306 S. WEI DM AN 



Van Hise as products of weathering, thus having an origin some- 

 what similar to these residual clays, but formed a short distance 

 below the land surface, being " regarded as the result of the 

 work of descending waters combined with progressive denuda- 

 tion." 



Probable slope of the buried pre-Cambriaji. — It will at once be 

 seen by the reader that, if, as believed by the writer on the evi- 

 dence here presented, the pre-Cambrian land of north-central 

 Wisconsin is an old peneplain of erosion formed in the period 

 preceding the deposition of the Potsdam sandstone, it is 

 extremely probable that this peneplain has a wide extension 

 beyond the border of the area here described. It is not the pur- 

 pose of the writer, however, to present in this paper the various 

 evidences for the belief in the wide extension of the pre-Potsdam 

 peneplain over the pre-Cambrian land. It might be of interest, 

 however, to point out as briefly as possible the general slope of 

 the surface of the pre-Cambrian beneath the adjacent area of the 

 Paleozoic rocks. It may be sufficient to state that fifty-five miles 

 south of Grand Rapids, at Kilbourn City, as shown in Plate I, 

 Fig. 3, the pre-Cambrian surface is struck at a depth of 385 feet, 

 or 515 feet above sea-level. About thirty-five miles farther 

 south, at Madison, the pre-Cambrian occurs at a depth of 810 

 feet, or 70 feet above sea-level. In the wells at both Kilbourn 

 City and Madison the pre-Cambrian rock struck was called a 

 shale, and it is apparently very similar to the decomposed 

 clayey schists about Grand Rapids. Furthermore, no conglom- 

 erate was found at the base of the sandstone. From Grand 

 Rapids to Madison, therefore, a distance of about ninety miles, 

 the surface of the pre-Cambrian descends from an elevation of 

 1,000 feet to 70 feet above the sea, and thus the slope of the 

 buried pre-Cambrian surface to the south continues in a remark- 

 able manner at the same rate of descent, about ten feet per mile, 

 that is exhibited by the uncovered and dissected peneplain 

 between Merrill and Grand Rapids (see Plate I, Fig. 3). 

 Between Grand Rapids and Kilbourn City is the Necedah pre- 

 Cambrian quartzite bluff with an elevation of 1,080 feet, and 

 between Kilbourn Cityand Madison arethe Baraboo pre-Cambrian 



