PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 439 



valley of the Rounclout. This marks the line of a fracture, which 

 has been vastly widened by denudation. This valley extends 

 from the Hudson through Virginia parallel to the Great Valley. 

 Upon the supposition that the upper devonian once covered this 

 valley, there have been removed from it, its entire length, 1,500 or 

 2,000 feet of strata. From some remarkable upthrows which lie 

 within it, at least 5,000 feet have been carried away. On some 

 other occasion, I hope to be able to present these wonderful 

 uplifts and erosions in detail. 



We have, it v/ill be seen, valleys under two dissimilar condi- 

 tions: one, where the rocks have not undergone disturbance, the 

 other, where they have been highly changed from their original 

 position. In both instances they have reached through every geo- 

 logical horizon, from laurentian to the carboniferous, and should 

 we extend our inquiries into the mesozoic, cretaceous, and tertiary, 

 we should find the same phenomena. Indeed the profoundest 

 cliasms of erosion, are in the cretaceous of the Colorado river. 



What has been the agent or agencies which has accomplished 

 this mighty work ? To shave ofl" thousands of feet from our 

 mountains, to scoop out wide valleys — to excavate the whole 

 length and breadth of the countiy ? The great American ex- 

 pounder of the Swiss system, says: Glaciers, nay 07Le glacier, over 

 a mile in thickness, covering, perhaps, the entire northern portion 

 of the North American continent. Another philosopher of the 

 Pennsylvania school, says: Some unknown agent accomplished ttie 

 work in one fell swoop of its most tremendous power. 



I am not prepared to assent to either of them, nor to any one 

 agency. It seems to me necessary to call into our aid all the 

 forces which now act to degrade, remove, and change the surface 

 of the globe. Time is the great element to take into our calcula- 

 tions. Next, molecular disintegralion and decomposition Third, 

 abrasion and removal by aqueous action. Fourth, oscillation and 

 change in the relative height of the land area over the oceanic. 



To obtain a clear conception of how this mighty work was done, 

 let us bear in mind that we are not gazing upon phenomena of any 

 recent date, as compared with geological time, nor upon physical 

 features, which were contemporaneous in the date of their history. 

 There is order and sequence in their history. One has succeeded 

 the other, in regular geological succession. None of our valleys, 

 with but few exceptions, save the insignificant excavations through 

 which our present streams flow are later than the drift period. 



