232 CORALS AND CORAL LSLANDS. 



is mostly thrown inward by the sea. The lagoon will con- 

 sequently 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 

 former 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 generally 

 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. 156), as stated by Darwin; a third had only "two or 

 three very small pieces of living reef rising to the surface," 

 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 orce 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 

 last sinking peak. Another period, and this had gone — the 

 island had sunk, leaving only the barrier at the surface and an 

 islet or two of coral in the inclosed lagoon. Thus the coral 

 wreath thrown around the lofty island to beautify and pro- 

 tect becomes afterward its monument, and the only record 

 of its past existence. The Paumotu Archipelago is a vast 



