76 THE GLACIAL LAKE AGASSIZ. 



Next beneath the drift is a thick formation of magnesian hmestone, 

 shown by comparison with the other wells to be the Galena and Trenton 

 strata, classed together as one formation under the second of these names 

 by Whiteaves and Tyrrell, which outcrops at a distance of 75 to S5 miles 

 northward, in the ^^cinity of Lower Fort Gany and East Selkirk, Mani- 

 toba. Its top and bottom at Humlioldt, however, are respectively 612 and 

 317 feet above the sea, the entire formation here being thus beneath the 

 level of Lake Winnipeg. Li southeastern Minnesota, southwestern Wis- 

 consin, and adjoining portions of Iowa and Illinois, the Galena and under- 

 lying Trenton limestones together range from 200 to 300 feet or more in 

 thickness. 



The sandstone next below, having a thickness of 71 feet, is evidently 

 the equivalent of the St. Peter sandstone, referable to the Chazy epoch, 

 which in southeastern Minnesota underlies the Trenton limestone, and 

 ranges in thickness there from about 75 feet to 164 feet. Its continuation 

 in Wisconsin, as described by Chamberlin and Irving, averages probably 

 between 80 and 100 feet thick, varying from a maximum of 212 feet down 

 to a fraction of 1 foot. In this and adjoining States, according to Irving, 

 it is continuous "over a region whose diameters are 500 and 400 miles."^ 



Beneath this the Humboldt Avell penetrated 92 feet of shales, partly 

 arenaceous and calcareous, which correspond to the Lower Magnesian or 

 Shakopee limestone of southeastern Minnesota, ranging from 96 feet in 

 thickness at Shakopee to 200 feet in Houston County, while in Wisconsin 

 it is from 65 to 250 feet thick. The reports of the geological surveys of 

 these States regard this formation as of Upper Cambrian age, but Walcott, 

 in his more recent review of the Cambrian,^ assigns the Lower Magnesian 

 limestone wholly or mainly to the base of the Lower Silurian system. Its 

 eastern equivalent is the Calciferous sandi'ock of New York. 



The entire Cambrian and Algonkian systems are wanting in this sec- 

 tion, and the Lower Silurian strata rest directly on the Archean crystal- 

 line rocks. 



' Geology of Wisconsin, Vol. I, pp. 145-150 ; Vol. II, p. 555. 

 2 U. S. Geol. Survey, Bulletin No. 81, 1891, p. 363. 



