AMERICAN GEOLOGY DECADE OF 1870-1879. 567 



mined by conditions not found in the rocks through which the chan- 

 nels are now carved, but which were in existence when the district 

 hist appeared above sea level, he called supt rim/posed valleys. 



In this same report, too, Powell made use for the first time of the 

 term hose level of erosion, which he defined as — 



an imaginary surface inclining slightly in all its parts toward the end of the prin- 

 cipal stream draining the area through which the level is supposed to extend, or 

 having the inclination of its parts varied in direction as determined by tiibutary 

 streams. 



He pointed out, further, that the region of the Grand Canyon was 

 a.ter all a region of lesser rather than greater erosion; that had the 

 country been favored with a rainfall equal to that of the Appalachian 

 country, the entire area might have been reduced to a base level which 

 would be the level of the sea, though the evidence of such erosion 

 might be almost wholly obliterated. 



J. P. Lesley, writing in 1874 on the iron ores of Spruce Creek, 

 Warriors Mark Run, and Half Moon River, in Huntingdon and Cen- 

 ter counties, Pennsylvania, took the ground that they were residual 

 deposits from the decomposition of ferriferous Lower 



J. P. Lesley on Iron . , , 



Ores and Denudation, Silurian beds, which were at one time buried beneath 



1874. 



more than 16,000 feet of Upper Silurian. Devonian, 

 and Carboniferous rocks, and had been subsequently exposed through 

 decomposition and erosion. 



At the end of the coal era the Middle States rose from the waves. * * The 



edges of the Bellefonte fault stood as a mountain range as high as the Alps, and the 

 backs of some of the great anliclinals of Pennsylvania must have'formed plateaus 

 then as high as Tibet and Bootan arc now. 



Erosion commenced and has continued through the Permian, Jurassic, Cretaceous, 

 and Tertiary ages to the present day, and still goes on. The high plateau was 

 gradually worn down to the present surface. Mountains once 30,000 or 40,000 feet 

 high are now but 2,000 or 3,000 above sea level. 



Such extreme views as to the possible height of mountain ranges 

 are equaled among modern geologists only by those of Clarence King 

 (p. 612). 



In 1874 N. H. Winchell accompanied, as geologist, the party headed 

 by Capt. W. Ludlow to the Black Hills of Dakota. His report, appear- 

 ing in 1875, occupied pages 21 to 66 of the quarto volume issued. 

 and was accompanied by a geological map. Winchell 

 u^e n Bi h a e ckHins, r ^874. recognized the occurrence of Cretaceous, Jurassic, 

 Triassic, Carboniferous, and Silurian (Potsdam) rocks, 

 overlying schists and slates, into and through which granite had been 

 intruded. He agreed in the main with Hayden's observations, as 

 given in his Second Annual Report (1868), though he failed to find 

 evidence of the unbroken conformability of the fossiliferous forma- 

 tions. On the contrary, he mentioned the Red Beds as lying uncon- 

 formably on the Carboniferous limestone. 



