Part 4 (1/2)
The remark made at the close of the last section, naturally leads to this division of our subject, which has not, I think, hitherto been considered under a right point of view. Ehrenberg (Ehrenberg, as before cited, pages 39, 46, and 50.) has stated, that in the Red Sea, the corals only coat other rocks in a layer from one to two feet in thickness, or at most to a fathom and a half; and he disbelieves that, in any case, they form, by their own proper growth, great ma.s.ses, stratum over stratum. A nearly similar observation has been made by MM. Quoy and Gaimard (”Annales des Sciences Nat.” tom. vi., page 28.), with respect to the thickness of some upraised beds of coral, which they examined at Timor and some other places.
Ehrenberg (Ehrenberg, ut sup., page 42.) saw certain large ma.s.sive corals in the Red Sea, which he imagines to be of such vast antiquity, that they might have been beheld by Pharaoh; and according to Mr. Lyell (Lyell's ”Principles of Geology,” book iii., chapter xviii.) there are certain corals at Bermuda, which are known by tradition, to have been living for centuries. To show how slowly coral-reefs grow upwards, Captain Beechey (Beechey's ”Voyage to the Pacific,” chapter viii.) has adduced the case of the Dolphin Reef off Tahiti, which has remained at the same depth beneath the surface, namely about two fathoms and a half, for a period of sixty-seven years. There are reefs in the Red Sea, which certainly do not appear (Ehrenberg, ut sup., page 43.) to have increased in dimensions during the last half-century, and from the comparison of old charts with recent surveys, probably not during the last two hundred years. These, and other similar facts, have so strongly impressed many with the belief of the extreme slowness of the growth of corals, that they have even doubted the possibility of islands in the great oceans having been formed by their agency. Others, again, who have not been overwhelmed by this difficulty, have admitted that it would require thousands, and tens of thousands of years, to form a ma.s.s, even of inconsiderable thickness; but the subject has not, I believe, been viewed in the proper light.
That ma.s.ses of considerable thickness have been formed by the growth of coral, may be inferred with certainty from the following facts. In the deep lagoons of Peros Banhos and of the Great Chagos Bank, there are, as already described, small steep-sided knolls covered with living coral.
There are similar knolls in the southern Maldiva atolls, some of which, as Captain Moresby a.s.sures me, are less than a hundred yards in diameter, and rise to the surface from a depth of between two hundred and fifty and three hundred feet. Considering their number, form, and position, it would be preposterous to suppose that they are based on pinnacles of any rock, not of coral formation; or that sediment could have been heaped up into such small and steep isolated cones. As no kind of living coral grows above the height of a few feet, we are compelled to suppose that these knolls have been formed by the successive growth and death of many individuals,--first one being broken off or killed by some accident, and then another, and one set of species being replaced by another set with different habits, as the reef rose nearer the surface, or as other changes supervened. The s.p.a.ces between the corals would become filled up with fragments and sand, and such matter would probably soon be consolidated, for we learn from Lieutenant Nelson (”Geological Transactions,” volume v., page 113.), that at Bermuda a process of this kind takes place beneath water, without the aid of evaporation. In reefs, also, of the barrier cla.s.s, we may feel sure, as I have shown, that ma.s.ses of great thickness have been formed by the growth of the coral; in the case of Vanikoro, judging only from the depth of the moat between the land and the reef, the wall of coral-rock must be at least three hundred feet in vertical thickness.
It is unfortunate that the upraised coral-islands in the Pacific have not been examined by a geologist. The cliffs of Elizabeth Island, in the Low Archipelago, are eighty feet high, and appear, from Captain Beechey's description, to consist of a h.o.m.ogeneous coral-rock. From the isolated position of this island, we may safely infer that it is an upraised atoll, and therefore that it has been formed by ma.s.ses of coral, grown together.
Savage Island seems, from the description of the younger Forster (Forster's ”Voyage round the World with Cook,” volume ii., pages 163, 167.), to have a similar structure, and its sh.o.r.es are about forty feet high: some of the Cook Islands also appear (Williams's ”Narrative of Missionary Enterprise,”
page 30.) to be similarly composed. Captain Belcher, R.N., in a letter which Captain Beaufort showed me at the admiralty, speaking of Bow atoll, says, ”I have succeeded in boring forty-five feet through coral-sand, when the auger became jammed by the falling in of the surrounding CREAMY matter.” On one of the Maldiva atolls, Captain Moresby bored to a depth of twenty-six feet, when his auger also broke: he has had the kindness to give me the matter brought up; it is perfectly white, and like finely triturated coral-rock.
In my description of Keeling atoll, I have given some facts, which show that the reef probably has grown outwards; and I have found, just within the outer margin, the great mounds of Porites and of Millepora, with their summits lately killed, and their sides subsequently thickened by the growth of the coral: a layer, also, of Nullipora had already coated the dead surface. As the external slope of the reef is the same round the whole of this atoll, and round many other atolls, the angle of inclination must result from an adaption between the growing powers of the coral, and the force of the breakers, and their action on the loose sediment. The reef, therefore, could not increase outwards, without a nearly equal addition to every part of the slope, so that the original inclination might be preserved, and this would require a large amount of sediment, all derived from the wear of corals and sh.e.l.ls, to be added to the lower part.
Moreover, at Keeling atoll, and probably in many other cases, the different kinds of corals would have to encroach on each other; thus the Nulliporae cannot increase outwards without encroaching on the Porites and Millepora complanata, as is now taking place; nor these latter without encroaching on the strongly branched Madreporet, the Millepora alcicornis, and some Astraeas; nor these again without a foundation being formed for them within the requisite depth, by the acc.u.mulation of sediment. How slow, then, must be the ordinary lateral or outward growth of such reefs. But off Christmas atoll, where the sea is much more shallow than is usual, we have good reason to believe that, within a period not very remote, the reef has increased considerably in width. The land has the extraordinary breadth of three miles; it consists of parallel ridges of sh.e.l.ls and broken corals, which furnish ”an incontestable proof,” as observed by Cook (Cook's ”Third Voyage,” book III., chapter x.), ”that the island has been produced by accessions from the sea, and is in a state of increase.” The land is fronted by a coral-reef, and from the manner in which islets are known to be formed, we may feel confident that the reef was not three miles wide, when the first, or most backward ridge, was thrown up; and, therefore, we must conclude that the reef has grown outwards during the acc.u.mulation of the successive ridges. Here then, a wall of coral-rock of very considerable breadth has been formed by the outward growth of the living margin, within a period during which ridges of sh.e.l.ls and corals, lying on the bare surface, have not decayed. There can be little doubt, from the account given by Captain Beechey, that Matilda atoll, in the Low Archipelago, has been converted in the s.p.a.ce of thirty-four years, from being, as described by the crew of a wrecked whaling vessel, a ”reef of rocks” into a lagoon-island, fourteen miles in length, with ”one of its sides covered nearly the whole way with high trees.” (Beechey's ”Voyage to the Pacific,” chapter vii. and viii.) The islets, also, on Keeling atoll, it has been shown, have increased in length, and since the construction of an old chart, several of them have become united into one long islet; but in this case, and in that of Matilda atoll, we have no proof, and can only infer as probable, that the reef, that is the foundation of the islets, has increased as well as the islets themselves.
After these considerations, I attach little importance, as indicating the ordinary and still less the possible rate of OUTWARD growth of coral-reefs, to the fact that certain reefs in the Red Sea have not increased during a long interval of time; or to other such cases, as that of Ouluthy atoll in the Caroline group, where every islet, described a thousand years before by Cantova was found in the same state by Lutke (F. Lutke's ”Voyage autour du Monde.” In the group Elato, however, it appears that what is now the islet Falipi, is called in Cantova's Chart, the Banc de Falipi. It is not stated whether this has been caused by the growth of coral, or by the acc.u.mulation of sand.),--without it could be shown that, in these cases, the conditions were favourable to the vigorous and unopposed growth of the corals living in the different zones of depth, and that a proper basis for the extent of the reef was present. The former conditions must depend on many contingencies, and in the deep oceans where coral formations most abound, a basis within the requisite depth can rarely be present.
Nor do I attach any importance to the fact of certain submerged reefs, as those off Tahiti, or those within Diego Garcia not now being nearer the surface than they were many years ago, as an indication of the rate under favourable circ.u.mstances of the UPWARD growth of reefs; after it has been shown, that all the reefs have grown to the surface in some of the Chagos atolls, but that in neighbouring atolls which appear to be of equal antiquity and to be exposed to the same external conditions, every reef remains submerged; for we are almost driven to attribute this to a difference, not in the rate of growth, but in the habits of the corals in the two cases.
In an old-standing reef, the corals, which are so different in kind on different parts of it, are probably all adapted to the stations they occupy, and hold their places, like other organic beings, by a struggle one with another, and with external nature; hence we may infer that their growth would generally be slow, except under peculiarly favourable circ.u.mstances. Almost the only natural condition, allowing a quick upward growth of the whole surface of a reef, would be a slow subsidence of the area in which it stood; if, for instance, Keeling atoll were to subside two or three feet, can we doubt that the projecting margin of live coral, about half an inch in thickness, which surrounds the dead upper surfaces of the mounds of Porites, would in this case form a concentric layer over them, and the reef thus increase upwards, instead of, as at present, outwards?
The Nulliporae are now encroaching on the Porites and Millepora, but in this case might we not confidently expect that the latter would, in their turn, encroach on the Nulliporae? After a subsidence of this kind, the sea would gain on the islets, and the great fields of dead but upright corals in the lagoon, would be covered by a sheet of clear water; and might we not then expect that these reefs would rise to the surface, as they anciently did when the lagoon was less confined by islets, and as they did within a period of ten years in the schooner-channel, cut by the inhabitants? In one of the Maldiva atolls, a reef, which within a very few years existed as an islet bearing cocoa-nut trees, was found by Lieutenant Prentice ”ENTIRELY COVERED WITH LIVE CORAL AND MADREPORE.” The natives believe that the islet was washed away by a change in the currents, but if, instead of this, it had quietly subsided, surely every part of the island which offered a solid foundation, would in a like manner have become coated with living coral.
Through steps such as these, any thickness of rock, composed of a singular intermixture of various kinds of corals, sh.e.l.ls, and calcareous sediment, might be formed; but without subsidence, the thickness would necessarily be determined by the depth at which the reef-building polypifers can exist.
If it be asked, at what rate in years I suppose a reef of coral favourably circ.u.mstanced could grow up from a given depth; I should answer, that we have no precise evidence on this point, and comparatively little concern with it. We see, in innumerable points over wide areas, that the rate has been sufficient, either to bring up the reefs from various depths to the surface, or, as is more probable, to keep them at the surface, during progressive subsidences; and this is a much more important standard of comparison than any cycle of years.
It may, however, be inferred from the following facts, that the rate in years under favourable circ.u.mstances would be very far from slow. Dr.
Allan, of Forres, has, in his MS. Thesis deposited in the library of the Edinburgh University (extracts from which I owe to the kindness of Dr.
Malcolmson), the following account of some experiments, which he tried during his travels in the years 1830 to 1832 on the east coast of Madagascar. ”To ascertain the rise and progress of the coral-family, and fix the number of species met with at Foul Point (lat.i.tude 17 deg 40') twenty species of coral were taken off the reef and planted apart on a sand-bank THREE FEET DEEP AT LOW WATER. Each portion weighed ten pounds, and was kept in its place by stakes. Similar quant.i.ties were placed in a clump and secured as the rest. This was done in December 1830. In July following, each detached ma.s.s was nearly level with the sea at low water, quite immovable, and several feet long, stretching as the parent reef, with the coast current from north to south. The ma.s.ses acc.u.mulated in a clump were found equally increased, but some of the species in such unequal ratios, as to be growing over each other.” The loss of Dr. Allan's magnificent collection by s.h.i.+pwreck, unfortunately prevents its being known to what genera these corals belonged; but from the numbers experimented on, it is certain that all the more conspicuous kinds must have been included.
Dr. Allan informs me, in a letter, that he believes it was a Madrepora, which grew most vigorously. One may be permitted to suspect that the level of the sea might possibly have been somewhat different at the two stated periods; nevertheless, it is quite evident that the growth of the ten-pound ma.s.ses, during the six or seven months, at the end of which they were found immovably fixed (It is stated by De la Beche (”Geological Manual,” page 143), on the authority of Mr. Lloyd, who surveyed the Isthmus of Panama, that some specimens of Polypifers, placed by him in a sheltered pool of water, were found in the course of a few days firmly fixed by the secretion of a stony matter, to the bottom) and several feet in length, must have been very great. The fact of the different kinds of coral, when placed in one clump, having increased in extremely unequal ratios, is very interesting, as it shows the manner in which a reef, supporting many species of coral, would probably be affected by a change in the external conditions favouring one kind more than another. The growth of the ma.s.ses of coral in N. and S. lines parallel to the prevailing currents, whether due to the drifting of sediment or to the simple movement of the water, is, also, a very interesting circ.u.mstance.
A fact, communicated to me by Lieutenant Wellstead, I.N., in some degree corroborates the result of Dr. Allan's experiments: it is, that in the Persian Gulf a s.h.i.+p had her copper bottom encrusted in the course of twenty months with a layer of coral, TWO FEET in thickness, which it required great force to remove, when the vessel was docked: it was not ascertained to what order this coral belonged. The case of the schooner-channel choked up with coral in an interval of less than ten years, in the lagoon of Keeling atoll, should be here borne in mind. We may also infer, from the trouble which the inhabitants of the Maldiva atolls take to root out, as they express it, the coral-knolls from their harbours, that their growth can hardly be very slow. (Mr. Stutchbury (”West of England Journal”, No.
I., page 50.) has described a specimen of Agaricia, ”weighing 2 lbs. 9 oz., which surrounds a species of oyster, whose age could not be more than two years, and yet is completely enveloped by this dense coral.” I presume that the oyster was living when the specimen was procured; otherwise the fact tells nothing. Mr. Stutchbury also mentions an anchor, which had become entirely encrusted with coral in fifty years; other cases, however, are recorded of anchors which have long remained amidst coral-reefs without having become coated. The anchor of the ”Beagle”, in 1832, after having been down exactly one month at Rio de Janeiro, was so thickly coated by two species of Tubularia, that large s.p.a.ces of the iron were entirely concealed; the tufts of this h.o.r.n.y zoophyte were between two and three inches in length. It has been attempted to compute, but I believe erroneously, the rate of growth of a reef, from the fact mentioned by Captain Beechey, of the Chama gigas being embedded in coral-rock. But it should be remembered, that some species of this genus invariably live, both whilst young and old, in cavities, which the animal has the power of enlarging with its growth. I saw many of these sh.e.l.ls thus embedded in the outer ”flat” of Keeling atoll, which is composed of dead rock; and therefore the cavities in this case had no relation whatever with the growth of coral. M. Lesson, also, speaking of this sh.e.l.l (Partie Zoolog.
”Voyage de la 'Coquille'”), has remarked, ”que constamment ses valves etaient engages completement dans la ma.s.se des Madrepores.”)
From the facts given in this section, it may be concluded, first, that considerable thicknesses of rock have certainly been formed within the present geological area by the growth of coral and the acc.u.mulation of its detritus; and, secondly, that the increase of individual corals and of reefs, both outwards or horizontally and upwards or vertically, under the peculiar conditions favourable to such increase, is not slow, when referred either to the standard of the average oscillations of level in the earth's crust, or to the more precise but less important one of a cycle of years.
SECTION 4.III.--ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE.
I have already described in detail, which might have appeared trivial, the nature of the bottom of the sea immediately surrounding Keeling atoll; and I will now describe with almost equal care the soundings off the fringing-reefs of Mauritius. I have preferred this arrangement, for the sake of grouping together facts of a similar nature. I sounded with the wide bell-shaped lead which Captain Fitzroy used at Keeling Island, but my examination of the bottom was confined to a few miles of coast (between Port Louis and Tomb Bay) on the leeward side of the island. The edge of the reef is formed of great shapeless ma.s.ses of branching Madrepores, which chiefly consist of two species,--apparently M. corymbosa and pocillifera,-- mingled with a few other kinds of coral. These ma.s.ses are separated from each other by the most irregular gullies and cavities, into which the lead sinks many feet. Outside this irregular border of Madrepores, the water deepens gradually to twenty fathoms, which depth generally is found at the distance of from half to three-quarters of a mile from the reef. A little further out the depth is thirty fathoms, and thence the bank slopes rapidly into the depths of the ocean. This inclination is very gentle compared with that outside Keeling and other atolls, but compared with most coasts it is steep. The water was so clear outside the reef, that I could distinguish every object forming the rugged bottom. In this part, and to a depth of eight fathoms, I sounded repeatedly, and at each cast pounded the bottom with the broad lead, nevertheless the arming invariably came up perfectly clean, but deeply indented. From eight to fifteen fathoms a little calcareous sand was occasionally brought up, but more frequently the arming was simply indented. In all this s.p.a.ce the two Madrepores above mentioned, and two species of Astraea, with rather large stars, seemed the commonest kinds (Since the preceding pages were printed off, I have received from Mr. Lyell a very interesting pamphlet, ent.i.tled ”Remarks upon Coral Formations,” etc., by J. Couthouy, Boston, United States, 1842.
There is a statement (page 6), on the authority of the Rev. J. Williams, corroborating the remarks made by Ehrenberg and Lyell (page 71 of this volume), on the antiquity of certain individual corals in the Red Sea and at Bermuda; namely, that at Upolu, one of the Navigator Islands, ”particular clumps of coral are known to the fishermen by name, derived from either some particular configuration or tradition attached to them, and handed down from time immemorial.” With respect to the thickness of ma.s.ses of coral-rock, it clearly appears, from the descriptions given by Mr. Couthouy (pages 34, 58) that Mangaia and Aurora Islands are upraised atolls, composed of coral rock: the level summit of the former is about three hundred feet, and that of Aurora Island is two hundred feet above the sea-level.); and it must be noticed that twice at the depth of fifteen fathoms, the arming was marked with a clean impression of an Astraea.
Besides these lithophytes, some fragments of the Millepora alcicornis, which occurs in the same relative position at Keeling Island, were brought up; and in the deeper parts there were large beds of a Seriatopora, different from S. subulata, but closely allied to it. On the beach within the reef, the rolled fragments consisted chiefly of the corals just mentioned, and of a ma.s.sive Porites, like that at Keeling atoll, of a Meandrina, Pocillopora verrucosa, and of numerous fragments of Nullipora.
From fifteen to twenty fathoms the bottom was, with few exceptions, either formed of sand, or thickly covered with Seriatopora: this delicate coral seems to form at these depths extensive beds unmingled with any other kind.
At twenty fathoms, one sounding brought up a fragment of Madrepora apparently M. pocillifera, and I believe it is the same species (for I neglected to bring specimens from both stations) which mainly forms the upper margin of the reef; if so, it grows in depths varying from 0 to 20 fathoms. Between 20 and 23 fathoms I obtained several soundings, and they all showed a sandy bottom, with one exception at 30 fathoms, when the arming came up scooped out, as if by the margin of a large Caryophyllia.
Beyond 33 fathoms I sounded only once; and from 86 fathoms, at the distance of one mile and a third from the edge of the reef, the arming brought up calcareous sand with a pebble of volcanic rock. The circ.u.mstance of the arming having invariably come up quite clean, when sounding within a certain number of fathoms off the reefs of Mauritius and Keeling atoll (eight fathoms in the former case, and twelve in the latter) and of its having always come up (with one exception) smoothed and covered with sand, when the depth exceeded twenty fathoms, probably indicates a criterion, by which the limits of the vigorous growth of coral might in all cases be readily ascertained. I do not, however, suppose that if a vast number of soundings were obtained round these islands, the limit above a.s.signed would be found never to vary, but I conceive the facts are sufficient to show, that the exceptions would be few. The circ.u.mstance of a GRADUAL change, in the two cases, from a field of clean coral to a smooth sandy bottom, is far more important in indicating the depth at which the larger kinds of coral flourish than almost any number of separate observations on the depth, at which certain species have been dredged up. For we can understand the gradation, only as a prolonged struggle against unfavourable conditions.
If a person were to find the soil clothed with turf on the banks of a stream of water, but on going to some distance on one side of it, he observed the blades of gra.s.s growing thinner and thinner, with intervening patches of sand, until he entered a desert of sand, he would safely conclude, especially if changes of the same kind were noticed in other places, that the presence of the water was absolutely necessary to the formation of a thick bed of turf: so may we conclude, with the same feeling of certainty, that thick beds of coral are formed only at small depths beneath the surface of the sea.