120 THE TORTUGAS AND FLORIDA REEFS. 



Plateau we may also fairly assume to have been built up, little by little, from its 

 orifinal level, by the accumulation of limestone formed in great part from the bodies 

 of the mass of animals which undoubtedly flourished upon the great submarine 

 plateau at a time when the Gulf Stream found its way out of the Gulf of Mexico 

 with less velocity than it now has as it passes through the narrow Straits of Bemini. 

 At that time it spread itself fan-shaped over the southern part of Florida and the 

 Bahama Bank, and flowed more gently northerly and easterly along the coast with 

 the additional reinforcement of the westerly equatorial flowing north of the Great 

 Antilles to the eastward of Cuba. The Bahama Banks then probably consisted of 

 a series of banks like Salt Key Bank, separated by channels like the Santarem and 

 St. Nicholas, undoubtedly kept open by the same currents as now form the old Bahama 

 Channel. These channels, like those between the keys on the Florida side, have 

 gradually become filled with the detritus driven into them by the trade winds, till 

 the whole formed the bank in its present state of consolidation. And yet it must 

 not be forgotten that, while in the western part of the Caribbean, the Gulf of 

 Mexico, and Florida a dead level prevails, at Havana, Hayti, and Barbados we 

 have reefs elevated to a great height, and others at considerable elevation on cer- 

 tain of the Greater and Lesser Antilles. The somewhat capricious distribution of coral 

 reefs may perhaps be explained by the action of the great equatorial currents. The 

 larger reefs occur in regions to which these currents bring in the track of their 

 course abundant supplies of food for the reef-building animals. On the eastern coast 

 of Africa, of Central America, or of Australia, for instance, extensive colonies of coral 

 reefs flourish, while on the western coast of the same continents in similar latitudes, 

 but not bathed by such powerful equatorial currents, the supply of food seems 

 insufficient for more than the isolated patches of corals existing there. 



Other naturalists, as Semper^ and Murray ,2 have already attempted to explain 

 the formation of coral reefs, in part at least, on grounds differing essentially from 

 those to which Darwin ascribed them, and similar in the main to those here 

 brought forward. Undoubtedly Darwin's theory of reef formation presents a sound 

 and admirable exposition of the grander causes which have brought about the 

 elevation or subsidence of large tracts to a level favorable for coral growth ; but at 

 the time he wrote upon this subject, the formation of these extensive limestone 

 banks, built up by the animals living on the bottom, and constantly strengthened 

 and increased by the attendant phenomena of winds and currents, was little under- 



1 Semper, C. Zeits. f. Wiss. Zool., 1863, XIII., p. 558. 



2 Murray, John. Proc. R. S. Edinb., >fo. 107, 1880, X., p. 505, On the Structure and Origin of Coral Reefs and Islands. 



