544 F- T. THWAITES 



with abundant specks of green shale occurs near the base of the 

 Oneota in western Wisconsin. In northeastern Wisconsin there 

 are several "stray sands" some of which have doubtless been mis- 

 taken by well drillers for St. Peter. These sandstones are not good 

 markers ; closely spaced records in the Fox River Valley show that 

 they pass horizontally within relatively short distances into floating 

 sand grains in a dolomite matrix. The well record in Table III 

 shows the character of the Lower Magnesian group in far north- 

 eastern Wisconsin, Not enough work has been done on the outcrops 

 in this region to permit of correlating any of these beds with the 

 western section. 



The maximum known thickness of the Lower Magnesian group 

 is over 700 feet in western Illinois; to the northeast it is much 

 thinner, in many places less than 100 feet. Throughout several 

 counties in eastern Wisconsin the dolomite is absent so far as 

 known from well records; within this district its horizon is repre- 

 sented in some places by a thin layer of chert and non-calcareous 

 clay, residium of the cherty dolomites, but near Milwaukee no 

 trace of the Lower Magnesian has been found. 



CAMBRIAN SYSTEM^ 



Madison Formation 



Distribution. — The type locality of the Madison formation is 

 the quarries just west of Madison. The formation, which lies 

 immediately beneath the Oneota, has been traced in outcrops from 

 that locality about 25 miles to the west. A few outcrops occur east 

 of Madison, and it is also known in a well record at Sun Prairie, 

 Wisconsin. Beds of somewhat similar character in the western 

 part of Wisconsin are beheved by Ulrich to represent the Madison. 



Character. — The Madison of the original locaHty is a fine 

 grained, buff, calcareous sandstone at the top, which passes below 

 to a pure white medium grained sandstone much like the Jordan. 

 In well samples it can only be distinguished from the Jordan, 

 which underlies it throughout western Wisconsin, by its yellow 

 color and dolomitic cement. The maximum thickness is about 

 50 feet. 



^ The Madison formation is regarded as the top of the Cambrian by most geolo^ 

 gists; Ulrich places the top of that system at a lower horizon. 



