AGASSIZ: BAHAMAS. 49 



has been formed which connects all these islets together, leaving only- 

 such entrances as Conch Cut, Wide Opening, and Wax Cay Cut for the 

 passage of craft of shallow draft. The light green color of the bank 



WATER-WORN ISLET, CON'CH CUT. 



connecting the islands indicates very markedly its position as contrasted 

 with the deeper waters of the passages and of the outer sea edge of the 

 bank. The larger cays are flanked on the western face by white sand 

 beaches, formed by the rapid disintegration of that side of the cays. 



All the way across the bank, from Conch Cut to Green Cay, we found 

 but little life upon the bottom ; there were no patches of weeds or of Gor- 

 gonians, the bottom being everywhere composed of coarse seolian sand. 

 But when about two to three miles off Green Cay, patches of coral heads 

 and of Gorgonians begin to appear, and become more numerous as we 

 approach the western edge of the bank forming the eastern side of the 

 Tongue of the Ocean. The absence of animal and vegetable life upon 

 the bottom of the interior of the banks is undoubtedly due to the con- 

 stant shifting of the coral sand from the action of the sea. At moderate 

 depths of one to three fathoms we could everywhere see that action 

 plainly indicated by the presence of ripple marks. In the shallower 

 parts of the banks this action forms great sand bores, which, exposed to 

 the action of the winds, also tends to increase them in size in the direc- 

 tion of the prevailing winds. To the eastward of Green Cay we could 

 see such a great sand bore, seven feet high, forming as it were a cay 

 consisting of nothing but a constantly shifting tract. In many localities 

 on the banks these great sand bores have assumed quite definite posi- 

 tions, which they retain, merely shifting north or south or advancing 

 eastward or westward within narrow limits. At our anchorage off Green 

 Cay the bottom consisted of fine hard coral sand, fairly well covered with 

 coralline a] gas. 



Green Cay. 



Plate XXII. 



The western extremity of Green Cay itself is the termination of one 

 of these shifting bores, and its frequent changes of position and dimen- 

 vol. xxvi. — no. 1. 4 



