CORAL REEFS, AND ISLANDS. 263 



ture of the ocean, there are other and important 

 conditions which influence the distribution of corals. 

 One of these is the proximity of the mouths of 

 rivers on account of the sediment which they bring' 

 down, and distribute over the sea bottom and the 

 neighbouring coast. No coral reefs can be formed 

 under such conditions. Too steep a shore and too 

 deep water, is another obstruction to the growth of 

 coral. Hereafter it will be shown that corals flourish 

 in comparatively shallow water. Finally, the proxi- 

 mity of volcanic action seems to be a great deterrent 

 to the growth of coral and the development of reefs.. 

 For instance, although other conditions are favour- 

 able, the island of Hawaii has active volcanoes, and. 

 but few traces of coral about it, whereas neighbouring- 

 islands, which have long been free from volcanic - 

 action, have considerable coral reefs. 



The depth at which living corals may be found in 

 the tropics, though varying somewhat in the estimates 

 of different naturalists, is generally admitted to be 

 comparatively small. There was a time when coral . 

 reefs were alluded to by voyagers as standing in 

 unfathomable ocean. This might have been only a 

 poetical licence, or, more probably, hazarded without 

 an attempt at sounding. Quoy and Gaymard, the 

 naturalists who explored the Pacific in 1 817-1 820^ 

 were the first to show that the guess was unfounded. 

 According to their observations the limit of distribu- 



