ON CORAL REEFS. 33 



edition in 1874). This work contains an account of 

 Darwin's well-known theory upon the origin of the 

 various coral formations fringing reefs, barrier reefs, 

 and atolls by the upward growth of the reef keep- 

 ing pace with the gradual sinking of the island upon 

 which it is based, so that the living corals always 

 remain at the surface under the most favourable 

 conditions, while beneath them is an ever-thickening 

 reef formed of dead coral, until at length, by con- 

 tinuing this process, the climax is reached in the 

 atoll, in which the original island has altogether 

 disappeared beneath the surface of a central lagoon 

 enclosed in a ring formed by the living edge of the 

 reef. This theory, after being accepted for many 

 years, has recently been disputed, chiefly as the 

 result of the observations made on the Challenger 

 expedition. It is contended by Dr. John Murray 

 " that it is not necessary to call in subsidence to 

 explain any of the characteristic features of barrier 

 reefs or atolls, and that all these features would exist 

 alike in areas of slow elevation, of rest, or of slow 

 subsidence" (Nature, August 12th, 1880, p. 337). 

 It cannot be said that this controversy is yet settled, 

 or that the supporters of either theory have proved 

 that the other does not hold at any rate, in certain 

 cases. 



Among his geological papers written at this time 

 was one describing the glacial phenomena observed 

 during a tour in North Wales. This paper (Philo- 

 sophical Magazine, 1842, p. 352) is placed by Sir 



