ANNOTATIONS AND ADDITIONS. 277 



being only 247 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 per centage 

 remaining in the sea- water. (Report of the 16th Meeting of the 

 British Association for the Advancement of Science, held in 1846, 

 p. 91.) 



Charles Darwin has developed in a very ingenious manner the 

 probable genetic connection between fringing or shore-reefs, island- 

 encircling 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 en- 

 closing reef, to one of the annular mountains of the moon, we must 

 not forget that those lunar mountains are not volcanoes, but wall- 

 surrounded districts. According to Darwin, the process of forma- 

 tion is the following : He supposes a mountainous island, sur- 

 rounded by a coral-reef (a lt 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 " encir- 

 cling reef" at some distance from the included islet, and subse- 

 quently, when the latter has entirely disappeared, an Atoll. Ac- 

 cording to this view, in which islands are regarded as the culmi- 

 nating 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. 

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