104 THREE CRUISES OF THE " BLAKE." 



The effect of strong currents and tides, at moderate depths, is 

 well shown in the topography of the Gulf of Maine, as mapped 

 on the Coast Survey charts. (Fig. 60.) Here the tides are high, 

 the currents powerful, and the effect on the bottom in pro- 

 ducing ledges, banks, etc., is in striking contrast to the bottom 

 in the tideless sea of the Gulf of Mexico. But no influence of 

 this kind is at work at a great depth in the ocean. The action 

 of currents is probably not limited by depth, as we see in the 

 exceptional case of the Gulf Stream ; but the action of the 

 winds and waves is felt at only very moderate depths ; and it is 

 only in volcanic regions, like those of the West Indies and of 

 Japan, that soundings hav^e revealed thus far any striking topo- 

 graphical features. The very absence of modifying and dis- 

 turbing conditions, such as chemical and aerial denudation, tends 

 to keep the original features due to submarine disturbances of 

 the earth's crust more or less intact ; and it is to the absence 

 of those conditions that we undoubtedly owe the existence of 

 such gigantic plateaux as the Yucatan Bank, and the great 

 Florida Bank, with its eastern extension of the Bahama Bank, 

 which reaches out till it nearly meets tlie comparatively nar- 

 row South American bank, that extends from the mainland to 

 Sombrero. 



What can be more impressive than the stupendous slope of 

 over twelve thousand feet that forms the eastern edge of the 

 Bahama Bank, stretching from the Great Abaco nearly unbro- 

 ken as far as the Virgin Islands, with high passes between Porto 

 Rico and San Domingo, and a deep canon between San Do- 

 mingo and the southern end of the Bahama Bank? The north- 

 ern extremity of this cliff, over seven hundred miles long, forms 

 the edge of a huge triangular plateau, five hundred miles by 

 two hundred and fifty, scarcely rising above the level of the sea, 

 and flanked on its western side by the high chains of Cuba. Its 

 eastern extremity falls into the edge of a sink, of a depth of over 

 four thousand fathoms, and culminates at a horizontal distance 

 of less than eighty miles in a summit on the island of Porto 

 Rico, no less than thirty thousand feet above the lowest point 

 of that depression. What can be more graceful than the com- 

 paratively narrow chain of the curve formed by the Windward 



