1899] FUNAFUTI: THE STUDY OF A CORAL ATOLL 



21 



DARWIN 



r rin ciMc Ree» 



CORAL rRACMEKTS 



SECTION 



25 fathoms in thickness (Fig. 2, first stage). But let us next suppose 

 that the island begins slowly to sink into the sea, carrying the reef with 

 it ; the upward limit to the 

 growth of the corals will be 

 displaced; they will commence 

 to flourish afresh, and the reef 

 will continue to extend up- 

 wards till the level of the low 

 tides is once more encountered, 

 and growth again arrested. 

 This process of submergence 

 and upward growth may of 

 course be repeated indefinitely, 

 and by the time the island has 

 descended 50 fathoms below 

 its original position, the reef will have acquired a corresponding 

 thickness. In such a case the unfavourable conditions to coral growth 



which prevail on the inner side 



Fig. 2.— First Stage. 



Encircling reef 



plan 

 Fig. 2. — Second Stage. 



of the reef, together with the re- 

 treating slope of the flanks of the 

 island, will have led to the forma- 

 tion of a channel of sea -water 

 between the reef and the shore 

 (Fig. 2, second stage). Finally, 

 let the submergence of the island 

 continue till it is completely 

 swallowed up by the sea, not a 

 vestige of its summit remaining 

 to mark its place ; the upward 

 growth of the corals, constantly proceeding, will bring them once more 

 to the level of low tides, and the result will be the formation of a 

 ring-shaped reef surrounding a 



•I / -1 -1 ATOLL 



central lagoon, or, m other words, 

 of an atoll (Fig. 2, third stage). 



If this hypothetical scheme 

 of the progress of events corre- 

 spond to the facts, we may expect 

 to find its various stages still 

 represented among the numerous 

 islands of the Pacific ; and this, 

 as Darwin endeavoured to show, 

 is clearly the case. The first 

 stage, in which the reef is no 



more than 25 fathoms thick, and forms a selvage accurately following 

 the margin of the land, is represented by that numerous class known 

 as " fringing " reefs. The second, in which a comparatively thick reef 



Fig. 2.— Third Stage. 



