Part 7 (2/2)
Lyell), that the areas of subsidence were likewise elongated; for the subsidence of a long, narrow s.p.a.ce of the bed of the ocean, including in it a transverse chain of mountains, surmounted by atolls, would only be marked on the map by a transverse blue band. But where a chain of atolls and barrier-reefs lies in an elongated area, between s.p.a.ces coloured red, which therefore have remained stationary or have been upraised, this must have resulted either from the area of subsidence having originally been elongated (owing to some tendency in the earth's crust thus to subside), or from the subsiding area having originally been of an irregular figure, or as broad as long, and having since been narrowed by the elevation of neighbouring districts. Thus the areas, which subsided during the formation of the great north and south lines of atolls in the Indian Ocean,--of the east and west line of the Caroline atolls,--and of the north-west and south-east line of the barrier-reefs of New Caledonia and Louisiade, must have originally been elongated, or if not so, they must have since been made elongated by elevations, which we know to belong to a recent period.
I infer from Mr. Hopkins' researches (”Researches in Physical Geology,”
Transact. Cambridge Phil. Soc., volume vi, part i.), that for the formation of a long chain of mountains, with few lateral spurs, an area elongated in the same direction with the chain, must have been subjected to an elevatory movement. Mountain-chains, however, when already formed, although running in very different directions, it seems (For instance in S. America from lat.i.tude 34 deg, for very many degrees southward there are upraised beds containing recent species of sh.e.l.ls, on both the Atlantic and Pacific side of the continent, and from the gradual ascent of the land, although with very unequal slopes, on both sides towards the Cordillera, I think it can hardly be doubted that the entire width has been upraised in ma.s.s within the recent period. In this case the two W.N.W. and E.S.E. mountain-lines, namely the Sierra Ventana and the S. Tapalguen, and the great north and south line of the Cordillera have been together raised. In the West Indies the N. and S. line of the Eastern Antilles, and the E. and W. line of Jamaica, appear both to have been upraised within the latest geological period.) may be raised together by a widely-acting force: so, perhaps, mountain-chains may subside together. Hence, we cannot tell, whether the Caroline and Marshall Archipelagoes, two groups of atolls running in different directions and meeting each other, have been formed by the subsidence of two areas, or of one large area, including two distinct lines of mountains. We have, however, in the southern prolongation of the Mariana Islands, probable evidence of a line of recent elevation having intersected one of recent subsidence. A view of the map will show that, generally, there is a tendency to alternation in the parallel areas undergoing opposite kinds of movement; as if the sinking of one area balanced the rising of another.
The existence in many parts of the world of high table-land, proves that large surfaces have been upraised in ma.s.s to considerable heights above the level of the ocean; although the highest points in almost every country consist of upturned strata, or erupted matter: and from the immense s.p.a.ces scattered with atolls, which indicate that land originally existed there, although not one pinnacle now remains above the level of the sea, we may conclude that wide areas have subsided to an amount, sufficient to bury not only any formerly existing table-land, but even the heights formed by fractured strata, and erupted matter. The effects produced on the land by the later elevatory movements, namely, successively rising cliffs, lines of erosion, and beds of literal sh.e.l.ls and pebbles, all requiring time for their production, prove that these movements have been very slow; we can, however, infer this with safety, only with respect to the few last hundred feet of rise. But with reference to the whole vast amount of subsidence, necessary to have produced the many atolls widely scattered over immense s.p.a.ces, it has already been shown (and it is, perhaps, the most interesting conclusion in this volume), that the movements must either have been uniform and exceedingly slow, or have been effected by small steps, separated from each other by long intervals of time, during which the reef-constructing polypifers were able to bring up their solid frameworks to the surface. We have little means of judging whether many considerable oscillations of level have generally occurred during the elevation of large tracts; but we know, from clear geological evidence, that this has frequently taken place; and we have seen on our map, that some of the same islands have both subsided and been upraised. I conclude, however, that most of the large blue s.p.a.ces, have subsided without many and great elevatory oscillations, because only a few upraised atolls have been observed: the supposition that such elevations have taken place, but that the upraised parts have been worn down by the surf, and thus have escaped observation, is overruled by the very considerable depth of the lagoons of all the larger atolls; for this could not have been the case, if they had suffered repeated elevations and abrasion. From the comparative observations made in these latter pages, we may finally conclude, that the subterranean changes which have caused some large areas to rise, and others to subside, have acted in a very similar manner.
RECAPITULATION.
In the three first chapters, the princ.i.p.al kinds of coral-reefs were described in detail, and they were found to differ little, as far as relates to the actual surface of the reef. An atoll differs from an encircling barrier-reef only in the absence of land within its central expanse; and a barrier-reef differs from a fringing-reef, in being placed at a much greater distance from the land with reference to the probable inclination of its submarine foundation, and in the presence of a deep-water lagoon-like s.p.a.ce or moat within the reef. In the fourth chapter the growing powers of the reef-constructing polypifers were discussed; and it was shown, that they cannot flourish beneath a very limited depth. In accordance with this limit, there is no difficulty respecting the foundations on which fringing-reefs are based; whereas, with barrier-reefs and atolls, there is a great apparent difficulty on this head; in barrier-reefs from the improbability of the rock of the coast or of banks of sediment extending, in every instance, so far seaward within the required depth;--and in atolls, from the immensity of the s.p.a.ces over which they are interspersed, and the apparent necessity for believing that they are all supported on mountain-summits, which although rising very near to the surface-level of the sea, in no one instance emerge above it. To escape this latter most improbable admission, which implies the existence of submarine chains of mountains of almost the same height, extending over areas of many thousand square miles, there is but one alternative; namely, the prolonged subsidence of the foundations, on which the atolls were primarily based, together with the upward growth of the reef-constructing corals. On this view every difficulty vanishes; fringing reefs are thus converted into barrier-reefs; and barrier-reefs, when encircling islands, are thus converted into atolls, the instant the last pinnacle of land sinks beneath the surface of the ocean.
Thus the ordinary forms and certain peculiarities in the structure of atolls and barrier-reefs can be explained;--namely, the wall-like structure on their inner sides, the basin or ring-like shape both of the marginal and central reefs in the Maldiva atolls--the union of some atolls as if by a ribbon--the apparent disseverment of others--and the occurrence, in atolls as well as in barrier-reefs, of portions of reef, and of the whole of some reefs, in a dead and submerged state, but retaining the outline of living reefs. Thus can be explained the existence of breaches through barrier-reefs in front of valleys, though separated from them by a wide s.p.a.ce of deep water; thus, also, the ordinary outline of groups of atolls and the relative forms of the separate atolls one to another; thus can be explained the proximity of the two kinds of reefs formed during subsidence, and their separation from the s.p.a.ces where fringing-reefs abound. On searching for other evidence of the movements supposed by our theory, we find marks of change in atolls and in barrier-reefs, and of subterranean disturbances under them; but from the nature of things, it is scarcely possible to detect any direct proofs of subsidence, although some appearances are strongly in favour of it. On the fringed coasts, however, the presence of upraised marine bodies of a recent epoch, plainly show, that these coasts, instead of having remained stationary, which is all that can be directly inferred from our theory, have generally been elevated.
Finally, when the two great types of structure, namely barrier-reefs and atolls on the one hand, and fringing-reefs on the other, were laid down in colours on our map, a magnificent and harmonious picture of the movements, which the crust of the earth has within a late period undergone, is presented to us. We there see vast areas rising, with volcanic matter every now and then bursting forth through the vents or fissures with which they are traversed. We see other wide s.p.a.ces slowly sinking without any volcanic outburst, and we may feel sure, that this sinking must have been immense in amount as well as in area, thus to have buried over the broad face of the ocean every one of those mountains, above which atolls now stand like monuments, marking the place of their former existence.
Reflecting how powerful an agent with respect to denudation, and consequently to the nature and thickness of the deposits in acc.u.mulation, the sea must ever be, when acting for prolonged periods on the land, during either its slow emergence or subsidence; reflecting, also, on the final effects of these movements in the interchange of land and ocean-water on the climate of the earth, and on the distribution of organic beings, I may be permitted to hope, that the conclusions derived from the study of coral-formations, originally attempted merely to explain their peculiar forms, may be thought worthy of the attention of geologists.
APPENDIX.
CONTAINING A DETAILED DESCRIPTION OF THE REEFS AND ISLANDS IN PLATE III.
In the beginning of the last chapter I stated the principles on which the map is coloured. There only remains to be said, that it is an exact copy of one by M. C. Gressier, published by the Depot General de la Marine, in 1835. The names have been altered into English, and the longitude has been reduced to that of Greenwich. The colours were first laid down on accurate charts, on a large scale. The data, on which the volcanoes historically known to have been in action, have been marked with vermillion, were given in a note to the last chapter. I will commence my description on the eastern side of the map, and will describe each group of islands consecutively, proceeding westward across the Pacific and Indian Oceans, but ending with the West Indies.
The WESTERN Sh.o.r.eS OF AMERICA appear to be entirely without coral-reefs; south of the equator the survey of the ”Beagle”, and north of it, the published charts show that this is the case. Even in the Bay of PANAMA, where corals flourish, there are no true coral-reefs, as I have been informed by Mr. Lloyd. There are no coral-reefs in the GALAPAGOS Archipelago, as I know from personal inspection; and I believe there are none on the COCOS, REVILLA-GIGEDO, and other neighbouring islands.
CLIPPERTON rock, 10 deg N., 109 deg W., has lately been surveyed by Captain Belcher; in form it is like the crater of a volcano. From a drawing appended to the MS. plan in the Admiralty, it evidently is not an atoll.
The eastern parts of the Pacific present an enormous area, without any islands, except EASTER, and SALA, and GOMEZ Islands, which do not appear to be surrounded by reefs.
THE LOW ARCHIPELAGO.
This group consists of about eighty atolls: it will be quite superfluous to refer to descriptions of each. In D'Urville and Lottin's chart, one island (WOLCHONSKY) is written with a capital letter, signifying, as explained in a former chapter, that it is a high island; but this must be a mistake, as the original chart by Bellinghausen shows that it is a true atoll. Captain Beechey says of the thirty-two groups which he examined (of the greater number of which I have seen beautiful MS. charts in the Admiralty), that twenty-nine now contain lagoons, and he believes the other three originally did. Bellinghausen (see an account of his Russian voyage, in the ”Biblioth. des Voyages,” 1834, page 443) says, that the seventeen islands which he discovered resembled each other in structure, and he has given charts on a large scale of all of them. Kotzebue has given plans of several; Cook and Bligh mention others; a few were seen during the voyage of the ”Beagle”; and notices of other atolls are scattered through several publications. The ACTAEON group in this archipelago has lately been discovered (”Geographical Journal”, volume vii., page 454); it consists of three small and low islets, one of which has a lagoon. Another lagoon-island has been discovered (”Naut. Mag.” 1839, page 770), in 22 deg 4' S., and 136 deg 20' W. Towards the S.E. part of the group, there are some islands of different formation: ELIZABETH Island is described by Beechey (page 46, 4to edition) as fringed by reefs, at the distance of between two and three hundred yards; coloured red. PITCAIRN Island, in the immediate neighbourhood, according to the same authority, has no reefs of any kind, although numerous pieces of coral are thrown up on the beach; the sea close to its sh.o.r.e is very deep (see ”Zool. of Beechey's Voyage,” page 164); it is left uncoloured. GAMBIER Islands (see Plate I., Figure 8), are encircled by a barrier-reef; the greatest depth within is thirty-eight fathoms; coloured pale blue. AURORA Island, which lies N.E. of Tahiti close to the large s.p.a.ce coloured dark blue in the map, has been already described in a note (page 71), on the authority of Mr. Couthouy; it is an upraised atoll, but as it does not appear to be fringed by living reefs, it is left uncoloured.
The SOCIETY Archipelago is separated by a narrow s.p.a.ce from the Low Archipelago; and in their parallel direction they manifest some relation to each other. I have already described the general character of the reefs of these fine encircled islands. In the ”Atlas of the 'Coquille's' Voyage”
there is a good general chart of the group, and separate plans of some of the islands. TAHITI, the largest island in the group, is almost surrounded, as seen in Cook's chart, by a reef from half a mile to a mile and a half from the sh.o.r.e, with from ten to thirty fathoms within it. Some considerable submerged reefs lying parallel to the sh.o.r.e, with a broad and deep s.p.a.ce within, have lately been discovered (”Naut. Mag.” 1836, page 264) on the N.E. coast of the island, where none are laid down by Cook. At EIMEO the reef ”which like a ring surrounds it, is in some places one or two miles distant from the sh.o.r.e, in others united to the beach” (Ellis, ”Polynesian Researches,” volume i., page 18, 12mo edition). Cook found deep water (twenty fathoms) in some of the harbours within the reef. Mr.
Couthouy, however, states (”Remarks,” page 45) that both at Tahiti and Eimeo, the s.p.a.ce between the barrier-reef and the sh.o.r.e, has been almost filled up,--”a nearly continuous fringing-reef surrounding the island, and varying from a few yards to rather more than a mile in width, the lagoons merely forming ca.n.a.ls between this and the sea-reef,” that is the barrier-reef. TAPAMANOA is surrounded by a reef at a considerable distance from the sh.o.r.e; from the island being small it is breached, as I am informed by the Rev. W. Ellis, only by a narrow and crooked boat channel. This is the lowest island in the group, its height probably not exceeding 500 feet. A little way north of Tahiti, the low coral-islets of TETUROA are situated; from the description of them given me by the Rev. J. Williams (the author of the ”Narrative of Missionary Enterprise”), I should have thought they had formed a small atoll, and likewise from the description given by the Rev. D. Tyerman and G. Bennett (”Journal of Voyage and Travels,” volume i., page 183), who say that ten low coral-islets ”are comprehended within one general reef, and separated from each other by interjacent lagoons;” but as Mr. Stutchbury (”West of England Journal,” volume i., page 54) describes it as consisting of a mere narrow ridge, I have left it uncoloured. MAITEA, eastward of the group, is cla.s.sed by Forster as a high encircled island; but from the account given by the Rev. D. Tyerman and G. Bennett (volume i., page 57) it appears to be an exceedingly abrupt cone, rising from the sea without any reef; I have left it uncoloured. It would be superfluous to describe the northern islands in this group, as they may be well seen in the chart accompanying the 4to edition of Cook's ”Voyages,” and in the ”Atlas of the 'Coquille's' Voyage.” MAURUA is the only one of the northern islands, in which the water within the reef is not deep, being only four and a half fathoms; but the great width of the reef, stretching three miles and a half southward of the land (which is represented in the drawing in the ”Atlas of the 'Coquille's' Voyage” as descending abruptly to the water) shows, on the principle explained in the beginning of the last chapter, that it belongs to the barrier cla.s.s. I may here mention, from information communicated to me by the Rev. W. Ellis, that on the N.E. side of HUAHEINE there is a bank of sand, about a quarter of a mile wide, extending parallel to the sh.o.r.e, and separated from it by an extensive and deep lagoon; this bank of sand rests on coral-rock, and undoubtedly was originally a living reef. North of Bolabola lies the atoll of TOUBAI (Motou-iti of the ”'Coquille's' Atlas”) which is coloured dark blue; the other islands, surrounded by barrier-reefs, are pale blue; three of them are represented in Figures 3, 4, and 5, in Plate I. There are three low coral-groups lying a little E. of the Society Archipelago, and almost forming part of it, namely BELLINGHAUSEN, which is said by Kotzebue (”Second Voyage,” volume ii., page 255), to be a lagoon-island; MOPEHA, which, from Cook's description (”Second Voyage,” book iii., chapter i.), no doubt is an atoll; and the SCILLY Islands, which are said by Wallis (”Voyage,” chapter ix.) to form a GROUP of LOW islets and shoals, and, therefore, probably, they compose an atoll: the two former have been coloured blue, but not the latter.
MENDANA OR MARQUESAS GROUP.
These islands are entirely without reefs, as may be seen in Krusenstern's Atlas, making a remarkable contrast with the adjacent group of the Society Islands. Mr. F.D. Bennett has given some account of this group, in the seventh volume of the ”Geographical Journal”. He informs me that all the islands have the same general character, and that the water is very deep close to their sh.o.r.es. He visited three of them, namely, DOMINICANA, CHRISTIANA, and ROAPOA; their beaches are strewed with rounded ma.s.ses of coral, and although no regular reefs exist, yet the sh.o.r.e is in many places lined by coral-rock, so that a boat grounds on this formation. Hence these islands ought probably to come within the cla.s.s of fringed islands and be coloured red; but as I am determined to err on the cautious side, I have left them uncoloured.
COOK OR HARVEY AND AUSTRAL ISLAND.
PALMERSTON Island is minutely described as an atoll by Captain Cook during his voyage in 1774; coloured blue. AITUTAKI was partially surveyed by the ”Beagle” (see map accompanying ”Voyages of 'Adventure' and 'Beagle'”); the land is hilly, sloping gently to the beach; the highest point is 360 feet; on the southern side the reef projects five miles from the land: off this point the ”Beagle” found no bottom with 270 fathoms: the reef is surmounted by many low coral-islets. Although within the reef the water is exceedingly shallow, not being more than a few feet deep, as I am informed by the Rev. J. Williams, nevertheless, from the great extension of this reef into a profoundly deep ocean, this island probably belongs, on the principle lately adverted to, to the barrier cla.s.s, and I have coloured it pale blue; although with much hesitation.--MANOUAI or HARVEY Island. The highest point is about fifty feet: the Rev. J. Williams informs me that the reef here, although it lies far from the sh.o.r.e, is less distant than at Aitutaki, but the water within the reef is rather deeper: I have also coloured this pale blue with many doubts.--Round MITIARO Island, as I am informed by Mr. Williams, the reef is attached to the sh.o.r.e; coloured red.
--MAUKI or Maouti; the reef round this island (under the name of Parry Island, in the ”Voyage of H.M.S. 'Blonde',” page 209) is described as a coral-flat, only fifty yards wide, and two feet under water. This statement has been corroborated by Mr. Williams, who calls the reef attached; coloured red.--AITU, or Wateeo; a moderately elevated hilly island, like the others of this group. The reef is described in Cook's ”Voyage,” as attached to the sh.o.r.e, and about one hundred yards wide; coloured red.--FENOUA-ITI; Cook describes this island as very low, not more than six or seven feet high (volume i., book ii., chapter iii, 1777); in the chart published in the ”'Coquille's' Atlas,” a reef is engraved close to the sh.o.r.e: this island is not mentioned in the list given by Mr.
Williams (page 16) in the ”Narrative of Missionary Enterprise;” nature doubtful. As it is so near Atiu, it has been unavoidably coloured red.-- RAROTONGA; Mr. Williams informs me that it is a lofty basaltic island with an attached reef; coloured red.--There are three islands, ROUROUTI, ROXBURGH, and HULL, of which I have not been able to obtain any account, and have left them uncoloured. Hull Island, in the French chart, is written with small letters as being low.--MANGAIA; height about three hundred feet; ”the surrounding reef joins the sh.o.r.e” (Williams, ”Narrative,” page 18); coloured red.--RIMETARA; Mr. Williams informs me that the reef is rather close to the sh.o.r.e; but, from information given me by Mr. Ellis, the reef does not appear to be quite so closely attached to it as in the foregoing cases: the island is about three hundred feet high (”Naut. Mag.” 1839, page 738); coloured red.--RURUTU; Mr. Williams and Mr.
Ellis inform me that this island has an attached reef; coloured red. It is described by Cook under the name of Oheteroa: he says it is not surrounded, like the neighbouring islands by a reef; he must have meant a distant reef.--TOUBOUAI; in Cook's chart (”Second Voyage,” volume ii., page 2) the reef is laid down in part one mile, and in part two miles from the sh.o.r.e. Mr. Ellis (”Polynes. Res.” volume iii., page 381) says the low land round the base of the island is very extensive; and this gentleman informs me that the water within the reef appears deep; coloured blue.--RAIVAIVAI, or Vivitao; Mr. Williams informs me that the reef is here distant: Mr.
Ellis, however, says that this is certainly not the case on one side of the island; and he believes that the water within the reef is not deep; hence I have left it uncoloured.--LANCASTER Reef, described in ”Naut. Mag.” 1833 (page 693), as an extensive crescent-formed coral-reef. I have not coloured it.--RAPA, or Oparree; from the accounts given of it by Ellis and Vancouver, there does not appear to be any reef.--I. DE Ba.s.s is an adjoining island, of which I cannot find any account.--KEMIN Island; Krusenstern seems hardly to know its position, and gives no further particulars.
ISLANDS BETWEEN THE LOW AND GILBERT ARCHIPELAGOES.
CAROLINE Island (10 deg S., 150 deg W.) is described by Mr. F.D. Bennett (”Geographical Journal”, volume vii., page 225) as containing a fine lagoon; coloured blue.--FLINT Island (11 deg S., 151 deg W.); Krusenstern believes that it is the same with Peregrino, which is described by Quiros (Burney's ”Chron. Hist.” volume ii., page 283) as ”a cl.u.s.ter of small islands connected by a reef, and forming a lagoon in the middle;” coloured blue.--WOSTOCK is an island a little more than half a mile in diameter, and apparently quite flat and low, and was discovered by Bellinghausen; it is situated a little west of Caroline Island, but it is not placed on the French charts; I have not coloured it, although I entertain little doubt from the chart of Bellinghausen, that it originally contained a small lagoon.--PENRHYN Island (9 deg S., 158 deg W.); a plan of it in the ”Atlas of the First Voyage” of Kotzebue, shows that it is an atoll; blue.-- SLARBUCK Island (5 deg S., 156 deg W.) is described in Byron's ”Voyage in the 'Blonde'” (page 206) as formed of a flat coral-rock, with no trees; the height not given; not coloured.--MALDEN Island (4 deg S., 154 deg W.); in the same voyage (page 205) this island is said to be of coral formation, and no part above forty feet high; I have not ventured to colour it, although, from being of coral-formation, it is probably fringed; in which case it should be red.--JARVIS, or BUNKER Island (0 deg 20' S., 160 deg W.) is described by Mr. F.D. Bennett (”Geographical Journal”, volume vii., page 227) as a narrow, low strip of coral-formation; not coloured.--BROOK, is a small low island between the two latter; the position, and perhaps even the existence of it is doubtful; not coloured.--PESCADO and HUMPHREY Islands; I can find out nothing about these islands, except that the latter appears to be small and low; not coloured.--REARSON, or Grand Duke Alexander's (10 S., 161 deg W.); an atoll, of which a plan is given by Bellinghausen; blue.-- SOUVOROFF Islands (13 deg S., 163 deg W.); Admiral Krusenstern, in the most obliging manner, obtained for me an account of these islands from Admiral Lazareff, who discovered them. They consist of five very low islands of coral-formation, two of which are connected by a reef, with deep water close to it. They do not surround a lagoon, but are so placed that a line drawn through them includes an oval s.p.a.ce, part of which is shallow; these islets, therefore, probably once (as is the case with some of the islands in the Caroline Archipelago) formed a single atoll; but I have not coloured them.--DANGER Island (10 deg S., 166 deg W.); described as low by Commodore Byron, and more lately surveyed by Bellinghausen; it is a small atoll with three islets on it; blue.--CLARENCE Island (9 deg S., 172 deg W.); discovered in the ”Pandora” (G. Hamilton's ”Voyage,” page 75): it is said, ”in running along the land, we saw several canoes crossing the LAGOONS;” as this island is in the close vicinity of other low islands, and as it is said, that the natives make reservoirs of water in old cocoa-nut trees (which shows the nature of the land), I have no doubt it is an atoll, and have coloured it blue. YORK Island (8 deg S., 172 deg W.) is described by Commodore Byron (chapter x. of his ”Voyage”) as an atoll; blue.--SYDNEY Island (4 deg S., 172 deg W.) is about three miles in diameter, with its interior occupied by a lagoon (Captain Tromelin, ”Annal. Marit.” 1829, page 297); blue.--PHOENIX Island (4 deg S., 171 deg W.) is nearly circular, low, sandy, not more than two miles in diameter, and very steep outside (Tromelin, ”Annal. Marit.” 1829, page 297); it may be inferred that this island originally contained a lagoon, but I have not coloured it.--NEW NANTUCKET (0 deg 15' N., 174 deg W.). From the French chart it must be a low island; I can find nothing more about it or about MARY Island; both uncoloured.--GARDNER Island (5 deg S., 174 deg W.) from its position is certainly the same as KEMIN Island described (Krusenstern, page 435, Appen.
to Mem., published 1827) as having a lagoon in its centre; blue.
<script>