64 THREE CRUISES OF THE " BLAKE." 



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, there is also evidence 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 which extend along the east- 

 ern edoe of the Caribbean Sea. In some cases these banks have 

 been elevated since the existing condition of things came about ; 

 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 the 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 West India 

 Islands of the formation 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 or littoral banks. 

 It is well known, also, that the " Challenger" and " Tuscarora " 

 soundings have developed a number of submarine elevations, 

 covered by deposits of pteropod and globigerina ooze, and these 

 deposits form extensive banks which serve as foundations for 

 barrier reefs and atolls, while the volcanic substratum has been 

 completely 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 submarine 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 pla- 

 teaux 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 have also the still older phenomenon of such 



