224 CORALS 



be left at the surface would be a ring of coral reef enclosing 

 a deep-water lagoon. 



The Darwinian theory is usually called the subsidence 

 theory, because it postulates a gradual sinking of the crust 

 of the earth over wide areas of the great ocean basins. 



Since the time when Darwin wrote, a great many more 

 facts have been ascertained concerning the character of the 

 floor of the great oceans, on the structure and distribution of 

 the upraised coral reefs of the tropical islands, and on the 

 construction and topography of living coral reefs and atolls ; 

 and many subsequent writers have expressed grave doubts 

 that the subsidence theory is not sufficient to account for 

 the occurrence of all barrier reefs and all atolls. Some 

 indeed, such as Alexander Agassiz, who spent many years 

 of his life in exploring and critically investigating the coral 

 reefs in all parts of the world, have come to the conclusion 

 that in no single instance can the presence of an atoll be 

 satisfactorily explained by the subsidence theory.^ 



It is possible that the truth lies between the two extreme 

 views, and that some barrier reefs and atolls have been 

 formed during subsidence and that others have been formed 

 during long periods of quiescence or even independently of 

 earth movements. Let us, then, consider very briefly some 

 reasons which have been brought forward as arguments 

 against complete acceptance of Darwin's hypothesis. 



The discovery of great masses of coral reef situated 

 several hundred feet above the sea-level, composed of the 

 same genera of coral as now occur on modern reefs, on islands 

 in the Pacific Ocean situated in close proximity to true 

 barrier reefs, proves that this land has been actually elevated 

 in geologically recent times, and it is difficult to reconcile this 

 fact of elevation with a theory which demands long-con- 

 tinued subsidence in the formation of the neighbouring 

 barrier reefs. Many of the typical atolls of the Indian Ocean 

 are raised to a height of nine or ten feet above high-water 



^ Prof. W. M. Davis of Harvard L'niv^ersity has recently given reasons 

 for believing that the subsidence theory is sufficient to account for the 

 occurrence of all atolls and barrier reefs (The Scientific Moutltly, vol. ii. 

 No. 4, 1916, and other publications). 



