ATOLL AND REEF FORMATION 219 



subsidence, and Darwin assumed tliat the corals of the outer 

 edge would grow best ; for he said that the corals love the 

 surf, Avhere they are better aerated and more abundantly fed. 

 There is no compensating factor for the sinking land, and if 

 this process continues for some time, the second stage is 



Fig. 59. 



^77771^' 



The Illustration used by Darwin to make clear the 

 Stages of "Subsidence" in Atoll Formation. 



This second stage 



arrived at, and the island remains, surrounded by a moat, the 

 lagoon channel, which is again encircled by the barrier reef, now 

 standing out some distance from the island 

 is again a geographic reality. (See Illustration B.) 



From this stage the atoll is easily derived ; the subsidence 

 continues, the reef outside compensates by coral growth, the 

 land, uncompensated, finally disappears within ; thus the 

 typical lagoon is formed : — a salt lake, in which a peak of 

 land is buried. Such, in brief, is the theory of subsidence. 



The basis of the atoll is therefore, on this theory, a sinking 



