44 ADDRESS TO THE BRITISH ASSOCIATION, 1881. 



so that the coral which had originally grown under 

 water had been raised above the sea-level. The circular 

 or oval shape of so many reefs, however, each having a 

 lagoon hi the centre, closely surrounded by a deep 

 ocean, and rising but a few feet above the sea-level, had 

 long been a puzzle to the physical geographer. The 

 favourite theory was that these were the summits of 

 submarine volcanoes on which the coral had grown. 

 But as the reef-making coral does not live at greater 

 depths than about twenty-five fathoms, the immense 

 number of these reefs formed an almost insuperable ob- 

 jection to this theory. The Laccadives and Maldives, 

 for instance meaning literally the ' lac of islands ' and 

 the ' thousand islands ' are a series of such atolls, and 

 it was impossible to imagine so great a number of craters, 

 all so nearly of the same altitude. Darwin showed, 

 moreover, that so far from the ring of corals resting on 

 a corresponding ridge of rocks, the lagoons, on the con- 

 trary, now occupy the place which was once the higheat 

 land. He pointed out that some lagoons, as for in- 

 stance 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 

 one thousand miles, along the north-east coast of Aus- 



