262 GENERAL ZOOLOGY 



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. 



Two important theories have been advanced to account 

 for the historical connection which in many cases is thought 

 to exist between fringing reefs, barrier reefs, and atolls. 

 Darwin believed that an atoll begins as a fringing reef sur- 

 rounding 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 

 increase in distance between it and the decreasing area of 

 sinking land. At last the top of the island disappears beneath 

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

 others, have published evidence in support of Darwin's sub- 

 sidence theory, but the erosion theory suggested by Dr. John 

 Murray, leader of the exploring expedition of H.M.S. 

 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 temporary 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 Avater partially connected with the sea. 



Dr. Murray suggested also the probability of atolls being 

 formed without the preliminary stages of fringing and barrier 



