STRUCTURE OF CORAL ISLANDS. 



35 



The Chagos Bank lies about ten degrees south of the Mal- 

 dives, and is ninety miles long and seventy broad. The rim is 

 mostly submerged from five to ten fathoms. 



Mr. Darwin confirms the opinion of Captain Moresby, that this 

 bank has the character of a lagoon reef, resembling one of the 

 Maldives; and he states on the evidence of extensive soundings, 

 that, if raised to the surface, it would actually become a coral 

 island, with a lagoon forty fathoms deep. In the words of Capt. 

 Moresby, it is in truth nothing more than a half-drowned atoll.* 



Metia and other elevated Coral Islands. — In the Chagos Group 

 we have an example of a sunken coral atoll. Metia affords an 

 instance of one that has been elevated by some force ; and several 

 such are met with in the Pacific. Metia, or Aurora Island, is one 

 of the western Paumotus. It is a small island about four miles 

 by two and a half in width, and two hundred and fifty feet in 

 height ; and it consists throughout of coral limestone. As we 

 approached it from the northeast, its high vertical cliffs were sup- 

 posed to be basaltic, and had much resemblance to the Palisades 

 of the Hudson. f This appearance of a vertical structure was 

 afterwards traced to vertical furrowings by the waters dripping 

 down its front, and the consequent formation of stalagmitic incrus- 

 tations. Deep caverns were also seen. 



The cliff, though vertical in some parts, is roughly sloping in 

 others, and on the west side, the surface of the island gradually 

 declines to the sea. 



The rock was found to be a white and solid limestone, seldom 

 presenting any traces of its coral origin. In some few layers 

 there were disseminated corals, looking like imbedded fossils, 

 along with beautiful casts of shells ; but for the most part it was 

 as compact as any secondary marble, and as uniform in texture. 

 Occasionally there were disseminated spots of crystallized calc- 

 spar. 



The caverns presented us with coarse stalactites, some of which 

 were six feet in diameter; and interesting specimens were ob- 

 tained containing recent land shells, which had been enclosed 

 while hibernating.J 



channels are narrow, or few in number, although the lagoon be of great size and 

 depth, (as in Suadiva,) there are no ring-formed reefs ; where the channels pre some- 

 what broader, the marginal portions of reef, and especially those close to the larger 

 channels are ring-formed, but the central ones are not so : where they a^e broadest, 

 almost every reef throughout the atoll is more or less perfectly ring-formed. Al- 

 though their presence is thus contingent on the openness of the marginal channels, 

 the theory of their formation, as we shall hereafter see, is included in that of the 

 parent atolls, of which they form the separate portions." 



* Darwin, op. cit, p. 39. 



f For a sketch of this island, see Narrative Exp. Exp., vol. i, p. 338. 



% It is probable that more extensive caverns would have been found, had there 

 been more than a few hours for the examination of the island. The Rev. Mr. Wil- 

 liams, in his work on Missionary Enterprises in the Pacific, gives very interesting 



