THE TORTUCtAS AND FLORIDA REEFS. 113 



prevailing winds bring to the bank. All the reefs on the south coast of Cuba be- 

 tween the Isle of Pines and the shore to the east and west may have a similar origin. 

 Among the West India Islands the barrier reefs of the windward side are built upon 

 plateaux of a similar structure. The Basse Terre of Guadeloupe is a fine example of 

 such a plateau, Avhich has been elevated slightly above the level of the sea. At 

 Barbados the whole shell of the island consists of a series of terraces, which have 

 been successively lifted by the trachytic centre forming the nucleus of the island. 

 These terraces are entirely composed of limestone formed of the species of Mollusks 

 and Radiates now living in the West India seas. , 



While there is thus undoubted evidence that a great part of the shore line of the 

 northeast extremity of South America has been washed away, yet there is also evi- 

 dence that the lines of the bank connecting the lesser West India Islands have been 

 built up by agencies similar to those which have formed the Yucatan and Florida 

 Banks, except that these latter have been formed around the volcanic islands or folds 

 extending along the eastern edge of the Caribbean Sea. In some cases these banks 

 have been elevated after the existing condition of things was in force ; in others their 

 elevation dates back to the period when the separation of the Caribbean from the 

 Pacific took place, at the time of the closing of the Isthmus of Panama. Evidence 

 of this action is found in iue elevated coral reefs and the raised earlier tertiary and 

 later cretaceous deposits of the West Indies and Central America. 



Nowhere do we find better examples than in the AVest India Islands of the forma- 

 tion of submarine banks in connection with volcanic peaks. A great number of peaks 

 of volcanic origin have risen nearly to the surface of the sea, or above it, and serve as 

 the foundation of great submarine banks. It is well known, also, that the " Challenger " 

 and " Tuscarora " soundings have developed a number of submarine elevations, covered 

 by deposits of Pteropods and Globigerina ooze, forming extensive banks serving as 

 foundations for barrier reefs and atolls, while the volcanic substratum has been com- 

 pletely hidden. In the West Indies, as at Martinique, there are volcanic peaks rising 

 to a height of over four thousand feet ; on their windward side are extensive sub- 

 marine plateaux, formed, I imagine, by agencies similar to those to which we ascribe 

 the formation of the Yucatan and Florida plateaux. Whenever such plateaux have 

 reached on their windward side the level at which corals prosper, there coral reefs 

 spring up and flourish. Side by side with such conditions we find plateaux at lower 

 levels, under a greater depth of water, covered only by the Invertebrates living upon 

 their surface, — as is the case, for instance, in the northern extremity of the plateau 

 of the Grenadines. These plateaux have probably never risen to the surface. We 

 VOL. XI 15 



