1888.] and the Coral Formations of the Indian Ocean. 447 



rock would be formed over the last layer of sand. I have seen quite 

 analogous formations in progress in a fathom of water a little way 

 above Point Marianne. Raise the formation to the surface, and you 

 get that stratification which occurs in so many parts of the island, a 

 stratification which cannot be explained on any theory of subsidence, 

 and is scarcely less difficult to explain on the supposition of rest. At 

 first I had some hesitation in extending to an island on the borders 

 of the lagoon, as is East Island, a view of the formation of layers of 

 sand and rock derived from an inspection of the interior of the 

 lagoon, but afterwards I saw that similar layers were being formed 

 just within the large reef known as Spurs' Reef, west of Middle 

 Island, so that no objection can be raised on that score. The whole 

 character of the Chagos Group is very much opposed to the theory 

 that atolls and barrier reefs are formed during subsidence. There 

 are several atolls rising above the waves,, that of Peros Banhos being 

 -55 miles in circuit, and composed of numerous small islets placed 

 upon a ring-shaped reef through which there are several large and 

 deep channels. Egmont or Six Islands is an instance of an atoll in 

 which the encircling reef is perfect and unbroken by any channels, 

 the laud consisting of six islets placed for the most part on the 

 southern and western sides of the reef. There are several submerged 

 banks, nearly all of which have an atoll form. Of these the best 

 known is the Great Chagos Bank, a huge submerged atoll 95 miles 

 long and 65 miles broad, having a depth of 4 — 10 fathoms over a 

 narrow rim around its periphery, and a central lagoon of a depth 

 varying up to 45 fathoms. South-west of the Great Chagos Bank, 

 distant less than 15 miles, lies the atoll of Six Islands, and on the 

 other side of these, scarcely 12 miles distant, lies another submerged 

 atoll, known as Pitt Bank. South-west of Pitt Bank are two smaller 

 banks, Ganges and Centurion's Banks. Darwin considered that the 

 Great Chagos Bank afforded particularly good evidence of the truth 

 of the subsidence theory. He regarded it as an atoll carried down by 

 a too rapid subsidence below the depth at which reef-building corals 

 flourish. The same would be the case for Pitts Bank and the two 

 others just mentioned. A more intimate knowledge of the Great 

 Chagos Bank, and of the relations of it and other submerged banks 

 to existing land, shows this view to be untenable. In the first place 

 the rim of the Great Chagos Bank is on an average not more than 

 6 fathoms below the surface, and therefore situated in a depth 

 eminently favourable for coral growth, and there are actually six islets 

 on the northern aud western edges rising above the water, and some 

 of them inhabited. Secondly, any such rapid subsidence could not 

 have affected areas only 30 miles apart without involving the Six 

 Islands atoll lying directly between them. A similar argument might 

 be extended to the- more northern islands of the Chagos group, and 



