78 THREE CRUISES OF THE '^' BLAKE." 



At the time when Darwin wrote, and when we knew httle of 

 the hmestone deposits formed by the accumulation of the debris 

 of moUusks, echinoderms, polyps, and the like, upon folds of 

 the earth's crust, the formation of the basal parts of barrier 

 reefs was diflicult of explanation. The evidence gathered by 

 Murray, Semper, and myself, partly in districts which Darwin 

 had already examined, and partly in regions where his theory of 

 reef formation never seemed to find its proper application, has 

 in a measure removed this difficulty. It all tends to prove that 

 we must look to many other causes than those of elevation and 

 subsidence for a satisfactory explanation of coral-reef formation. 

 All-important among these causes are the prevailing winds and 

 currents, the latter charged with sediment which helps to build 

 extensive plateaux from lower depths to levels at which corals 

 can prosper. This explanation, tested as it has been by pene- 

 trating the thickness of the beds underlying the coral reefs, 

 seems a more natural one, for many of the phenomena at least, 

 than that of the subsidence of the foundation to which the 

 ofreat vertical thickness of barrier reefs has been hitherto re- 

 ferred. Still, it is difficult to account for the great depth of 

 some of the lagoons — forty fathoms — on any other theory 

 than that of subsidence. 



If, however, we have succeeded in showing that great sub- 

 marine plateaux have gradually been built up in the Gulf of 

 Mexico and the Caribbean by the decay of animal life, we shall 

 find no difficulty in accounting for the formation of great piles 

 of sediment on the floors of the Pacific and Indian Oceans, 

 provided these banks lie in the track of a great oceanic current. 

 Certainly the coral reefs of the Caribbean and the Gulf of 

 Mexico, of Florida and the Bahamas, are distributed upon banks 

 which lie directly in the path of the great Atlantic equatorial 

 currents and of the Gulf Stream, — banks which we know to 

 have been formed by the agency of these currents. 



The fact that the coral reef at the extremity of Florida is the 

 most recent of the coral formations found on the Florida shores 

 plainly shows that these reefs, as well as those of Yucatan, 

 Cuba, the Bahamas, and the Caribbean Sea, though not all of 

 the same age, were yet of modern origin, since we find them 



