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 voleanic regions, like those of the West Indies and of 
Japan, that soundings have 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 the 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 сайоп 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 a£ 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 
