386 DAVIS — PLAINS OF MARINE AND SUBAERIAL DENUDATION. 



The erosion followed uplift, the deposition followed submergence when 

 the erosion was essentially completed. Along the surf^ice of contact 

 there are — 



"A few bosses of Silurian strata rising higher than the hard quartzitic sandstone 

 which forms the base of the Carboniferous. These are Paleozoic hills, which were 

 buried by the growing mass of sediment. But they are of insignificant mass, rarely 

 exceeding two or three hundred feet in height, and do not appear to have rufhed 

 the parallelism of the sandstones and limestones of the massive Red Wall gi'oup 

 above them " (page 209). 



On another page (181) Dutton says : 



" The meaning of this great unconformity obviously is that after a vast body of 

 early Paleozoic strata had been laid down they were distorted by differential ver- 

 tical movements, were flexed and faulted, and were elevated ab(n'e the sea. They 

 were then enormously eroded. . . Still later the region was again submerged." 



Over the rugged country thus ravaged, tlie later strata, perhaps 15,000 

 feet thick, were laid down. 



Many other examples of the American view may l)e given. Most of 

 them, as in the cases already cited, take no account of the possibility that 

 the evenly abraded surface of the older terrane might be essentially the 

 product of wave work, but tacitly assume that it resulted from sul)acrial 

 erosion, followed b}^ depression, with more or less tilting, so that the sub- 

 mer":ed area comes to be sheeted over with waste derived from some non- 



'&' 



submerged area. 



'to^ 



Irving concludes that in Wisconsin — 



"An amount of material vast beyond computation was removed from this ancient 

 land l^eforc the encroachment u^xtn it of the sea within which the [Potsdam] sand- 

 stone was deposited."* 



The buried oldland is referred to as a " sul)-Pots(lam land surface." f 

 Van Hise, writing of the great unconformities below and above the 

 Penokee series of Wisconsin and upper Micliigan, implies great subaerial 

 erosion, b}" which an uplifted region was reduced to a pene})lain ; depres- 

 sion, submergence and deposition of material eroded elsewhere then fol- 

 low^ed. The essentials of the explanation are that the Penokee series 

 rests upon an ancient land surface, more or less modified b}' w^ave action 

 at the time of submergence, but worn down from its constructional form 

 almost entirely by subaerial agents.| 



Walcott, recognizing wave work at the margin of an encroaching sea 

 as contributing to the formation of basal conglomerates, nevertheless ex- 

 plains the great pre-Cambrian land area of our country as " approaching 



♦ Seventh Ann. Rep. U. S. (Jeol. Survey, 1HS8, p. 402. f Il»i<l., p. 409. 



X U. S. Geo!. Survey, monograph xix, 1S92, pp. 404-400. 



