OCR Text |
Show ~86 LINEAR DIRECTION OF CORAL ISLES. [Ch. XVlll. No.4. One ~a lud.f de_?ree Cllannel. X -----1--------.~,----1 ~··:t•,.~··· (fJ ·0· ----,;------1 E'fUitlorial (!ITallllel 7:J.• divas, on the north, as do the isles of the Chagos Archipelago, on the south, so that these may be continuations of the same chain of submarine mountains, crested in a similar manner by coral limestone. It would be rash to hazard the hypothesis, that they are all the summits of volcanos, yet we might imagine, that if Java and Sumatra were submerged, they would give rise to a somewhat similar shape in the bottom of the sea ; for the volcanos of those islands observe a linear direction, and are often separated from each other by intervals, corresponding to the atolls of the Maldivas; and as they rise to various heights, from five to ten thousand feet above their base, they might leave an unfathomable ocean in the intermediate spaces. In regard to the thickness of the masses of coral, MM. Quoy and Gaimard are of opinion, that the species which contribu_te most active] y to the formation of solid masses do not grow where the water is deeper than twenty-five or thirty feet. But the branched madrepores, which live at considerable depth, may form the first foundation of a reef, and raise a platform on which other species may build *, and the sand and ~~' JO\lrn, of Geogru.ph. Soc, of Lonuon, 18311 I>· 218, Ch. XVIII.] RATE OF TI-lE GROWTH OF CORAL. ~87 b.r oken fragments was 11 e d b y t h e waves from reefs may, in time, produce calcareous rocks of great thickness. '!'he. rapidity of th e growt h of coral I.s by no means great, acco. rdmg to the rep or t of th e nati·v es to Cap tam. Beechey. In an Island west of GambI' er ' s group, our navi·g ators observed t~~ Chama gigas (Tridacna, Lam.) while the animal was yet hvm~, so completely overgrown by coral, that a space only of two mches was left for the extremity of the shell to open and sh~t * · But conchologists suppose, that the chama may require th1:ty yea~s or more to attain its full size, so that the fact is qmte consistent with a very slow rate of increase in the calca~ eous re~fs. In the late expedition to the Pacific no positive mformat10~ ~ould ~e obtained, of any ci1annel having been filled up w1thm a given period, and it seems established that several reefs had remained for more than half a century, at about the same depth from the surface. Th~ increase of .coral limestone, however, may vary greatly accordmg to the sites of mineral springs, for these we know often issue in great numbers at the bottom of the sea in volcanic regions, as in the Mediterranean, for example, where they sometimes cause the sea at great depths to be fresher than at the surface, a phenomenon also declared by the South Sea islanders to be common in the Pacific. But when we admit the increase of coral limestone to be slow, we are merely speaking with relation to the periods of human observation. It often happens, that parasitic testacea ~ive and die on the shells of the larger slow-moving gasteropods In the South Seas, and become entirely inclosed in an incrustation of compact limestone, while the animal, to whose habitation they are attached, crawls about and bears upon his back these shells, which may be considered as already fossilized. It is, therefore, probable, that the reefs increase as fast as is compatible with the thriving state of the organic beings which chiefly contribute to their formation; and if the rate of augmen~ ation th~s implied be called, in conformity to our ordinary ldeas of time, gradual and slow, it does not diminish, in the II< :Beechey's Voyage to the Pacific, &c. p. 157, |