310 S. WEIDMAN 



not at their tops. Professor Van Hise has since intimated his 

 belief in the probability of the principal conclusion concerning 

 the age of the peneplain as here presented. Whether the tops 

 of the buttes in the sandstone district represent a Cretaceous or 

 any other post-Paleozoic peneplain I shall not discuss. But in 

 the pre-Cambrian region studied, although this part of the conti- 

 nent must have been near sea-level several times since Potsdam 

 time, there appears to be no evidence for the belief in the degra- 

 dation of the pre-Cambrian rocks to a peneplain later than the 

 period immediately previous to the deposition of the Potsdam 

 sandstone. 



SUMMARY. 



In the pre-Cambrian area of north-central Wisconsin the main 

 feature of the land surface is the even-summited, flat-topped 

 upland area forming an even sky-line sloping upward to the 

 north and having the appearance of a plain or plateau below 

 which lie the valleys of the region and above which rise a few 

 pointed hills and ridges. The rocks of the pre-Cambrian area 

 are of various kinds of igneous formations and metamorphic 

 sedimentaries, which are much folded and crumpled and have a 

 typical mountain structure. The schistosity and bedding of the 

 various formations dip in all directions, and the formations are 

 cut off abruptly at the even sky-line of the main upland area. 

 The indifference of surface form to internal structure shown in 

 the plain-like feature of the upland area and the mountain 

 structure of the rocks, can be explained in only one way: by the 

 degradation or wearing down by erosion of a once mountainous 

 region to an approximate plain. Hence it is concluded that the 

 even summits of the main upland area of the pre-Cambrian dis- 

 trict represent the surface of an old peneplain of erosion. The 

 pointed hills and ridges consisting of hard resistant quartzite, 

 like the Mosinee hills and Rib hill, were not worn down to the 

 level of the surrounding flat-topped uplands consisting of softer 

 rocks, and these stood up as monadnocks in the peneplain. 



The slope of the peneplain, as shown by the elevations of 

 the flat-topped uplands is downward to the south at the rate of 

 ten feet per mile. The slope of the plain of the sandstone dis- 



