270 C0RAL8 AND CORAL ISLANDS. 



size of the atoll, owing to the fact already noted, that the de- 

 tritus is mostly thrown inward by the sea. The lagoon will 

 consequently become smaller and shallower, and the outline of 

 the island in general, more nearly circular. Finally, the reefs 

 of the different sides may so far approximate by this process, 

 that the lagoon is gradually obliterated, and the large atoll is 

 thus reduced to a small level islet, with only traces of a for- 

 mer depression about the centre. Thus subsidence aids 

 detritus accumulations in filling up the lagoon ; and as filled 

 lagoons are found only in the smallest islands, such as Swain's 

 and Jarvis's, the two agencies have beyond doubt been gen- 

 erally united. 



This subsidence, if more rapid than the increase of the 

 coral reef, would become fatal to the atoll, by gradually sink- 

 ing it beneath the sea. Such a fate has actually befallen 

 two atoll-formed reefs of the Chagos Group, in the Indian 

 Ocean (p. 192), as stated by Darwin; a third has only "two 

 or three very small pieces of living reef rising to the sur- 

 face," and the fourth has a portion nine miles long, dead and 

 submerged. Darwin calls such reefs dead reefs. The south- 

 ern Maldives have deeper lagoons than the northern, fifty 

 or sixty fathoms being found in them. This fact indicates 

 that subsidence was probably most extensive to the south, and 

 perhaps also most rapid. The sinking of the Chagos Bank, 

 which lies farther to the south, in nearly the same line, may 

 therefore have had some connection with the subsidence of 

 the Maldives. 



In view of the facts which have been presented, it appears 

 that each coral atoll once formed a fringing reef around a high 

 island. The fringing reef, as the island subsided, became a bar- 

 rier reef, which continued its growth while the land was slowly 

 disappearing. The area of waters within finally contained the 



