CORAL REEFS. APPENDIXT 207 



of Madagascar are considered to be " proof that atolls are 

 formed in areas of elevation, and if the facts which I have 

 already stated concerning Diego Garcia are of any weight, it 

 would seem that most of the coral formations of the Indian 

 Ocean mark areas of elevation rather than of rest, certainly 

 they are not evidence of subsidence." That the last move- 

 ment in the region may have been one of elevation need not 

 be disputed ; and, as far as any general theory of coral forma- 

 tion is concerned, the same movement, or a reversed one, may 

 be taking place to-day. But Mr. Bourne does not show that 

 the characteristic structure of the islands under special con- 

 sideration was not formed during a period of subsidence, or 

 that no subsidence has really taken place; the fact that ele- 

 vation may be now taking place in no way precludes the pos- 

 sibility of an antecedent subsidence. The raised marine strata 

 of continental areas might as well be taken in evidence of non- 

 submergence or subsidence. It would indeed be difficult to 

 prove, from what evidence Mr. Bourne has placed before us, 

 that Diego Garcia is not to-day subsiding, instead of rising, 

 despite the positive proof that is given of a recent elevation of 

 four feet. Assuming the correctness of Mr. Darwin's hypothe- 

 sis of subsidence I fail to see what condition would be brought 

 about by a change of movement i.e., if such subsidence as 

 caused the formation of " drowned-atolls " were followed by ele- 

 tion other than that which is presented by Diego Garcia and 

 the other reefs which Mr. Bourne describes. 



Mr. Bourne does not believe that the solution-theory of the 

 formation of lagoons is tenable, and he challenges "the state- 

 ment that the destructive agencies within an atoll or a sub- 

 merged bank are in excess of the construction " (vid. ant., note). 



R. Irvine. " Coral Formations." Nature, Apl. 26, 1888. 



The author furnishes the following results as to the solubility 

 of different coral fragments (and other limestones) in sea-water, 

 in grammes per litre, for an exposure of 12 hours: dead 

 Forties, 0'395; coral sandO'032; Bermuda harbor-mud 0'041 ; 

 Isophyllia dipsacea, from Bermuda, 0041 ; Millepora ramosa 



