xx PREFA TOR Y NOTE. 



they are unable to extend higher, without the help of elevation ; 

 that the islands north of St. Christoval, called the "Three 

 Sisters," commenced their growth as two flat-topped and sub- 

 merged reefs ; and that coral-reefs may grow from a depth 

 greater than 25 fathoms, — the conditions necessary being the 

 state of the water, and more particularly as to whether it carries 

 suspended mud, which is often fatal to the life of the polypes. 

 And in a recent letter to Dr. Murray, which has been published 

 in Nature (vol. xxxix. p. 236), the same observer states that, in 

 his opinion, many features of importance were overlooked by 

 Darwin when examining the Keeling atoll, and that these 

 give no support whatever to the theory of subsidence. 



Lastly, Mr. G. C. Bourne has printed a very interesting 

 paper 1 on the Chagos group, in which he arrives at the con- 

 clusion that the majority of the reefs in the Indian Ocean show 

 evidences of elevation 'rather than of rest' ; and that 'certainly 

 they are not evidences of subsidence. 5 He challenges the two 

 chief features in Murray's theory — the shape and character of 

 lagoons depending on the more vigorous growth of the polypes 

 on the periphery of the reef owing to ocean-currents, and the 

 solution of its interior by the carbonic acid in the water ; and 

 states that it must be realised 'that the laws governing the 

 formation of coral-reefs are exceedingly complex, and that 

 many circumstances have to be taken into account before any 

 perfect explanation of their structure can be obtained. 5 ' That 

 sea-water exercises a solvent action upon carbonate of lime 

 does not admit of a doubt, and that the scour of tides, com- 

 bined with the solvent action of the water, does affect the 

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

 statement 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 crystalline limestone in the interstices 

 of dead corals or coral debris. Any one who is acquainted with 

 the structure of coralline rock, knows how such a porous mass 

 1 Proc. Roy. Soc, xliii. p. 440, 1888. 



