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Show ANNOTATIONS AND ADDITIONS. 277 being only 24 7 parts in ten thousand, while in the Categat it amounts to 371 parts in ten thousand. He is disposed to attribute this difference to the many coral-banks among the West Indian Islands, which appropriate the lime, and lower the percentage remaining in the sea-water. (Report of the 16th Meeting of the British Association for the Advancement of Science, held in 1846, p. 91.) Cha.rles Darwin has developed in a very ingenious manner the probable generic connection between fringing or shore-reefs, islandencircling reefs, and lagoon-islands, i. e. narrow ring-shaped reefs enclosing interior lagoons. According to his views, these three varieties of form are dependent on the oscillating condition of the bottom of the sea, or on periodic elevations and subsidences. The hypothesis which has been several times put forward, according to which the closed ring or annular form of the coral-reefs in Atolls or Lagoon Islands marks the configuration of a submarine volcano, -the structure having been raised on the margin of the crater, is opposed by their great dimensions, the diameters of many of them being 30, 40, and sometimes even 60 geographical miles. Our fire-emitting mountains have no such craters; and if we would compare the lagoon, with its submerged interior and narrow enclosing reef, to one of the annular mountains of the moon, we must not forget that those lunar mountains are not volcanoes, but wallsurrounded districts. According to Darwin, the process of formation is the following : He supposes a mountainous island, surrounded by a coral-reef (a "fringing reef" attached to the shore), to undergo subsidence: the "fringing reef" which subsides with the island is continually restored to its level by the tendency of the coral-animals to regain the surface of the sea, and becomes thus, as the island gradually sinks and is reduced in size, first an "encircling reef" at some distance from the included islet, and subsequently, when the latter has entirely disappeared, an Atoll. According to this view, in which islands are regarded as the culminating points of a submerged land, the relative positions of the different coral islands would disclose to us that which we could hardly learn by the sounding line, concerning the configuration of the land which was above the surface of the sea at an earlier epoch. 24 |
