THEORY OF CORAL-REEFS 



there is generally a shallow sand)- clianncl a few feet in depth. 

 AA'liere banks of sediment liave 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 

 satisfactor}' which docs not include the three great classes. 

 We ]ia\'e seen that we are driven to believe in the subsidence 

 of those \ast areas, interspersed with low islands, of which not 

 one rises above the height to which the \\-ind and waves can 

 tlirow up matter, and )-et arc constructed b\' animals requiring 



^iliiiiililiiiiiiiiii" "' ''''''I'/lmmim'- 



AA, Outer cd'-e-; of the frin^ina-reef, at the level of the sea. I'T!, The >^ho^es of the friiiElcJ 

 i^la^,J. 



\ .\! , Outer edges of tlie reef, after it^ iip\\-ard growth durin,t; a period uf stibsidence, now coii- 

 \erted into a b.arrier, with islets on it. IVK', I'he 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 [he Ie\el of the sea. 



a foundation, and that foundation to lie at no great depth. 

 Let us then take an island surrounded b}' fringing-reefs, which 

 offer no difficulty in their structure ; and let this island with its 

 reef, represented b)' the unbroken lines in the woodcut, s!owl_\- 

 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 b)- the surf on the margin of the reef, 

 will soon regain the surface. The water, ho\\-evcr, 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 proportional!)- broader. A section of the reef and 

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

 is gi\-en b\' the dotted lines. Coral islets are supposed to have 

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



