CORAL REEFS AND ISLANDS 203 



extend either upwards on to the shore or downwards into 

 deeper and colder water, and so it ends by encircling the 

 land. 



The theories we have to consider, then, are to account 

 for the formation of barrier reefs and atolls, and, omitting 

 purely fanciful speculations, the first and most celebrated 

 is that of Charles Darwin (1842), who based his view of the 

 matter upon two facts, one physical and the other physi- 

 ological. The physical fact is that many parts of the land 

 are not stationary, but are undergoing slow movements of 

 elevation or subsidence ; and the physiological is that 

 the coral polypes can only live in shallow water of a certain 

 temperature. Darwin's theory is, in effect, that if a 

 fringing reef (Fig. 11, F.) has become estabHshed round 

 the shore of an island that is slowly subsiding, then as the 

 land sinks the coral animals will build the reef upwards, 

 so as to keep near the surface within the zone of shallow 

 warm water, and so in course of time, because of the 

 natural slope of the land and the more or less vertical 

 upgrowth of the coral, the reef will become separated from 

 the shore by a wide and moderately deep lagoon, and the 

 fringing reef will have become converted into a barrier reef 

 (B). Let these processes continue and eventually the 

 original island will be completely submerged, and an atoll 

 or ring of coral (A) will surround the lagoon which now 

 occupies the position of the sunken land. According to 

 this view, the three forms of reef are merely stages in one 

 process of growth, which begins as a fringing reef and ends as 

 an atoll (Fig. 11, Darwin). 



The simpUcity and the comprehensive nature of this 

 theory proved very fascinating, and led to its wide accept- 

 ance by biologists and geologists alike. It was adopted in 

 every textbook of physical geography, and the existence 

 of an atoll came to be usually stated as one of the proofs 

 of subsidence. The American geologist, J. D. Dana, from 

 independent observations made during Wilkes's expedition, 



