96 THE SHORE 



the regular contour of the ocean floor is reached,* though 

 it must be slightly altered by the deposition of coral 

 mud and rubble from the reef above. The cause of the 

 existence of the first slope to 40 fathoms is not known, 

 though it has been supposed to be connected with the 

 depth to which the effects of wind-produced waves are 

 felt. 



Further observations on the currents, bottom con- 

 ditions, and fauna of the outer slope, are urgently 

 needed. The steep is clearly a " talus" formation, 

 resulting from the outwash of loose masses of coral 

 from the reef above, its angle of slope being the angle 

 of rest of such masses in comparatively calm water. 



The theory of the formation of atoll and barrier reefs 

 propounded by Charles Darwin in his " Coral Reefs," 

 a book of genius in its line as great as his " Origin 

 of Species," depends on the assumption that the land, 

 round which a fringing reef has formed, is subsiding 

 at such a rate that the coral and other organisms upon 

 the reef can, by their growth, keep it at the surface 

 of the sea (Fig. 29). Such organisms will grow more 

 vigorously at the ocean edge of the reef, where they will 

 have more food. Thus it will be easily understood 

 that, as subsidence proceeds, a channel (at first a boat 

 channel, and then a lagoon) might be formed between 

 the reef and the land. This lagoon will attain its 

 breadth by the gradual submergence of more and more 

 of the land, which finally might be completely sub- 

 merged, the formerly fringing reef becoming successively 

 a barrier reef and an atoll reef, the contours of these 

 roughly representing the former extension of the land 

 mass. It is a theory delightful in its simplicity, but it 

 depends on the assumption that the land continues to 

 subside slowly for periods of time even geologically 



* Compare this with the slopes off continents, p. 208. 



