CORAL REEFS 279 



Stated roughly, Agassiz believed that corals, especially 

 modern corals, have played a far less important role in 

 the formation of atolls and barrier reefs than Darwin's 

 theory would imply. The corals merely take up their 

 work after the banks and shoals prepared for them have 

 reached a suitable depth, or height, for their growth. 

 These shallows owe their existence to various causes, 

 which differ in different localities ; they may be due either 

 to accumulations of silt, deposits of the shells of marine 

 animals, the erosion of volcanic islands, or aeolian lime- 

 stone hills, or other non-coralline limestones, and in 

 some cases they appear to have been formed from what 

 is apparently Tertiary coral rock. It is an essential part 

 of Agassiz's theory, as will be explained in a later chap- 

 ter, that he considered the origin of these Tertiary coral 

 formations an entirely separate question from the method 

 of formation of modern coral growths. 



It will readily be seen that Darwin's theory demands 

 a very considerable thickness of coral formation. Thus 

 in any locality where the existence of only a compara- 

 tively thin layer of coral can be proved, it may be as- 

 sumed that Darwin's theory does not hold good. On the 

 other hand, a thick mass of coral rock would not ne- 

 cessarily indicate subsidence ; for, as both Murray and 

 Agassiz have shown, such a formation might have orig- 

 inated from the growth of a coral reef pushing out to 

 sea on its own talus the debris of the reef, which, broken 

 off by the waves, had rolled down and built up its outer 

 slope. Furthermore, where a modern reef had obtained 

 a foothold on an eroded platform of older limestone con- 

 taining corals, it would often be exceedingly difficult by 

 boring to detect from the core the difference between 

 them. 



