PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 441 



latter the crettiCeous, and this in turn the tertiary. The whok 

 interior of the continent has been mainly builded from the waste 

 of older strata lying on the outer edge. Some minor uplifts of 

 ancient rock in the interior, contributed their mite. 



I consider, then, that the valleys of the disturbed portion of our 

 country are as old in their origin, as the rock system to which they 

 belonir, and have sufiered denudation from that time to this, by 

 the same agencies, and no other, which now act through the whol* 

 year, upon the North American continent. Through each geolo- 

 gic change, the older valleys suffered, from widening and dee^^eu- 

 iug, while hill tops were lowered. 



In the undisturbed portion, running water has been the most 

 efficient agent to break up and remove the rock masses. With 

 the elevation of the carboniferous, along the Atlantic, drainage 

 commenced. The channels of all the great rivers rising within it, 

 were immediatel}'' marked out. Rain descending upon the plateau, 

 gathered in pools, until it broke over its bounds, and descending th« 

 edges of the strata hewed out for itself a passage to the sea. In 

 the early stages, there were many water-falls and cascades, but as 

 the streams wore their channels backwards cascades were chanixed 

 into rapids, and rapids into gently flowing streams. With the 

 deepening of channels, banks would be undermined, the superim- 

 posed material would crumble and fall into the stream, currents 

 would bear it along, estuaries, deltas, and the ocean would receive 

 it. Thus, while the valleys increased in width, and the hills suf- 

 fered diminution, the seas were filled up on the shore line by tidal 

 deposits, the Atlantic was narrowed, and the interior seas filled up. 



When I stand at the confluence of the Alleghany and the Monon- 

 gahela, and behold those noble rivers in spring flood, turbid with 

 sediment and debris, flowing through wide and deep valleys — and 

 when I ascend their slopes to their summits, and there find l)ut a 

 narrow plateau to cross before I am compelled to descend into 

 another vallev as profound fis the one from which I have emero'exl, 

 I am not surprised at their depth, nor width; nor at the remnant 

 which is left of the original high land, when I reflect that in the 

 Permian early days these streams were but sluggish reedy watei"s, 

 like the upper Mississippi; but they had a mission to perform, to 

 fill up the cretaceous seas, which then had a shore line at Paducah, 

 Ky., and Cairo, 111. That was the work to be done. The Ohio 

 has done its part. No wonder the carboniferous has been lost. It hiis 

 been gnawed into fragments, and these fragments are the concrete 

 of the new. 



