8 PHYSIOGRAPHY AND GEOLOGY 



free communication with the ocean beyond, by another bank which connects 

 the coast of South America with Florida. 



Although these banks are for the most part submerged, they are none the 

 less real; for, not only do they prevent the cold water at the bottom of the 

 Atlantic from getting over into the basins of the Caribbean-Gulf region, but 

 they also exclude free circulation of the waters within the region itself. 



The islands of the West Indies are nothing more than the superficial por- 

 tions of these banks or ridges which happen at the present epoch to rise above 

 the surface of the water. These dry land summits are, in terms of geology, 

 transitory and uncertain. They have not always been as they are to-day, nor 

 will they remain so in the future; but they change their shapes and positions 

 in response to movements of the earth's crust of which they form a part. The 

 Bahama Islands which are to be considered as the summits of a portion of 

 the eastern ridge connecting South America with Florida are no exception. 



Contour and slope. — The flat-topped ridge or platform from which the 

 Bahama Islands rise extends from southern Florida to eastern Haiti. By an 

 examination of the map which accompanies this chapter" (Plate X) it will 

 be seen that this platform rises rapidly from the deeper regions which surround 

 it on all sides. The steepest ascent is along the eastern face of the platform, 

 where it abruptly rises from the bottom of the Atlantic to the surface of the 

 ocean — a vertical distance of 2500 fathoms — in a little less than 25 miles. 

 On the south, west and north, the ascent, although rapid, is not so pronounced as 

 towards the Atlantic for the reason that the waters east of Florida, Cuba, and 

 Haiti are not as deep. On all sides, however, the platform is so well marked 

 that it stands out as a great submerged tableland from the surrounding ocean- 

 bottom. 



The northwestern half of the Bahama platform varies greatly from 

 the southeastern. Not only is it shallower, lying for the most part, as in 

 the Great and Little Bahama Banks, only a few feet or fathoms beneath the 

 surface of the ocean, but also it is less broken than the latter, and carries the 

 largest islands. The islands of the southeastern half are arranged in small 

 groups and rise rapidly on all sides from a lower portion of the platform. 

 They are also separated from each other by wide passages. 



^° The base of this map was engraved from transfers kindly furnished by the 

 Hydrographic Office. To this the bathymetric contours and colors have been 

 added. The general appearance of this map is similar to the one published by 

 Professor Agassiz in his " Reconnoissance of the Bahamas," but it differs from it in 

 that the area included is not the same, more contours and details have been intro- 

 duced, and a different system of colors employed. 



