498 CHARLES DARWIN 



that the submarine prolongation of the land is gently inclined 

 In fact, the reefs extend only to that distance from the shore, 

 at which a foundation within the requisite depth from 20 to 

 30 fathoms is found. As far as the actual reef is concerned, 

 there is no essential difference between it and that forming 

 a barrier or an atoll : it is, however, generally of less width, 

 and consequently few islets have been formed on it. From 

 the corals growing more vigorously on the outside, and from 

 the noxious effect of the sediment washed inwards, the outer 

 edge of the reef is the highest part, and between it and the 

 land there is generally a shallow sandy channel a few feet in 

 depth. Where banks or sediments have accumulated near to 

 the surface, as in parts of the West Indies, they sometimes 

 become fringed with corals, and hence in some degree resem- 

 ble lagoon-islands or atolls, in the same manner as fringing- 

 reefs, surrounding gently sloping islands, in some degree 

 resemble barrier-reefs. 



No theory on the formation of coral-reefs can be con- 

 sidered satisfactory which does not include the three great 



AA. Outer edges of the fringing- reef, at the level of the sea. BB. 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. B'B*. The 

 shores of the now encircled island. CC. 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. 



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 ani- 

 mals requiring a foundation, and that foundation to lie at 

 no great depth. Let us then take an island surrounded by 

 f ringing-reefs, which offer no difficulty in their structure; 



