sometimes thousands of miles fiom land and surrounded 

 by very deep water, the earlier naturalists assumed that 

 ring-shaped atolls grew from the edges of extinct vol- 

 canoes lying close beneath the surface of the ocean. 

 This, however, does not explain barrier reefs and it 

 would require something beyond mere coincidence to 

 account for the existence of the innumerable atolls 

 which are scattered about the tropical Pacific and Indian 

 Oceans. 



Darwin produced, as a result of careful study, a 

 theory which fits all the known facts to a remarkable 

 degree and accounts for the presence not only of atolls 

 but also of barrier reefs. This theory is in the main 

 still held today but a number of later writers have 

 added to it on the basis of new information which has 

 come to light. According to Darwin, all the coral reefs 

 then known were situated in areas where, at some time 

 in the past, a sinking of the earth had taken place. This 

 subsidence theory explains the formation of the three 

 types of reef with admirable simplicity. 



The first stage in the formation of an atoll or a barrier 

 reef was thought by Darwin to be a fringing reef. A 

 reef of this type might be expected to grow on the shore 

 of any continent or island in tropical seas where other 

 conditions were suitable. As the land began slowly to 

 subside the reef would continue to grow upwards and, 

 unless the subsidence were too rapid, the growth of 

 coral would keep pace with it. The continual sinking 

 of the land would increase the distance between the reef 

 and the shore so that a deep wide channel would de- 

 velop and the reef would now have all the character- 

 istics of a barrier reef (see Fig. 7) . If the land happened 

 to be a small island, it would eventually disappear be- 



28 



