300 S. WEI DM AN 



Immediately south of the Wisconsin Central Railway bridge at 

 Stevens Point on the west side of the Wisconsin river relations 

 similar to those appearing in the above sections are shown. At 

 this place is lO or 12 feet of decomposed pre-Cambrian rock, 

 overlain by sandstone, along the river bank, while 40 feet west 

 of the river is a well showing 4 feet of sandstone overlying 12 

 feet of clay, and 100 feet still farther west, on the west side of 

 the wagon road, on higher ground, is a well showing 12 feet of 

 sandstone and below this 12 feet of the kaolinized pre-Cambrian 

 rock overlying hard crystalline rock. Similar thicknesses of 

 the clay formation were noted beneath the sandstone mounds in 

 which the sandstone quarries are located, on the west side of the 

 river at Stevens Point. An instance of a sandstone mound over- 

 lying the clay is shown in the diagram (Fig. 8). About a mile 

 north of Stevens Point is located the Langenberg brickyard, the 

 source of the clay here used being a thickness of 15 to 20 feet 

 of the decomposed pre-Cambrian schists. This clay bed is 

 about a mile and a half from the Wisconsin river and is not 

 overlain with sandstone, though the latter formation lies at a 

 higher level, forming a low broad hill one-fourth of a mile to 

 the north. 



It seems hardly necessary to multiply instances of the occur- 

 rence of a variable thickness of decomposed pre-Cambrian 

 lying beneath an equally variable thickness of the Potsdam 

 sandstone. The relations shown in the above sections are found 

 wherever the sandstone and pre-Cambrian occur, whether it be 

 along the river banks or in the well sections through the saad- 

 stone far from the streams. 



As already stated, it was Irving's belief that the clays were 

 formed after the deposition of the sandstone, two possible expla- 

 nations^ being offered by him: one, that the clays were formed by 

 processes of weathering during the existing cycle of erosion, but 

 mainly in pre-glacial times ; and the other, that "surface waters, 

 percolating through the porous sandstone — in ancient times 

 much thicker than now — have formed natural watercourses 

 along the junction between it and the less easily penetrable 



' Geol. of Wis., Vol. II, p. 464. 



