78 THREE CRUISES OF THE “ BLAKE." 
At the time when Darwin wrote, and when we knew little of 
the limestone deposits formed. by the accumulation of the débris 
of mollusks, echinoderms, polyps, and the like, upon folds of 
the earth's crust, the formation of the basal parts of barrier 
reefs was difficult of explanation. The evidence gathered by 
Murray, Semper, and myself, partly in distriets 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 
great 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 — оп 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 
Mexieo 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 thé 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 
