40 



PHYSIOGRAPHIC GEOLOGY. 



Fig. 



30. 





N 



A 



60° 



jr^m 





•^30° 



X. ^^ 











'W 



yv. :• 





^0° 



s 



$ 





or south of the equator, is the same. This system is illustrated 



in the annexed figure (fig. 30), in which all minor movements 



are avoided in order to present only the 



predominant courses. WE is the equator 



in either ocean ; 30°, 60°, the parallels so 



named ; N, S, the opposite polar regions : 



the arrow-heads show the direction of the 



movement. 



The main facts are as follow : — 



(1.) A flow in either tropic (see figure) from 

 the east, and in the higher temperate latitudes 

 from the west, the one flow turning into the 

 other, making an elliptical movement. The 

 tropical waters may pass into the extratro- 

 pical regions in all longitudes, but the move- 

 ment is appreciable only towards the sides of 

 the oceans. 



(2.) A flow of a part of the easterly-flowing extratropical waters 

 (see fig. 30) outward towards the polar region, to return thence 

 with the polar waters mainly along the western side of the ocean 

 (though partly by the eastern). 



(3.) A flow of the colder current under the warmer when the 

 two meet, since cold water, is heavier than warm. 



(4.) A lifting of the deep-seated cold currents to the surface 

 along the sides of a continent or island, or over a submerged bank, 

 as on the west coast of South America. 



(5.) A movement of the circuit, as a whole, some degrees to the 

 north or south with the change of the seasons, or as the sun passes 

 to the north or south of the equator. 



(6.) On the west side of an ocean (see fig. 30) the cold northerly 

 current is mainly from the polar latitudes ; on the east side it is 

 mainly from the high temperate latitudes, being the cooled extra- 

 tropical flow on its return. 



(7.) The tropical current has great depth, being a profound 

 movement of the ocean, and it is bent northward in its onward 

 course by ihe deep, submerged sides of the continents. The Gulf 

 Stream has consequently its main limit 80 to 100 miles from the 

 American coast, where the ocean commences its abrupt depths 

 (§ 17). Hence, a submergence of a portion of a continent suffi- 

 cient to give the body of the current a free discharge over it 

 would have to be of great depth, — probably two thousand feet at 

 least. 



42. The usual explanation of the courses is as follows : — As the 



