152 



Louis Agassiz. 



having at all times been areas of gradual upheaval 

 with comparatively slight oscillations of rise and 

 subsidence, and the oceans at all times areas of 

 gradual depression with equally slight oscillations. 

 Now that the geological constitution of our conti- 

 nent is satisfactorily known over the greatest part of 

 its extent, it seems to me to afford the strongest evi- 

 dence that this has been the case ; while there is no 

 support whatever for the assumption that any part 

 of it has sunk again to any very great depth after its 

 rise above the surface of the ocean. The fact that 

 upon the American continent, east of the Rocky 

 Mountains, the geological formations crop out in 

 their regular succession, from the oldest azoic and 

 primordial deposits to the cretaceous formation, 

 without the slightest indication of a great subse- 

 quent subsidence, seems to me the most complete 

 and direct demonstration of my proposition. Of 

 the western part of the continent I am not prepared 

 to speak with the same confidence. Moreover, the 

 position of the cretaceous and tertiary formations 

 along the low grounds east of the Alleghany range is 

 another indication of the permanence of the ocean 

 trough, on the margin of which these more recent 

 beds have been formed. I am well aware that in a 

 comparatively recent period, portions of Canada 

 and the United States, which now stand six or seven 

 hundred feet above the level of the sea, have been 

 under water ; but this has not changed the configu- 

 ration of the continent, if we admit that the latter 

 is in reality circumscribed by the two-hundred- 

 fathom curve of depth." 



