THE PRE-POTSDAM PENEPLAIN 



299 



no evidence of sedimentation, and hence could not have been 

 deposited by water. On the other hand, as pointed out by 

 Irving a number of years ago,' there is every evidence, as shown 

 by correspondence in structure and composition, as well as by 

 gradation downward into the hard rock beneath, that the clay 



W/iSCO^SIA< RIVER 



Fig. 5. — Section across the Wisconsin river valley near Grand Rapids. (After 

 Irving.) 



formation had its origin in the weathering and decomposition of 

 the crystalline rocks. It was Irving's belief that the clay, 

 though confined to the pre-Cambrian region, having more or less 

 of a sandstone covering and often occurring beneath a few layers 

 at least of sandstone — which fully agrees with the writer's defi- 



SAND5T0NE. 

 DECOMPOSED atslEISS. 

 PARTLY DECOMPOSED G.NEISS. 

 GRAVE L% SAND. 

 TALUS. 



LlNKNO\/VN 



Fig. 6. — Ideal section across the Black river at Halcyon showing the shale or 

 partly decomposed gneiss. (After Buckley.) 



nition of the area — had nevertheless been formed subsequent to 

 the deposition of the Potsdam sandstone which covers it, instead 

 of antecedent, as believed by the writer. 



In the two sections, after Irving and Buckley (Figs. 5 and 6), 

 the position of the clay beneath the sandstone is clearly shown. 



' Trans. Wis. Acad, of Sci., Vol. II, pp. 13-17 ; and Geol. of IVis., Vol. II, p. 46S. 



