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Show ~90 ORIGIN OF THE FORM [Ch. XVIII. represented just rising above the waves, covered with the cocoa-nut and other trees, and inclosing within, a lagoon of tranquil water. The accompanying section will enable the reader to com-prehend the usual form of such islands. (No.6.) No.6. S ecticm of a Coral Illand. (a a) Habitable part of the island, consisting of a strip of coral, inclosing the lagoon. ( b b) The lagoon. The subjoined cut (No. 7) exhibits a small part of the section of a coral island on a larger scale. No.7. S ection of part of a Coral Ialand. (a b) Habitable part of the island. (be) Slope of the side of the island, plunging at an angle of forty-five to tbe depth of fifteen hundred feet. ( c c) Part of the lagoon. ( d d) Knolls of coral in the lagoon, with over-hanging masses of coral, re-sembling the capitals of columns. The circular or oval forms of the numerous coral isles of the Pacific, with the lagoons in their centre, naturally suggest the idea that they are nothing more than die crests of submarine volcanos, having the rims and bottoms of their craters overgrown by corals. This opinion is strengthened by the conical f.orm of the submarine mountain, and the steep angle at which it plunges on all sides into the surrounding ocean. It is also well known that the Pacific is a great theatre of volcanic action, and every island yet examined in the wide region termed Eastern Oceanica, consists either of volcanic rocks or coral limestones. It has also been observed that, although within the circular coral reefs, there is usually nothing discernible but a lagoon, Ch. XVIII.] OF CORAL ISLANDS. ~91 the bottom of which 1• 5 covere d w·t t h cora1, yet w.t t hm' some of these basins ' as in G am bt'e r •s group, rocks composed of porous lava an~ other volcanic substances, rise up, resembling the two Kamem s, and oth.er eminences of igneous origin, which have been thrown up wtthin the times of history, in the midst of the Gulf of Santorin *. We mentioned that in volcanic archipelagos there is generally one large habitual vent, and many smaller volcanos for~ed at different points and at irregular intervals, all of whtch have usually a linear arrangement. Now in several of t~e gr~~ps of · Eastern Oceanica there appears to be a similar dispositiOn, the gr~~t islands, such as Otaheite, Owhyhee, ~nd Terra del Sptrtto Santo, being habitual vents, and the lm~s of small ·circular coral isles which are dependent on them bemg. very pr~bah!y trains of minor volcanos, which. may have been m eruptiOn smgly and at irregular intervals. The absence of circular groups in the West Indian seas and the tropical parts of the Atlantic, where corals are nume;ous, has be.en adduced as an additional argument, inasmuch as vol.can~c ~ents, though existing in those regions, are very inferlOr m tmpo.rtance to those in the Pacific and Indian seas t. It may be obJected that the circles formed by some coral reefs or. groups of coral islets, varying as they do from ten to thirty ~mles and ~pwards in diameter, are so great as to preclude the l~ea of their being volcanic craters. In regard to this objectiOn we may refer to what we have said in a former volume respecting the size of the so-called craters of elevation many of which, .we conceive, may be the ruins of truncated c:nes:j:. There Is yet another phenomenon attendinO' the circular reefs, to which we have not alluded, viz., th: deep narrow passage which almost invariably leads from the sea into the lagoon, and is kept open by the efllux of the sea at low tides. It is sufficient that a reef should rise a few feet above low-water mark to cause the waters to collect in the laO'oon at hiO'h tide * See vol. i. p. 386. t Dela Beche, Geol, Man. p. 141. b 0 ' t See vol. i. p. 388. U2 |
