66 THE BERMUDA ISLANDS. 



of coral growth is concentrated in the first zone of some fifty 

 or sixty feet, which would practically mark the depth at which 

 a bounding rim of accelerated growth would be formed, and 

 also fix the depth of the lagoons.* But as has already been 

 seen, the depth of nearly all extensive lagoons is very much 

 greater, in some cases six times as great, or more. 



The difficulty in the premises disappears almost entirely if 

 we accept Mr. Darwin's hypothesis of subsidence, for here the 

 accelerated outer growth is assumed to depend no less upon 

 interior retardation (as the result of the accumulation of in- 

 jurious sediment), as upon an actual increase in the quantity 

 of the food-supply. The depth and size of the lagoon will 

 then depend upon the extent of land that has undergone sub- 

 sidence, and upon the measure of its submergence. Where 

 the descent is very gradual the upward development of the 

 coral structures may by overgrowth completely close out the 

 lagoon ; where, on the other hand, the descent is unusually rapid, ' 

 more rapid than the compensating upward growth of the corals, 

 a " drowned " atoll may be the result. The great Chagos Bank, 

 which is situated some 700 miles to the south of the Maldives 

 and has a length of about 90 miles with a greatest width of 

 70 miles, has generally been assumed to be only a completely 

 submerged or drowned atoll. If raised to the surface it would 

 be in the form of a true atoll, with a depth of water in the 

 lagoon of 40-50 fathoms. At the present time the bounding 

 reef is covered with water of from 4 to 10 fa-thoms depth. The 

 Bermuda Islands have also been instanced as an example of a 

 partially drowned atoll, but, as has been shown in the preced- 



*It is surprising that this consideration in the assumed fornialion of deep lagoons 

 through accelerated marginal growth should be so generally overlooked. Prof. Hick- 

 son, in liis address before the British Association (1888) on " Theories of Coral Reefs 

 and Atolls," furnishes an instance of such oversight. He says : " It seems very 

 probable then that when a large submarine' bank, by accumulation of sediment or by 

 elevation, comes within the limit of coral growth, the growth commences and is 

 almost confined to the edges of the bank, and that in course of time the edges of the 

 bank reach the surface, whilst the centre of the bank has made little or no progress. 

 This seems to be a very reasonable explanation of the deep lagoons of largt atolls, 

 and one to which at present I can see no valid objection." 



