CORAL REEFS 225 



mark. This was well known to Darwin, who accounted for 

 it bv the supposition that the dry land of the atolls had been 

 formed bv boulders of coral cast up by the waves in great 

 storms. But if it had been formed in this way, the corals 

 of which it is composed would be found lying in various 

 positions, some upright, some on their sides, and some upside 

 down. A critical examination, however, of some of these 

 rocks has shown that the corals are all upright and in the 

 position in which they grew on the living reef. This proves 

 that even in the Indian Ocean, which was considered to 

 provide the most conclusive evidence in favour of the sub- 

 sidence theory, a recent elevation of a few feet has actually 

 taken place. 



The question of the foundation on which the atolls and 

 barrier reefs rest is obviously an important one, and various 

 attempts have been made to answer it by making deep bore 

 holes through the coral rock. 



Darwin considered that the many widely scattered atolls 

 must rest on rockv bases, ^ and if it could be proved by boring 

 that the atolls and barrier reefs do rest on rocky bases we 

 should be in possession of the most conclusive evidence of 

 the truth of the subsidence theory. But it has been shown 

 that in very manv cases the reefs rest not on a terrigenous 

 base but upon a submerged platform composed of a hard 

 limestone formed by calcareous organisms other than reef- 

 building corals, which has been planed down by wave 

 action in prehistoric times to a moderately level surface. 



Sluiter ^ showed many years ago how it is possible for a 

 coral reef to be formed even on the soft volcanic mud of the 

 submerged slopes of Krakatoa, and borings through the coral 

 islands Edam and Onrust led to the discovery that they rest 

 on the muddy bottom of the Java Sea. 



There seems to be, in fact, no direct evidence either from 

 borings or soundings, or by the study of elevated reefs, of the 

 existence of great thicknesses of coral rock, formed by the 

 typical reef-building corals resting on a land foundation such 

 as the Darwin theory of subsidence demands. 



^ Darwin, C, Coral Reefs, 3rd ed., p. 125. 

 - Sluiter, Biol. Centralblatt, ix. 1S90, p. 738. 



