CCELEXTERATA ACTIXOZOA. 129 



Its distance from the Australian coast is about 30 miles. 

 The sea on the inner side is 300 feet deep ; in some places 

 outside it is 1,800 feet. Sometimes a barrier reef sur- 

 rounds an island, which on this account is called a lagoon 

 island, the intervening water being called a lagoon. 



185. (3.) Atolls are rings of coral, surrounding a cen- 

 tral lagoon, in which there is no island. They are usually 

 pierced by several openings, and form excellent natural 

 harbours. As the coral animal cannot subsist in fresh or 

 brackish water, these gaps are believed to have been 

 originated by rivers which once flowed from the island 

 round which the polypes began to build. There are 

 several groups of atolls both in the Indian and Pacific 

 Oceans. The ring of coral is sometimes 90 miles in 

 diameter, but is seldom more than a quarter of a mile 

 broad. Although only raised a few feet above sea level, 

 many of them have human inhabitants. 



The coral animal, it is evident, could never raise a reef 

 above the sea level, as it cannot exist out of the water. 

 The reefs are raised by the action of the waves which 

 detach masses of rock, and pile them up an island in 

 the course of ages is thus formed. The interstices of the 

 masses are gradually filled up by calcareous sand, also 

 washed off by the action of the waves, and the whole 

 mass is cemented together by water containing carbonate 

 of lime in solution. 



A coral reef, as has been stated, sometimes extends 

 downwards as much as 1,800 feet, and it was formerly 

 believed that the coral polypes commenced to build at 

 these depths, and gradually raised their structures until 

 they reached the surface. Darwin has proved the incor- 

 rectness of this theory, and has shown that the coral 

 animals cannot live at a greater depth than from 15 to 

 30 fathoms. He accounts for the occurrence of coral 

 reefs at greater depths, by the gradual subsidence of the 

 gea bottom. He believes that all coral reefs were origin- 

 ally constructed in shallow water, near the shore of some 

 island or continent; that is, they were once in the 



15 a j 



