SOME ANIMAL ARCHITECTS. 151 



begin to subside, the sea will flow in upon the land, and 

 the width of the reef (R E) will be increased, especially on 

 the outer face (E G) by the upward growth of the coral, so 

 that in time the belt or sheet of water between R and E 



FIG. 15. Diagram of coral-reefs (Jukes) : A u, fringing reef; c D, barrier reef. 



becomes of considerable extent, and a barrier reef is formed. 

 In the case of land of limited extent, the formation of an 

 atoll readily takes place as represented in the second figure. 

 The various lines (s s) represent the unaltering sea-level. 

 The shaded central portion represents the original land. 

 The letters B B indicate the successive upward growth of 



6. Diagram of atoll or lagoon reef (Jukes). 



the coral as the land subsided ; and as the land disappears 

 in the deep it becomes an atoll (A A), surrounded by a great 

 wall or cup of coral-structure. 



The final processes which the atoll undergoes, consist in 

 the filling up of the lagoon by debris derived from the reef, 

 and in the formation of a soil on the coral-ring by the .action 

 of the sea, which detaches fragments of coral-rock, and heaps 

 up sand on the surface of the new land. The sea will drift 

 its weeds on the coral-rock, and these will decay and form 



