R. Irving—Primordial and Canadian Rocks of Wisconsin. 441 
Ill. The St. Peter's Sandstone—80-100 feet in thickness; for 
the most part a coarse quartzose sandstone; at times with ferra- 
ginous cement, at others loose and incoherent. 
IV. and Buff Limestones. 
During the past season I have been able to make some 
modifications in this scheme, as far as the*Potsdam, Lower 
Magnesian, and St. Peter’s are concerned, for the ire of south 
central Wisconsin, especially in Dane and Columbia Counties. 
The succession now made out is as follows, beginning below: 
I, The Lower or Rips Sandstone___..-_.. ....800 feet. 
IL: The Mendota Limestone... 2 a a 30 
Ill. The Madison Sandciae Se ee back se ce ee 36. « 
IV. The Main Body of Ties. AR ee 80-120 “ 
V. The St. Peter’s Sandstone._.._........... 80-100 * 
VI. The Buff and Blue Limestones. 
I. The Potsdam Sandstone.-—In the region of the “Four 
Lake Country,” about Madison, in Dane County, only the upper 
layers of this great formation are to be seen. Farther north, in 
southern Columbia County, the northerly rise of the rocks brings 
lower layers to view. In all this region the upper 50-100 fee 
of the formation are decidedly dolomitic, but nevertheless “to 
very different and distinguished from the overlying 
which is entered by two th at Madison. The lowest lay- 
ers seem, however, to be a coarse quartzose, non-dolomitic mate- 
rial. The dolomitic admixture in the upper layers amounts to 
15 or 20 per cent. In the Janesville well there was some indi- 
cation of the existence of a distinct layer of limestone at a 
very considerable depth in the Potsdam. Scolithi are the only 
indications of life 
II. The Mendota Limestone.—This limestone has every where 
in the region around Madison, and in fact wherever recognized, 
very ma rked characters. It has always a peculiar yellowish, 
dirty appearance, and is frequent stained by reddish patches 
and seams of non-hydrated oxide of iron. Layers of green sand 
occur, especially about the junction with the ican a sand- 
stone. This limestone is usually heavy-bedded below and thin- 
bedded to shaly above. It is always a some extent arenaceous. 
In localities it is fossiliferous, carrying trilobites especially, and 
most common among them the Dzkellocephalus Minnesotensis, 
or some very similar “form. The fossil locality at Mazomanie, 
on the west side of Dane County, which is alluded to by Hall 
