FRESH-WATER POLYP AND SOME ALLIES 281 



fringing reef. If by subsequent changes, or as originally 

 formed, a navigable channel lies between the ridge and the 

 land, it is called a barrier reef. If the formation surrounds 

 a body of water, which it nearly or completely cuts off from 

 the sea, it is called an atoll. The most remarkable examples 

 of atolls are the Maldive Islands. 



Many theories have been advanced to account for the 

 historical connection which is thought to exist between 

 fringing reefs, barrier reefs, and atolls. Two of the more 

 important of these theories will be discussed. Darwin be- 

 lieved that an atoll begins as a fringing reef surrounding an 

 oceanic island, which may be of volcanic or other origin, 

 and that as the island sinks from internal causes, the coral 

 polyps build up carbonate of lime as long as they are within 

 their range of favorable depth. The reef that at first was 

 a fringing reef becomes a barrier reef, on account of the in- 

 crease in distance between it and the decreasing area of 

 sinking land. At last the top of the island disappears be- 

 neath the water, and the atoll remains. Professor J. D. 

 Dana, and others, have published evidence in support of 

 Darwin's subsidence theory, but the erosion theory suggested 

 by Sir John Murray, leader of the exploring expedition of 

 the Challenger (1850), has probably more adherents at the 

 present day. The erosion theory has been modified and 

 extended, chiefly by Dr. Alexander Agassiz. According to 

 this theory coral polyps may form a fringing reef about an 

 oceanic island. The reef continues to grow, while the soil or 

 rock of the island is carried away by rains and by rivers. 

 The solution of the soil and rock is caused by the tempo- 

 rary chemical union of these substances with carbon dioxide 

 derived from dead animals and plants. The idea is that in 

 the course of a few centuries an entire island could be worn 

 away, and the atoll left with its lagoon of water partially 

 connected with the sea. 



