68 bulletin: museum of comparative zoology. 



the other narrow fringes of limestone by the reefs surrounding them. 

 As we pass northwards from South America we find that these islands 

 become largei', or rather that they are the summits of larger plateaus, 

 forming banks of greater or less extent and separated by channels of 

 various depths. We also see that on some of the banks there are no 

 volcanic islands, the whole surface of the plateau being covered by coral 

 sand, in part the remnants of limestone tracts which had a greater ex- 

 tension, or in part perhaps of limestone banks which during periods of 

 great volcanic activity have gradually formed upon the folds of the bot- 

 tom of the ocean. Even granting for the Bahamas the greatest possible 

 subsidence as indicated by the deepest ocean-holes, the outlines of the 

 banks at the time of their greatest elevation could hardly have been 

 materially different from that of the present charts, as the whole change 

 of level is taken to be well inside the 100 fathom line, at not more than 

 fifty to sixty fathoms. Take the sea face slope as we find it to-day, it 

 would not have changed materially the position of the coral reefs which 

 must have been growing there, perhaps as barrier reefs exposed to the 

 disintegrating action of the sea, and supplying by their own disintegra- 

 tion the material needed for the formation of the a^olian hills from which 

 the Bahama Land was built. Or, more probably, these reefs existed as 

 fringing reefs, much as they do in our days along some parts of the 

 coast of the Sandwich Islands, and from them were formed the immense 

 stretches of coral sand beaches which, swept alternately by the trades 

 and the winds prevailing at other seasons, supplied the sand to build up 

 the gigantic dunes of former days. These formed the highest hills of the 

 Bahamas, and they in their turn have, from varioup causes mentioned 

 in this account of the Bahamas, been reduced to their present limits. 



The Bank from Great Abaco to Bahama Island. 



Plate X. Fig. 1. 



Between Rocky Point (Great Abaco) and the southeastern extremity 

 of Bahama Island only a few cays exist, — Gorda, Channel Cay, Black 

 Rock, Lily, and Burrow Cays. The reef can be crossed at Burrow 

 Cay and at Mores Island Channel. Between Burrow Cay and Ba- 

 hama Island we found numerous sand bores extending five or six miles 

 from the edge of the bank. Inside of Gorda Cay and to the east of 

 Southern Cay and of Mores Island extends a great tract filled with 

 banks and numerous sand ridges reaching towards the ill defined low 



