334 THE GEOLOGY OF MINNESOTA. 



[St- Lawrence limestone. 



other, the notably distinct lithological characters of some of the forma- 

 tions, and the gradual and regular dip of the strata, when taken with 

 the erosion, enable one to decide with certainty the rock over which 

 he is standing, even when it is hidden from view. All these enable one to 

 read the stratigraphical enigma of the county with little trouble. In this 

 study the intimate knowledge of the county possessed by Mr. W. D. Hurl- 

 but aided greatly, as he cheerfully rendered all the assistance in his power. 

 Many of the details of the map illustrating this county were supplied by 

 him. 



Olmsted county furnishes an excellent field for teaching stratigraphy 

 to a class of students. The strata are interesting. The characters men- 

 tioned above make the reading of them, under their varied degrees of ex- 

 posure and erosion, easy and instructive. For instrution in geological field- 

 work no district could be better adapted. 



The formations found in the county are the following, as known in 

 geological nomenclature. 



g ( Hudson Tlivei group. .Shales, shaly sandstones and impure limestones, the prob- 

 able equivalent of the Maquoketa shaks of Iowa. Seen.. 15ft. 



f 1. Dolomitic limestones of the Galena formation 40-50 ft. 



'2. Calcareous strata, less dolomitic and more argillaceous, 



Trenton group < sometimes designated Upper Trenton 100 ft. 



3. Green shales with limestone strata 40 ft. 



[ 4. Limestone. Trenton 15 20 ft. 



I Sandstone St. Peter about 110ft- 



, . j Dolomitic limestone Shakopee 30-40 ft. 



m ' ' ] Sandstone Jordan 20ft. 



[ Dolomitic limestone St. Lawrence about 200 ft. 



Perhaps it would be more in keeping with the actual state of our 

 knowledge to include the Lower Silurian strata all under one designation 

 the Trenton period than to attempt to express the parallelisms between its 

 parts and any of the New York members of that period, since there is some 

 reason to include not only the Hudson River and Trenton epochs but also 

 the Utica slate and the Black River limestone, among our strata. 



The St. Lawrence limestone. The actually known area in which this 

 limestone forms the surface in Olmsted county is small, and on the plate 

 representing the geology of the county it has been designated by horizon- 

 tal purple bars. It is found in the town of Oronoco, and is abundantly 

 exposed along the banks of the Zumbro and its north-middle branch. In 

 its lithological characters it does not differ essentially from the descriptions 



