43G TRANSACTIONS OF THE AMERICAN INSTITUTE. 



200, 300, to GOO feet deep. In the short distance of thirty miles 

 from the source of the Alleghany, it flows through a valley half 

 to three-quarters of a mile in width, and from seventy-five to one 

 hundred feet deeper than the meadows. This is in the soft shales 

 of the Chemung. In the harder strata of the carboniferous, the 

 valley is much narrower. After emerging from the carboniferous, 

 at Portsmouth, Ohio, the Ohio river begins to widen its valley — 

 until before debouching into the Mississippi, it is from five to 

 eleven miles wide. Its greatest depth, at any point I am ac- 

 quainted with, is at the mouth of Yellow creek, where it makes 

 its great northwestern bend. It is here 750 to 800 feet deep to 

 the water, and about 100 feet deeper beneath. All lateral streams 

 flowing into it have a corresponding depth and breadth. The 

 Muskingum is, I judge, somewhat broader, in correspondence 

 with the magnitude of the stream. 



The Delaware river has a much narrower Valley, and also the 

 Lehigh; but the Susquehanna attains a corresponding breadth 

 with the Ohio. The Mississippi is the broadest of all — twenty 

 miles is not uncommon for it; and Harper puts it at 100 in some 

 places of the lower portion. 



Take the entire area of any given portion of the carboniferous, 

 devonian, and upper silurian strata, drained by any of our large 

 streams, west of the mountains, and I judge that full nine-tenths 

 of the upper portion has been swept away along their valleys. 

 There is another sj^stem of denudation which seems to have 

 aflected the entire surface of the country, the mountain tops, the 

 hill slopes, the auti-clinal folds and the level champaign country. 

 No portion of our country, which I have visited, appears to have 

 been exempt from this erosion. Virginia and Kentucky have suf- 

 fered, as Avell as New York, Maine and Canada. 



Examples of this denudation can best be seen travelling south- 

 w^ards from Lake Ontario. From this body of water the traveler 

 ascends by a series of terraces to plateaus succeeding each other, 

 until he attains the height of the Catskiil mountains. Sometimes 

 these terraces rise boldly, 90 or 100 feet — sheer precipices — 

 ancient sea walls were they? By means of one of these in Onon- 

 daga, I was enaljled to obtain data, for the amount of denudation 

 the soft strata of the salt group had undergone. From six to 

 eight miles in width could clearly be made out, and for the harder 

 limestones of the Ilelderburgh group, not as much. 



The want of outliers prevents that accuracy of our calculations 



