THE TORTUGAS AND FLORIDA REEFS. 121 



stood. These focts have been brought into notice and emphasized by recent deep- 

 sea explorations. Darwin, however, when examining maps of the West Indies, had 

 been struck by the probable connection between the areas of deposition of the great 

 banks marked upon the charts and the course of the sea currents. He naturally 

 explained the steep slopes abruptly dropping from comparatively shallow plateaux 

 to great depths, by what is known to occur wherever great masses of sediment are 

 found, and he therefore considered these plateaux to be submerged mountains. 

 Such they undoubtedly are, in a certain sense ; not wholly built, however, as Dar- 

 win supposes, of sediment, but in great part also of the remains of the innumerable 

 animals living and dying upon them. The nucleus of these banks has probably 

 been formed around the shores of promontories subjected to the most active play 

 of the great oceanic currents. 



At the time when Darwin wrote, and when we knew little of the limestone 

 deposits formed by the accumulation of the debris of Mollusks, Echinoderms, Polyps, 

 and the like, upon folds of the earth's crust, the basal parts of barrier reefs were dif- 

 ficult 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 part re- 

 moved this difficulty. It all tends to prove that we must look to many other causes 

 than those of elevation and subsidence for a satisiixctory 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 penetrating into 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 great vertical thickness of barrier reefs has been 

 hitherto referred. It is, however, 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 submai'ine 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 Carib- 

 bean 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, which we know to have been formed by the agency of these cur- 

 VOL. XI. 16 



