168 P. A. Daly— Problem's of the Pacific Man d$. 



example is the well known Funafuti atoll of the Ellice group. 

 (See fig. 11, page 172.) Much larger are the atolls of the 

 western Paumotus, which rival some of the marvellous atolls 

 of the Indian ocean. 



The origin of all three classes of reefs is a fascinating prob- 

 lem, affecting our understanding of the whole intertropical 

 zone of the earth. It has to do with first principles regarding 

 the stability of the earth's crust, the constancy of the general 

 sea-level, the kinds of climate enjoyed or suffered by the 

 organisms of the past, and with the march of organic evolu- 



Fig. 2. 



Fig. 2. Composite copy of Darwin's sections, illustrating his subsidence 

 theory. Successive sea-levels at L, U and L" ; fringing reefs at F ; barriers 

 at B ; atoll reefs at A. 



tion. If there were no other good reasons for systematically 

 studying the gems of the Pacific, this one should lead to 

 speedy action. 



Here as usual in productive science, the guide to good obser- 

 vation is theory. The half-score of theories, so far offered in 

 explanation of coral reefs, differ greatly in strength. I shall 

 not attempt to mention them all but will stress two theories, 

 both of which postulate a recent deepening of the water on 

 shoal or island wherever atoll or barrier reef is found. Such 

 deepening may occur in two different ways. On the one hand, 

 the island or shoal may sink, the sea-level remaining sensibly 

 constant. On the other hand, the general sea-level may rise, 

 the shoal or island being undisturbed. The first assumption is 

 made in the celebrated subsidence theory of the reefs ; the 

 second is made in the newer Glacial-control theory. Each 

 explanation offers hints for future field work. To appreciate 

 their value, we must review the theories themselves. For two 

 reasons they will be discussed in some detail : first, because 

 they relate to a topic which has specially engaged my attention 



