IX THE SEA 361 



now occupy the place which was once the 

 highest land. He pointed out that some 

 lagoons, as for instance that of Vanikoro, 

 contain an island in the middle ; while other 

 islands, such as Tahiti, are surrounded by a 

 margin of smooth water separated from the 

 ocean by a coral reef. Now if we suppose 

 that Tahiti were to sink slowly it would 

 gradually approximate to the condition of 

 Vanikoro; and if Vanikoro gradually sank, 

 the central island would disappear, while on 

 the contrary the growth of the coral might 

 neutralise the subsidence of the reef, so that 

 we should have simply an atoll with its 

 lagoon. The same considerations explain the 

 origin of the "barrier reefs," such as that 

 which runs for nearly a thousand miles, along 

 the north-east coast of Australia. Thus 

 Darwin's theory explains the form and the 

 approximate identity of altitude of these 

 coral islands. But it does more than 

 this, because it shows that there are great 

 areas in process of subsidence, which though 

 slow, is of great importance in physical 

 geography. 



