AQUEOUS AGENCIES. 47 



often recognize the old shore-lines of previous geological 

 epochs. 



Oceanic Currents. 



The earth is covered with two oceans, an atmospheric 

 and an aqueous. The former covers the whole of it, fifty 

 or more miles deep ; the latter covers three quarters of it, 

 three miles deep. On the surface of the one we swim and 

 sail, on the bottom of the other we crawl. .Both of these 

 oceans are in constant circulation in every part. The cur- 

 rents in the one are winds, in the other oceanic streams. 

 The cause of circulation and the general directions of the 

 currents are also the same. There is, indeed, some dif- 

 ference of opinion as to the immediate cause of oceanic 

 currents ; but there can be no doubt that, directly or indi- 

 rectly, they, like winds, are caused by difference of tem- 

 perature between the equator and the poles. In both 

 cases, too, there are disturbing causes which complicate 

 the result. In the one case, local variations of tempera- 

 ture and extreme mobility of the medium ; in the other, 

 the existence of unseen submarine banks, and especially 

 of impassable barriers, the continents. As our sole object 

 is to discuss their geological agency, we shall describe but 

 one of these great oceanic currents as an example. 



Gulf Stream. This stream takes its origin in the equa- 

 torial current which, stretching across the Atlantic from 

 the coast of Africa, strikes the wedge-shaped eastern point 

 of South America and divides north and south. The 

 larger northern branch runs along the coast of South 

 America into the Caribbean, and thence through the 

 Straits of Florida into the North Atlantic. After passing 

 the point of Florida it turns north, runs along the coast of 

 the United States, turns eastward from the coast of Lab- 

 rador, and, after sending a branch toward the Arctic re- 

 gion north of Europe, turns southward to join again the 

 equatorial current on the coast of Africa. The amount of 



