472 THE ICE AGE IN NORTH AMERICA. 



graphical position and contour of South America give special 

 significance to this cause in its relation to the Gulf Stream 

 and to all the regions dependent on it for warmth of climate. 

 Cape St. Roque, the easternmost point of South America, 

 is only five degrees south of the equator, and is fifty degrees, 

 or three thousand miles, east of the longitude of Florida. 

 With the present relation of the trade- winds to each other, 

 this situation of the continent of South America is favorable 

 to the production of the Gulf Stream. For it is evident at 

 a glance that the movement of water caused in the South 

 Atlantic by the southeast trades will be at its maximum over 

 the tropical belt in the vicinity of Cape St. Roque, and that 

 the movement there will be in a general northwest direction. 

 Hence there is great significance in the present contour of 

 the continent, it being such that there is nothing to impede 

 the movement of water once begun by the trade-winds in a 

 northwest direction (at least, so much of it as is north of 

 Cape St. Roque) ; but the current thus started must keep 

 on its way until it reaches the cul-de-sac formed by the Gulf 

 of Mexico, and from this there is no escape except through 

 the passage between the West India Islands and Florida. 

 Here we have a deep, strong current, produced by the same 

 hydrostatic law which propels the comparatively small stream 

 in the hydraulic ram. Or, to draw an illustration from a 

 grander spectacle, the movement is like that of the tides 

 when passing up through gradually restricted channels. In 

 such cases the thin onward movement of the tidal wave over 

 a wide space is translated into a narrow but deeper and more 

 powerful current up the gradually restricted channel into 

 which it is forced ; so that, whereas the general height of 

 the tidal wave is but two or three feet, it sometimes in re- 

 stricted channels, like the Bay of Fundy, rises to the height 

 of seventy feet, and the so-called " bore," characteristic of 

 many tidal rivers, like the Orinoco and the Amazon, becomes 

 the terror of navigators. Through such a translation of the 

 gentle but steady pressure of the southeast trades over the 

 wide area of the South Atlantic Ocean, the waters of the 



