THE LOWER FORMS OF LIFE. 125 



deep, but generally there is enough water to admit ships of all sizes. 

 This constitutes what is called a " fringing reef," the best examples 

 of which are found at the Salomon Isles, the New Hebrides, the 

 Friendly and Navigator's Islands in the Pacific, and at Sumatra, 

 Nicobar, Ceylon, Mauritius, Madagascar, and the African coast in 

 the Indian Ocean. 



Now let us imagine this fringing reef to be a coral wall surround- 

 ing the island, and covered with living polyps to the bottom of the 

 lagoon channel on one side, and from twelve to twenty fathoms on 

 the other. And let us further suppose that a sudden sinking, of 

 say six feet, took place ; the immediate effect of this would be first 

 to kill all the polyps on the sea-side below thirty fathoms ; and 

 supposing the coral wall on the other hand to be built up six feet 

 .suddenly ; it is quite clear that, as the side of the mountain (a) 



Fig. 175. 



slopes considerably to the sea, by the above imaginary movement 

 the coral wall would be removed further from the shore, and the 

 lagoon channel would become wider. Now what I have supposed 

 might take place suddenly does actually occur slowly, and the result 

 of this subsidence of the land and the now languishing volcano is to 

 convert a "fringing" into what is known as a "barrier reef," as 

 seen in Fig. 175. The reef is still farther removed from the shore, 

 and the lagoon channel widens, as seen at Tahiti, Fidji, New 

 Caledonia, New Ireland, Oualan, Louisade, and the west coast of 

 New Guinea, and a large extent of coast from near Cape Sunday to 

 Torres Straits in the N.W. of New South Wales. 



Barrier reefs are also found surrounding the Cormoro Islands in 

 the Indian Ocean and both sides of the shores of the Bed Sea, from 

 16 to 21 N. lat., occupying nearly the middle of the coasts of that 



