1^8 THE SEAS 



exceptional interest. Kno\\Ti as the " glacial-control " 

 theory, this states that during the great ice ages so much 

 water was locked away in the form of ice that the surface 

 of the sea was lowered by between twenty-five and thirty- 

 five fathoms in the tropics. During this period the pre- 

 existing coral reefs (for coral reefs are known from the 

 earliest times of which we have any geological evidence) 

 were destroyed, but when the glacial period passed the water 

 which returned to the sea gradually raised it to its former 

 level and at the same time became warmed up, and in this 

 warm water new coral colonies became established on the 

 flat platforms into which those regions of the sea bottom 

 exposed during the glacial period had been converted. 

 As the water rose the corals growing on these platforms 

 would grow upwards with it, in just the same manner as 

 Darwdn supposed, only in the former case it was the sea 

 which was changing its level and in the latter the earth. 



There is support for this ingenious theory from Dr. 

 Mayor w^ho calculated from the growth of corals in Samoa 

 that the reefs in that region at any rate, could all have 

 been formed since the last glacial epoch. On the other 

 hand, we have no definite knowledge as to the amount of 

 water which was taken from the sea during the ice age, 

 while there is abundant evidence that many reefs are not 

 founded on platforms which could have been formed in 

 this way. 



