260 GEOLOGY OF EASTERN WISCONSIN. 



thickness of the formation is reduced by the inequalities of its Arch- 

 aean bottom, it is by the loss of the lower members of the group and 

 not by the thinning of all. 



/. Madison Sandstone. This name has been assigned, by Prof. 

 Irving, to the uppermost subdivision, from its occurrence in the vi- 

 cinity of the capital, where it is extensively used, under that name, as 

 a building stone. He regards this as a member of the Calciferous 

 group above, rather than of the Potsdam, in respect to which, howev- 

 er, I feel compelled to differ from him, for reasons given below. 



At its more typical localities, this sandstone is a rather coarse 

 grained, thick bedded, compact, but soft, slightly calcareous, light 

 colored sandstone. It is best shown at Lucas Point, on the southern 

 shore of Green Lake, a few miles west of Ripon, where, however it is 

 more than usually fine-grained. 



At this point it is horizontally laminated, and marked by wavy 

 lines of reddish yellow iron stains, though these are probably not con- 

 stant characters. In its upper portion, immediately beneath the low- 

 er magnesian limestone, it is at most locations coarse, and the topmost 

 layer is of ten broken up and mixed with calcareous material, giving it 

 a coarsely brecciated structure. This layer, or its equivalent, usually 

 marks the upper limit of the formation with distinctness, though more 

 or less of sand mingles with the lower ledges of the limestone above. 



2. Mendota Limestone. This name has been given to this group of 

 strata, by Prof. Irving, from its occurrence on Lake Mendota, near 

 Madison. The term limestone is applicable, however, to this division 

 in eastern Wisconsin, only in a qualified sense. It consists really of a 

 group of alternating strata of arenaceous magnesian limestone, sandy 

 calcareous shales, and shaly and calcareous sandstones. The lime- 

 stones are soft, granular, porous, friable, rather thin bedded, buff 

 colored, and frequently contain searns of greensand. 



They resist erosion to a greater degree than the sandstones above 

 and below, and so sometimes form the protecting cap of terraces. 



The shales are variegated with yellow, red and purple. Those of the 

 latter class are quite characteristic, though something very similar 

 occurs at a few exceptional localities in the St. Peters sandstone. 

 The purple portion, which only makes up a part of the mass of the 

 rock, consists of irregular layers, lumps and patches mixed with red- 

 dish and yellowish portions, giving the whole a peculiar mottled ap- 

 pearance. The lighter colored shales occur intimately associated with 

 these. Both classes are more or less arenaceous and calcareous, are 

 soft and brittle, easily crushed, and readily decomposed under the ac- 

 tion of atmospheric agencies. 



