MUSEUM OF COMPARATIVE ZOOLOGY. 377 



These facts have an immediate bearing upon the question of the 

 origin of submarine basins as compared with the inequalities of the 

 mainland. The configuration and relief of our continents, as far as 

 they are not the result of later denudations, have been determined by 

 uplifts and the gradual rise of the land above the level of the sea, and 

 hence have arisen the fractured ridges of mountain ranges, with their 

 upright crests ; while the areas of the great oceanic basins are surfaces 

 of depression or subsidence, upon which prominent inequalities would 

 of necessity be wanting, from the very fact that the breaks, where 

 any occurred, must be turned downward. If this view is correct, it 

 naturally follows that the main outlines and circumscription of the con- 

 tinents and of the oceans must have been determined at the very be- 

 ginning of the formation of inequalities upon the earth's surface, and 

 remained essentially the same through all geological ages, varying only 

 as to their relative height and depth, as well as to their respective 

 extension. 



Such considerations enable us now to raise the question of the age 

 of the Gulf Stream. Our present knowledge of the atmospheric and 

 oceanic currents justifies the assumption that, — owing to the rotation 

 of the earth upon its axis, and taking for granted that the latter has 

 never changed its poles, — the great equatorial currents, fostered by 

 the trade-winds, must flow in an east-westerly direction and be fed 

 by northerly and southerly polar currents slanting westwards towards 

 the equator. As long as the chain of the Andes did not intercept 

 the Atlantic equatorial current, it must have been continuous with the 

 great Pacific current ; and, as stated by A. Agassiz, in another report, 

 p. 305, there is palajontological evidence that during the cretaceous 

 period the through channel was still open. I may add that I have my- 

 self seen the evidence, along the base of the Eocky Mountains, and on the 

 western borders of the Amazonian Valley, of the post-cretaceous ele- 

 vation of the great mountain range which rises like a huge barrier on 

 the western side of the North and South American Continents, dividing 

 the Pacific water-shed from that which feeds the Atlantic. We are thus 

 justified in assuming that, even during the cretaceous period, there ex- 

 isted a great North Atlantic current, flowing from the northeast in a 

 southwest direction, and that the Gulf Stream has assumed its present 

 course in the opposite direction since that period ; that is. since the Rocky 



Mountains and Andes have joined hands across Central America. This 



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