DENUDATION, OR GENERAL EROSION. 



275 



any extent greater. The accompanying section across Middle Tennes- 

 see shows a vertical erosion of 1,200 to 2,400 feet, over the whole 

 valley of Middle Tennessee, which is sixty miles across and one hun- 

 dred miles long. In most cases the removed matter is not so easily 

 estimated as in those mentioned. The strata in mountain-chains are 



^^^?^^ 



Fig. 251.— Section through Portions of England. 



usually folded in a very complex way, and then denuded. But the 

 ideal restoration of these may be effected, and the amount of erosion 

 approximately estimated. Figs. 250 and 251 are sections across the 

 mountainous parts of England, as restored by Prof. Eamsay. 



Average Erosion. — By these methods Prof. Ramsay estimates the 

 denudations over many portions of England to be not less than 10,000 

 to 11,000 feet in thickness.* Over the whole Appalachian region the 

 denudation has probably been enormous, in some places 8,000 to 20,000 

 feet. Over the whole region of the high Sierra Eange, as we have 

 shown,f erosion has removed the whole of the Jurassic and Triassic 

 slates, and bitten deep into the underlying granite. The thickness of 

 these slates is not known, but it must be many thousand feet. In the 

 Uintah Mountain region, according to Powell, over an area of 2,000 

 square miles, an average thickness of three and a half miles has been 



Fig. 252.— Uintah Mountains— Upper Part restored, showing Fault : Lower Part showing the 

 Present Condition as produced by Erosion rafter Powell). 



taken away (Pig. 252), the extreme thickness removed being nearly 

 five miles. From the Wahsatch have been removed 32,000 feet, or 



* Geological Observer, p. 819. 



f American Journal of Science and Arts, vol. v, p. 325. 



