464 THEORY OF CORAL REEFS. 



more vigorously on the outside, and from the noxioi 

 effect of the sediment washed inwards, the outer edge of 

 the reef is tlie highest part, and between it and the land 

 there is generally a shallow sandy channel a few feet in 

 depth. Where banks of sediment have accumulated near 

 to the surface, as in parts of the West Indies, they some- 

 times become fringed with corals, and hence in some degree 

 resemble lagoon-islands or atolls ; in the same manner as 

 fringing-reefs, surrounding gently-sloping islands, in so me 

 degree resemble barrier-reefs. i^K 



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

 sidered 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 

 \he wind and waves can throw up matter, and yet are 



i 



~^^M 



I 



»•• A 



ifi,el of Jei 



AA. Outer edges of the fringtng-reef, at the level ot the sea. BB. The 

 shores of the fringed island. 



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

 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. 



constructed by animals requiring a foundation, and that 

 foundation to lie at no great 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, repre- 

 sented by the unbroken lines in the woodcut, slowly 

 subside. Now as the island sinks down, either a few feet 

 at a time or quite insensibly, we may safely infer, from 

 what is known of the conditions favourable to the growth 

 of coral, that the living masses, bathed by the surf on the 

 margin of the reef, will soon regain the surface. The 

 water, however, will encroach little by little on the shore, 



