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Show ~88 ORIGIN OF frm FORM OF [Ch. XVIII. ]east degree, the geological importance of such calcareous masses. Suppose the ordinary growth of coral limestone to amount to six inches in a century, it will then require three thousand years to produce a reef fifteen feet thick; but have we any ground for presuming that, at the end of that period, or of ten times thirty centuries, there will be a failure in the supply of lime, or that the polyps and molluscs will cease to act, or that the hour of the dissolution of our planet will first arrive, as the earlier geologists were fain to anticip.llte ? Instead of contemplating the brief annals of human events, let us turn to some natural chronometers, to the volcanic isles of the Pacific, for example, which shoot up ten or fifteen thousand feet above the level of the ocean. These islands bear evident marks of having been produced by successive volcanic eruptions ; and coral reefs are sometimes found on the volcanic soil, reaching for some distance from the sea-shot·e into the interior. V\Then we consider the time required for the accumulation of such mountain masses of igneous matter according to the analogy of known volcanic agency, all idea of extenuating the comparative magnitude of coral limestones, on the ground of the slowness of the operations of lithogenous polyps, must instantly vanish. The information collected during the late expedition to the Pacific throws much additional light on the peculiarities of form and structure of coral islands. Of thirty-two of these, examined by Captain Beechey, the largest was thirty miles in diameter, and the smallest less than a mile. They were of various shapes, all formed of living coral, except one, which, although of coral formation, was raised about eighty feet above the level of the sea, and encompassed by a reef of living coral. All were increasing their dimensions by the active operations of the lithophytes which appeared to be gradually extending and brinO'ing the immersed parts of their structure to the surface. Tw;1ty-nine of the number had lagoons in their centres, whi:h had probably existed in the others, until they were filled, m the course of time, by zoophytic and other substances. Ch. XVIII.] OF COitAL ISLANDS, ~89 . I~ the above-mentioned islands, the strips of dry coral eu-circhn( l' the laO'oons 1 d' d f 1 . o o w 1en tveste o oose sandy materials heaped upon them, are rarely elevated more than two feet above the level of the sea; and were it not for the abrupt descent of the external rnarOo' in wl11'ch causes th e sea to b rea k upon it, these strips would be wholly inundated. '' Those parts of the strip which are beyond the reach of th e waves are no lon?'er inhabited by the animals that reared them, but have their cells filled with a hard calcareous SLibst ance, an d present a brown rugged appearance. The parts which are still im-mersed, or are dry at low water only, are intersected by small channels, and are so full of hollows that the tide, as it recedes, leaves small lakes of water upon them. The width of the plain or strip of dead coral, in the islands which fell under our obset·vation, in no instance exceeded half a mile from the usual wash of the sea to the edge of the lagoon, and in general was ~n1! about. three or four hundred yards*." Beyond these hm1ts t~e s1de.s ?~ the island descend rapidly, apparently by a s~ccesswn of mclmed ledges, each terminating in a precipice. 'I he ~epth of th: lagoons is various ; in some entered by Captam Beechey, It was from twenty to thirty-eight fathoms. In the annexed cut (No. 5), one of these circular islands is J"iow of W hit¨ay l sla11<l t. . * Captain Beechey, part i. p. 188. ta't ThJs plate and the section which follows are copied, by permission of ca11• m Beechey, from the illustrations of his valuable work befo1·e alluded to. Vot, II. u |
