XV.] CORAL LAND. 259 



great depth of water, varying according to the extent to 

 which subsidence has gone on. By continued subsidence 

 of an island encircled by a barrier, the lagoon, C C, becomes 

 wider and wider. At length, only a few rocks may stand 

 up in the centre of the lake ; even these may at last dis- 

 appear, leaving nothing but a sheet of water surrounded by 

 the reef, and the barrier in this way becomes converted into 

 an atoll, as shown in Fig. 75. Here the original land, A, has 

 entirely disappeared beneath the growth of coral, B B, which 

 surrounds the lagoon, C. 



Assuming then that, where barrier-reefs and coral-islands 

 occur, they indicate areas of subsidence, Mr. Darwin has 

 been able to map out the Pacific and Indian oceans into 

 zones in which the land is, or has been, slowly sinking.^ 

 These zones alternate with areas in which elevation is pro- 

 bably going on, as indicated by the occurrence of active 

 volcanoes. Fringing-reefs tell less about movements of the 

 sea-bottom, for they may occur where the land is either 

 stationary or rising. In some cases, an old fringing-reef is 

 found standing high and dry above water, like a raised 

 beach, and thus showing clearly that the land has been 

 subject to elevation. 



1 The Stnicture and Distribution of Coral Reefs. By Charles Darwin, 

 M.A., F.R.S. Second Edition. 1874. 



S 2 



