R. Irving—Primordial and Canadian Rocks of Wisconsin. 448 
and brecciated layers. In none of these subdivisions have any 
fossils been seen. 
V. St. Peter's Sandstone—This formation appears with its 
usual characters, except that east of Madison it presents a very 
marked deviation from its usual very uniform thickness of 100 
feet, being reduced as low as 40 feet. 
On the Mississippi River bluffs, in the vicinity of Winona 
and La Crosse, and again farther south, there exists, so far as now 
wn, between the Potsdam and St. Peter’s sandstone, 200- 
250 feet of dolomite without sandstone layers. Farther north- 
west, there occurs in the valley of the Minnesota River—accord- 
ing to Professor N. H. Winchell, State geologist of Minnesota— 
the following succession of strata, beginning below: 
I. The Lower or Potsdam Sandstone. 
IL The St. Lawrence Limestone. 
UL The Jordan panastone .-- < 2665.0 os eka. coe 50 feet. 
IV. ‘The Shakopee Limestone . -...----2....1: 1.8% 
V. The St. Peter’s Sandstone. 
Sandstone, and Mendota Limestone. Future investigations may 
show that the Jordan and Madison ‘sandstones are somewhere 
nown, however, the two regions in which these similar alterna- 
tions occur are separated by an area where all merge into a mass 
of limestone whose thickness is much greater than their com- 
bined thickness. 
The following schedule serves to show the probable equiva- 
lence of the strata in these different regions: 
South Central Wisconsin.| Mississippi Bluffs. Minnesota River. 
| St. Peter’s Sandstone. | St. Peter’s Sandstone. | St. Peter’s Sandstone. 
i) 
2 Main Body of Limestone, Shakopee Sandstone. 
= 1 Lower Magnesian 70 feet. 
|Madison Sandstone, 35 ft. 
Limestone, 
Mendota Limestone. 200 to 250 feet. ist. La 
30 feet. 
Jordan Sandstone, 50 ft. 
St. Lawrence Limestone. 
Lower or Potsdam 
Sandstone. 
Lower or Potsdam Lower or Potsdam 
Sandstone. Sandstone. 
Terpsourtsg | 
University of Wisconsin, February 13, 1875. 
Am. Jour Sct. Tatrp Series A IX, No. 54.—Junz, 1875. 
