THE OCEAN AND ITS WORK 



245 



Subsidence Theory of Darwin. — This theory holds that on a slowly 

 sinking island (Fig. 236) a fringing reef would be built to the surface 

 by the accumulation of the calcareous remains of the corals and other 

 animals, and plants which nourish under similar conditions. It is 

 apparent that since corals grow best on the outside of a reef where 

 the waves beat freely and there is an almost complete absence of sedi- 

 ment, a fringing reef would in time, if the sea bottom slowly sub- 

 sided, become a barrier reef, separated from the island by a lagoon. 



FRINGING REEF STAGE 



ENCIRCLING REEF STAGE 



PLAN 



Fig. 236. — Diagram illustrating Darwin's theory of coral islands, showing the 

 fringing reef stage before subsidence, the encircling reef stage after some subsidence 

 (dotted line), and the atoll stage after the island had been completely submerged. 



It is also readily seen that under such conditions the lagoons would 

 become deeper and wider as the subsidence proceeded. The distance 

 of the barrier reef from the island, under these conditions, would de- 

 pend upon the slope of the island and the amount of sinking. If sub- 

 sidence continued, the peak of the original island would eventually 

 disappear and an atoll would be left. The following have been offered 

 as proofs of this theory. Islands surrounded by barrier reefs are 

 characterized (1) by an embayed shore line which, as has been seen 

 (p. 228), indicates subsidence ; (2) by the absence of delta plains in 

 the indentations, such as would be present if the island had stood at 

 the same level for a long period of time; and (3) by ridges that do 



