and the Evidence of the Funafuti Borings. 83 



Funafuti report. In a succeeding paper he proposes to recall 

 the attention of workers in this field to the support given to 

 the subsidence theory of atoll formation by a consideration of 

 the apparently unrelated problem of the origin and distribu : 

 tion of dolomite in coral and other limestones. 



Some of these facts and the legitimate conclusions which 

 follow from them were pointed out • by Professor Sollas in 

 1905,* but subsequent writers on the subject have not com- 

 mented on them and apparently have not realized their signifi- 

 cance. 



Von LendenfeUVs and Daly's Diagrams of Atoll Development. 



Darwin's and Dana's diagrams of atoll development show 

 a centripetal displacement of the outcrop of the reef of 

 an atoll as the result of growth during subsidence. Yon Len- 

 denfeldf and Daly:]: have published diagrams showing a cen- 

 trifugal displacement of the outcrop of the atoll reef during 

 subsidence. This latter view can only be tenable on the 

 assumption that the debris broken from the seaward face of 

 the reef accumulates as a submarine talus within the limits of 

 coral groioth, and that fresh coral reef material grows upwards 

 on that talus. 



On " a priori " grounds the writer would expect the follow- 

 ing factors to determine in large measure the nature of the 

 submarine profile of an atoll. 



1. The relative rates of subsidence and of upward growth 

 of corals. § 



2. Rate of abrasion of the seaward face of the growing reef 

 by wave attack. 



3. Rate of lateral regrowth of corals on the seaward side 

 of the abraded reef. 



With regard to the first point, if the rates of subsidence and 

 of upward growth are nearly equal a very steep outer wall to 

 the reef will develop and might continue for many hundreds 

 of feet. If this were the only factor involved a slight centri- 

 petal shift of the outcrop of the reef would be expected, for 

 only submarine talus formed within the limits of coral growth 

 can sustain growing corals and these will fail to reach the sur- 

 face. If the rate of subsidence is less than that of upward 

 growth! of coral, the latter will be checked on reaching sea level 

 and more material will be abraded from the seaward face, sub- 

 marine talus will collect and if within the limits of coral growth 

 upward growth of coral on the talus will commence. The 



* Sollas, The Age of the Earth, T. Fisher Unwin, London, pp. 121-132. 



t R. Von Lendenfeld, Gaca, Jahrg. 26, 196, 1890 ; Westerrnann's Monats- 

 hefte, p. 505, Jan. 1896. 



{Op. cit., p. 247. 



§ One may neglect the case where the rate of subsidence exceeds that of 

 upward growth of coral, since the atoll would in that case be drowned. 



