308 S. WEI DM AN 



and the sandstone district south of the pre-Cambrian area have 

 already been pointed out. The sandstone district is apparently 

 a graded valley plain sloping upward to the north, and the 

 isolated mounds are disappearing remnants of the divides of an 

 early and less eroded sandstone region. In the sandstone dis- 

 trict the main surface feature is the plain of the valley bottom 

 with an entire absence of the valley sides, but as one follows 

 north along the Wisconsin river into the pre-Cambrian area, 

 valley sides are seen to close in upon the river, the Wisconsin 

 valley becomes narrow and is carved deeper and deeper into the 

 peneplain toward the north. The slope of the valley plain of 

 the sandstone district from Kilbourn City to Grand Rapids, as 

 shown in Plate I, Fig. 3, is upward to the north at the rate of 

 about two feet per mile ; but in the pre-Cambrian district the 

 narrow valley bottom of the Wisconsin river rises from Grand 

 Rapids, with an elevation of 1,000 feet, to Merrill, with an eleva- 

 tion of 1,250 feet, as indicated in the same figure, thus showing 

 a rate of ascent of about four and one-half feet per mile in the 

 dissected peneplain. Throughout the course of the Wisconsin 

 river in the pre-Cambrian district there are numerous rapids. 

 From Nekoosa to Stevens Point across the border of the sand- 

 stone district there is a series of almost continuous rapids where 

 the river flows southward, there being no rapids in the westward 

 course of the river flowing parallel to the border of the two dis- 

 tricts. North of Stevens Point the rapids are not so numerous, 

 though extensive rapids occur at intervals of ten to twenty miles, 

 as at Mosinee, Wausau, and Merrill. The broad valley plain of 

 the sandstone district thus assumes a steeper gradient as it 

 changes to the narrow Wisconsin valley, with numerous rapids, in 

 the dissected peneplain. It is believed that the steeper gradient 

 and narrowness of the valley of the Wisconsin, and likewise of the 

 other valleys in the pre-Cambrian area, as compared with the 

 same and similarly related valleys in the sandstone area, are 

 fully explained by the greater resistance to erosion of the crystal- 

 line rock than of the sandstone ; and if this be true, it would follow 

 that the degradation of the sandstone district and the dissection 

 of the pre-Cambrian peneplain were very probably contempo- 



