502 THEORY OF CORAL-REEFS chap. 



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 sometimes 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 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 



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 con- 

 verted 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. 



SECTION OF CORAL-REEF. 



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, 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 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, the island becoming lower and 

 smaller, and the space between the inner edge of the reef and 

 the beach proportionally broader. A section of the reef and 

 island in this state, after a subsidence of several hundred feet, 

 is given by the dotted lines. Coral islets are supposed to have 

 been formed on the reef ; and a ship is anchored in the lagoon- 



