurrence; more usually a snow-white line of great breakers, with here and there an islet crowned by cocoa-nut trees, separates the smooth waters of the lagoon-like channel from the waves of the open sea. The barrier reefs of Australia and of New Caledonia... |
excited much attention: in structure and form they resemble those encircling many of the smaller islands in the Pacific Ocean. With respect to fringing, or shore reefs, there is little in their structure which needs explanation; and their name expresses their comparatively small extension. They differ from barrier-reef... |
within a broad channel of deep water. Reefs also occur around submerged banks of sediment and of worn-down rock; and others are scattered quite irregularly where the sea is very shallow: these in most respects are allied to those of the fringing class, but they are of comparatively little interest. I have given a separ... |
above classes, and have described some one reef or island, on which I possessed most information, as typical; and have afterwards compared it with others of a like kind. Although this classification is useful from being obvious, and from including most of the coral reefs existing in the open sea, it admits of a more fu... |
atoll-formed reefs on the one hand, where there is a great apparent diffi- culty with respect to the foundation on which they must first have grown; and into fringing reefs on the other, where, owing to the nature of the slope of the adjoining land, there is no such difficulty. The two blue tints and the red colour on ... |
map (Plate III.) represent this main division, as explained in the beginning of the last chapter. In the Appendix, every existing coral reef, except some on the coast of Brazil not included in the map, is briefly described in geographical order, as far as I possessed information; and any particular spot may be found by... |
have been advanced to explain the origin of atolls or lagoon-islands, but scarcely one to account for barrier-reefs. From the limited depths at which reef-building polypifers can flourish, taken into consideration with certain other circumstances, we are compelled to conclude, as it will be seen, that both in atolls an... |
has subsided; and that during this downward movement, the reefs have grown upwards. This conclusion, it will be further seen, explains most satisfactorily the outline and general form of atolls and barrier-reefs, and likewise certain peculiarities in their structure. The distribution, also, of the different kinds of co... |
to the points subject to volcanic eruptions, fully accord with this theory of their origin.* * A brief account of my views on coral formations, now published in my Journal of Researches, was read May 31, 1837, before the Geological Society, and an abstract has appeared in the Proceedings. ATOLLS OR LAGOON-ISLANDS. SECT... |
H.M.S. Beagle, is given in Plate I. fig. 10. The greatest width of this atoll is nine miles and a half. Its structure is in most respects characteristic of the class to which it belongs, with the exception of the shallowness of the lagoon. The accompanying woodcut represents a vertical section, supposed to be drawn at ... |
outer coast across one of the low islets (one being taken of average dimensions) to within the lagoon. A—Level of the sea at low water: where the letter A is placed, the depth is 25 fathoms, and the distance rather more than 150 yards from the edge of the reef. B—Outer edge of that flat part of the reef, which |
dries at low water: the edge either consists of a convex mound, as represented, or of rugged points, like those a little farther seaward, beneath the water. C—A flat of coral-rock, covered at high water. D—A low projecting ledge of brecciated coral-rock, washed by the waves at high water. E—A slope of loose fragments, ... |
gales: the upper part, which is from six to twelve feet high, is clothed with vegetation. The surface of the islet gently slopes to the lagoon. F—Level of the lagoon at low-water. The section is true to the scale in a horizontal line, but it could not be made so in a vertical one, as the average greatest height of |
the land is only between six and twelve feet above high-water mark. I will describe the section, commencing with the outer margin. I must first observe that the reef-building polypifers, not being tidal animals, require to be constantly submerged or washed by the breakers. I was assured by Mr. Liesk, a very intelligent... |
by some chiefs at Tahiti (Otaheite), that an exposure to the rays of the sun for a very short time invariably causes their destruction. Hence it is possible only under the most favourable circumstances, afforded by an unusually low tide and smooth water, to reach the outer margin, where the coral is alive. I succeeded ... |
part, and found it almost entirely composed of a living Porites, 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 inter... |
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 furt... |
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 su... |
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 protec... |
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, assure... |
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 ... |
the breakers. The woodcut shews the form of the bottom off the reef: the water deepens for a space between one and two hundred yards wide, very gradually to 25 fathoms (A in section), beyond which the sides plunge into the unfathomable ocean at an angle of 45°.† To the depth of ten or twelve fathoms, the bottom is exce... |
rugged, and seems formed of great masses of living coral, similar to those on the margin. The arming of the lead here invariably came up quite clean, but deeply indented, and chains and anchors which were lowered, * This Millepora, (Palmipora of Blainville,) as well as the M. alcicornis, possesses the singular property... |
delicate, as on the face and arm. † The soundings from which this section is laid down were taken with great care by Capt. FitzRoy himself: he used a bell-shaped lead, having a diameter of four inches, and the armings each time were cut off and brought on board for me to examine. The arming is a preparation of tallow, |
placed in the concavity at the bottom of the lead. Sand, and even small fragments of rock will adhere to it; and if the bottom be of rock, it brings up an exact impression of its surface. in the hopes of tearing up the coral, were broken. Many small fragments, however, of Millepora alcicornis were brought up; and on th... |
arming from an eight-fathom cast, there was a perfect impression of an Astræa, apparently alive. I examined the rolled fragments cast on the beach during gales, in order further to ascertain what corals grew outside the reef. The fragments consisted of many kinds, of which the Porites already mentioned and a Madrepora,... |
As I searched in vain in the hollows on the reef and in the lagoon, for a 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 e... |
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... |
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 an... |
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 violent... |
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, ... |
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 sec... |
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 so... |
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 prot... |
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 pec... |
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 dirt... |
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 ... |
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 gr... |
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 tow... |
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 h... |
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 ... |
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 appeara... |
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... |
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, incr... |
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 fee... |
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. Th... |
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 ... |
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,... |
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 rounde... |
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... |
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 struc... |
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 considerab... |
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 d... |
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... |
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 thes... |
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, po... |
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... |
* 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... |
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 wate... |
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 ... |
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 anim... |
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 s... |
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 prob... |
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 t... |
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 wh... |
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 ... |
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 mention... |
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,... |
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.... |
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 ... |
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 ... |
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 ... |
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... |
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