TllEORV OF CORAL-KEEFS. 



269 



feet in depth. Where banks of sediment have 

 accumulated near to the surface, as in parts of the 

 West Indies, they sometimes become fringed with 

 corals, and hence, in some degi-ee, resemble lagoon- 

 islands or atolls ; in the same manner as fringing- 

 reefs, suiTounding gently-sloping islands, in some 

 degree resemble barrier-reefs. 



No theory on the formation of coral-reefs can be 

 considered satisfactory which does not include the 

 three great classes. We have seen that we are 

 driven to believe in the subsidence of those vast 

 areas, interspersed with low islands, of which not 

 one rises above the height to which the wind and 

 waves can throw up matter, and yet are constructed 

 by animals requiring a foundation, and that founda- 

 tion to lie at no gi'eat depth. Let us, then, take an 

 island surrounded by fringing-reefs, which offer 

 no difficulty in their structure ; and let this island 

 with its reef, represented by the unbroken lines in 

 the woodcut, slowly subside. Now, as the island, 

 sinks down, either a few feet at a time or quite in- 

 sensibly, we may safely infer, from what is known 



A A. Outer edges of the fringing- reef, at the level of the sea. B B. The 

 shores of the fringed island. 



A'A'. Outer edges of the reef, after its upward growth during a period 

 of subsidence, now converted into a barrier, with islets on it. Ji'B'. The 

 shores of the now encircled island. C C. Lagoon-channel. 



N.B. In this and the following woodcut, the subsidence of the land could 

 be represented only by an apparent rise in the level of the sea. 



of the conditions favourable to the gTow1;h of coral, 



that the living masses, bathed by the surf on the 



Z2 



