CORAL REEFS AND ISLANDS 209 



to conclusions (1919) favourable to Darwin's theory. He 

 lays stress upon embay ments of the coast -lines due to 

 erosion and the haK-drowned valleys as proof of submergence, 

 and he points to the unconformity between the coral reef 

 and the underlying rock which is eroded, and therefore was 

 once exposed to the air, which again is evidence of sub- 

 mergence. But these characters only prove that subsidence 

 took place before the coral reef was formed upon the imder- 

 lying rock, and do not show that the land was still sinking 

 while the fringing reef was growing up to become a barrier 

 reef or an atoll — which is the theory put forward by Darwin. 

 It is unnecessary to discuss every view that has been 

 put forward by investigators of the coral-reef problem, but 

 one other of outstanding importance must be mentioned. 

 R. A. Daly, of Harvard, in a series of papers since 1915, 

 has advocated what is known as the " glacial-control " 

 theory, which is, that existing coral reefs are very recent, 

 and have been formed only during late glacial and post- 

 glacial times ; that the pre-existing tropical reefs had been 

 exterminated in glacial times, when, he estimates, the water 

 withdrawn from circulation and locked up in the form of 

 ice may have lowered the level of the ocean in tropi- 

 cal regions by as much as 50 to 70 metres ; that the 

 melting of the glaciers set free a great volume of water, ^ 

 becoming rapidly warmer, which caused the tropical oceans 

 to deepen gradually and permit the newly estabhshed coral 

 reefs to form as thin veneers upon the numerous shallow 

 platforms which had been produced by erosion or wave- 

 action during the previous pre-glacial and glacial periods. 

 As the water became warmer, reefs would be formed round 

 the edges of these platforms as a consequence of the newly 

 established coral colonies growing upwards to keep pace 

 with the gradual deepening caused by the water set free 

 from the ice slowly raising the level of the ocean. 



^ But the question arises whether this water may not have been 

 locked up again by increasing glaciation in the Antarctic. 



P 



