674 DE. F. WOOD JONES ON A [Juiie 15, 



a " drowned atoll." The theory is wonderfully complete and 

 embraces every form of coral structure. Many observed facts have, 

 however, been accumulated since its first proposal, and many of 

 these observations tend to make the theory untenable. 



Atolls are known to exist on land areas actually rising — " high 

 islands " and " low islands " exist in the same neighbourhood ; 

 and some atolls bear unmistakable signs in their own structure 

 of actual elevation having taken place during their formation. 

 (Semper.) 



The undermining of trees and the denudation of shore-lines 

 do not necessarily indicate subsidence, for they are inconstant 

 effects, and an area of land denudation is compensated for by an 

 area of land construction at another part of the island ring. The 

 lagoon does not tend to become deeper as time goes on, nor do 

 its shores tend to become constantly denuded by their sinking 

 beneath the waves ; but lagoons tend to shoal, and lagoon shores 

 to "encroach upon the waters of the lagoon. " Drowned atolls '' 

 are not necessarily final stages of atoll sinking, for they may be 

 early stages of atoll making. 



It is not to be assumed that subsidence, like a conflagration, 

 obliterates its own evidences, for were subsidence to have been a 

 factor in the formation of the Cooos atoll, the Breccia Platform 

 would inevitably show its workings. 



Since the outer edge of the Breccia Platform is the most 

 recently formed pai-t, and its inner edge is its most ancient part ; 

 and since, in its whole extent, it embraces portions laid down 

 throughout an enormous period of time : it is evident that the 

 level of its outer edge should be higher than that of its inner 

 edge, if subsidence had occurred — and this is not found to be 

 the case. 



The fact that atolls tend to be elongated along the line in which 

 the group to which they belong is stretched (Sollas, Brit. Ass. 

 1893) does not necessarily indicate that subsidence has caused the 

 sinking of a long ridge of oceanic land, for since the wind has a 

 great influence in atoll shaping (Kramer, Hedley, &c.), the wind 

 that shapes the individual atoll tends to shape the whole group. 



(b) The Theory of Solution ; first brought forward by Sir John 

 Mru-ray in 1880 (Proc. Roy. Soc. Edin. April 5th, 1880). Between 

 the date of the publication of Darwin's theory and the framing of 

 this hypothesis, several new observations had been made, and 

 some of these were of such a natui'e as to tend to disprove the 

 earlier theory, and some of them greatly simplified the problem. 

 It was known that banks did exist in the sea upon which reef- 

 corals might conveniently start their building. This knowledge 

 was not available in Darwin's time. 



In Sir John Murray's hypothesis these banks are assumed to 

 be probably volcanic in origin and to be afterwards clothed -with 

 Globigerina-ooze. The reef is formed upon the bank when the bank 

 is of a convenient depth, and the corals of the outer edge grow 

 more luxuriantly because they are better fed. The central parts 



