1888.] and the Coral Formations of the Indian Ocean. 449 



formations, situated near lat. 10° S., north of Madagascar, in which 

 are found raised atolls — atolls whose dry land just rises above the 

 waves and submerged banks. There can be no clearer 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. 



Those who have felt that the evidence brought against Darwin's 

 subsidence theory is too strong to be resisted, must often have felt 

 that no satisfactory explanation of the lagoons of atolls or the lagoon 

 channels of barrier reefs has been given in its place. Semper was the 

 first to suggest that the lagoon was formed by a solution of the 

 interior parts of the reef, and more recently this view has been urged 

 with great force by Murray, who points out jn addition, that corals 

 on the periphery of a reef must, from their position, get the advantage 

 over those more interiorly situated, being more directly in the track 

 of food-bearing currents. Neither of these explanations has com- 

 pletely satisfied me. That sea-water exercises a solvent action upon 

 carbonate of lime does not admit of doubt, and that the scour of 

 tides, combined with this solvent action of the water, does affect the 

 extent and depth of a lagoon is obvious. But I challenge the state- 

 ment that the destructive agencies within an atoll or a submerged 

 bank are in excess of the constructive. It would be nearer the mark 

 to say that they nearly balance one another. In the first place the 

 carbonate of lime held in solution by sea- water is deposited as crystal- 

 line limestone in the interstices of dead corals or coral dibris. Anyone 

 who is acquainted with the structure of coralline rock knows how 

 such a porous mass as a Meeandrina head becomes perfectly solid by 

 the deposition of lime within its mass. This deposition can only be 

 effected by the infiltration of sea-water. In reckoning the solvent 

 action of sea-water, therefore, account must be taken of the fact that 

 a not inconsiderable proportion of the carbonate of lime held in solu- 

 tion is redeposited in the form, of crystalline limestone. Of this, it 

 seems, Mr. Murray has not taken sufficient account, and has, there- 

 fore, overstated the destructive agency of the sea. Secondly, the 

 growth of corals, and the consequent formation of coral rock within 

 the lagoon, is generally overlooked. 



Whilst diving for corals at Diego Garcia I had abundant oppor- 

 tunities of studying the formation of coral rock within the lagoon, in 

 depths under 2 fathoms. The layers of tolerably compact rock thus 

 formed are of no mean extent or thickness; they soon become covered 

 with sand, and are thus protected from the solvent action of the 

 water. I have found it impossible to reconcile Mr. Murray's views 

 with what I saw of coral growth within a lagoon. Not only do the 



