THE PRE-POTSDAM PENEPLAIN 



301 



crystalline rocks, and have thus exerted an unusual disintegrat- 

 ing action ; whilst the sandstone itself has subsequently acted 

 as a preserver of the kaolinized rock from the ordinary eroding 



//ff// 



/^// 



PoticJesm Sandstone 

 Residual Clatj 

 Decomposed Granite 

 Hard Granite 



Fig. 7. — Section across the Wisconsin river at Stevens Point. 



agencies." It seems hardly necessary to enter into the details of 

 the explanations offered by Irving '^ and accepted by Buckley^ for 

 the post-Potsdam development of these clay deposits. It is pur- 

 posed to present in a later publication a fuller discussion^ of the 



l ?v°'-'^'' l River Gravel and 5and 

 Potsdam Sancisfone 

 Residual Clai^ 

 ParilLf Decomposed Rock 



mum Hard CrLjstal line Rock 



wrnwmwm 



Fig. 8. — Section on west side of Wisconsin river at Stevens Point. 



clay and associated formations. It should be borne in mind 

 that the evidence of the peneplain character of the pre-Cambrian 

 land of central Wisconsin before the deposition of the Potsdam 

 sandstone, which is believed to be a fact of great significance in 



^ Ibid., pp. 463, 464. 



'^ Bull. VII, Wis. Geol. and Nat. Hist. Surv., p. 217; also discussion before Wis. 

 Acad, of Sci., December, 1901. 



3 A report on the geology of north-central Wisconsin, Wis. Geol. and Nat. 

 Hist. Surv. 



