312 S. WEIDMAN 



complex variety of pebbles found in the pre-Cambrian con- 

 glomerates, is pointed out as significant evidence in full accord 

 with all the other indications pointing to the pre-Potsdam age of 

 the peneplain. The occurrence of the thick deposits of residual 

 clay beneath the Potsdam sandstone, indicating the deep weath- 

 ering of the crystalline rocks without erosion, and thus the 

 approximate base-leveled condition of the pre-Cambrian land a 

 long time before the encroachment of the Potsdam sea, is 

 adduced as significant evidence pointing to the degradation of 

 the pre-Cambrian land to a peneplain by sub-aerial erosion and 

 not by submarine erosion. The occurrence of the deposits of 

 residual clay at the surface of the pre-Cambrian rocks suggests 

 the probability that similar residual deposits may be found 

 between other unconformable series, and that such residual 

 deposits may be used as evidence of unconformity in other parts 

 of the geological column just as the old soils and weathered 

 zones in the Pleistocene deposits are used for determining the 

 stratigraphy of the glacial formations. 



The probable wide extension of the peneplain over other 

 portions of the pre-Cambrian area is suggested, and the contin- 

 uation of the peneplain slope beneath the surrounding Paleozoic 

 to the south is pointed out. The degrading of the sandstone 

 district to a broad valley plain, the uncovering of the pre-Pots- 

 dam peneplain at Grand Rapids at the border of the crystalline 

 and sandstone districts, and the deep dissecting of the uncovered 

 peneplain farther north, about Wausau, are pointed out as con- 

 temporaneous processes, this work being continued through a 

 long period into the present time. The probability of the former 

 extension of the Potsdam sandstone over a large part of the 

 peneplain of the pre-Cambrian area, if not the whole of it, is 

 suggested. 



S. Weidman. 



Madison, Wis. 



EXPLANATION OF PLATE I. 



Fig. I. — Profile extending approximately east and west through the 

 deeply dissected peneplain at Wausau. The uplands rise to the same level 

 to an elevation of about 1,400 feet representing the surface of the peneplain. 



