VIEWS OF AMERICAN WRITERS. 387 



the baselevel of erosion over large portions of its surface."* Moreover, 

 it was a result of continental depression and not of erosive encroachment 

 of the Avaves that the upper Cambrian sea gained its extension over the 

 great interior of the continent (page 565). The relation of subaerial and 

 marine agencies are here, as in so many instances, just reversed from their 

 proportionate activities in Richthofen's scheme. 



McGee was the first to present a clear statement of the vast subaerial 

 denudation of our Atlantic slope in Mesozoic time : 



"Before the initiation of Potomac deposition, but subsequent to the accumula- 

 tion of the Triassic and Rhaetic deposits and to the displacement and diking by 

 which they are affected, there was an eon of degradation during which a grand 

 mountain system was obliterated and its base reduced to a plain which, as its topog- 

 raphy tells us, was slightly inclined seaward and little elevated above tide. . . 

 There followed a slight elevation of the land, when the rivers attacked their beds 

 and excavated valleys as deep as those today intersecting the Piedmont plain. 

 . . . Then came the movement by which the deposition of the Potomac forma- 

 tion was initiated; the deeply ravined baselevel plain was at the same time sub- 

 merged and tilted oceanward."t 



It appears from the foregoing examples that, in denuded plains over 

 which unconformable sediments have been deposited, some late and small 

 share in the work of denudation may be allowed to the shore waves as 

 they advance over an already prepared peneplain when depression occurs ; 

 but it is otherwise with those uplifted and dissected plains of denudation 

 upon which there is no reason to think that unconformable sediments' 

 have ever been deposited. The plateau in which the Grand can^^on of 

 the Colorado is cut is an extraordinary exam pie of this kind. It is, 

 moreover, notable from consisting of nearly horizontal strata, where acute 

 observation has been needed to detect evidence of the long cycle of ero- 

 sion passed through before the region was uplifted to its present altitude. 



The great plateau is beveled obliquely across the Carboniferous and 

 Permian strata, so that the undulating surface of the upland in its medial 

 part presents Permian beds on the hills and Carboniferous beds in the 

 hollows ; but to the south, where the strata gently rise, the whole surface 

 is Carboniferous ; to the north, where the strata sink, the surface is en- 

 tirely Permian. 



*' We may suppose that this entire region, at the epoch at which the great denuda- 

 tion of the Mesozoic system approached completion, occupied a level not much 

 above the sea. Under such circumstances it would have been at what Powell terms 

 baselevel of erosion. The rivers and tributaries would no longer corrade their 

 channels. The inequalities which are due to land sculpture and the general process 

 of erosion would then no longer increase, and the total energy of erosion would be 



* Twelfth Ann. Rep. U. S. Geol. Survey, 1891, p. 5G2. 



t Three formations of the middle Atlantic slope. Am. Jour. Sci., vol. xxxv, 18SS, p. 142. 



