CORAL REEFS 227 



Murray's explanation of the formation of the deep lagoons 

 by solution seems to be the least acceptable part of his theory, 

 as it has been shown that in the coral seas the water does 

 not contain free carbonic acid and there is definite evidence 

 that in many instances the lagoons are slowly silting up 

 instead of deepening, as they should do if they are subject 

 to solution. Notwithstanding these objections, however, it 

 is still possible that some of the lagoons have been formed, 

 not perhaps by solution but by the scouring action of the 

 tides, which do carry great quantities of the fine detritus 

 formed by the natural disintegration of the corals through 

 the channels into the deep water beyond the outer edge 

 of the reefs. 



There are two processes going on continuously in the 

 lagoons, the accumulation of silt and the scouring action of 

 the tides, and these, in general, counteract one another ; but 

 it is probable that under changing conditions accumulations 

 may at one time gain the upper hand and at another the 

 scouring action may become dominant. The evidence that 

 a particular lagoon is at the present day silting up, is, at 

 any rate, no decisive proof that the lagoon has not formerlv 

 undergone a process of deepening by the scouring action of 

 the tides. 



The existence in many parts of the world of extensive 

 submarine banks or platforms on which the modern coral 

 reefs rest has been the basis of another theory of coral reef 

 formation which has met with some support. 



In the consideration of previous theories the question of 

 any possible changes in the sea-level does not necessarily 

 arise ; but it is clear that if the crust of the earth remained 

 stationary and the level of the seas rose, the coral reefs and 

 atolls might have been formed in precisely the same way 

 as if the crust of the earth subsided and the sea-level 

 remained constant. 



It has been suggested ^ that during the Glacial Period so 

 much water was piled up on the continental lands in the 

 form of ice, that the level of the sea was lowered to the 



1 See R. A. Daly, " The Glacial Control Theory," Proc. Amevican 

 Acad. Arts and Sci., \'ol. 51, 1915. 



