122 Magnesian Series of the NortJnve stern States 



In Iowa the small extent of surface exposures is productive of 

 fewer descriptions. The name Potsdam sandstone included all 

 formations below the Lower Magnesian limestone, and the name 

 Lower Magnesian covered all rocks to the St. Peter sandstone 

 until recently. McGee uses the name Oneota for the formation 

 exclusive of the Shakopee and New Richmond.* Charles R. 

 Keyes follows McGee in his studies of the current year.t 



The further topics of the paper are summarized as follows : 



1. In Minnesota there are two well defined dolomite forma- 

 tions, Oneota and Shakopee. Below these is a great thickness of 

 sandstone and green shales into which a few stray bands of 

 dolomite enter and give dolomitic character to certain layers of 

 the St. Lawrence. Above the Shakopee, after more than one 

 hundred feet of sandstone, comes the Galena series characterized 

 by a weakening of the dolomitic habit through the occurrence 

 of a limestone with less than fifteen per cent, of magnesium car- 

 bonate. 



2. A marked faunal break separates the St. Lawrence forma- 

 tion from the sandstone beneath ; another break occurs between 

 the Oneota and Shakopee, the two dolomites named above, and 

 a third betweeen the Shakopee and St. Peter sandstone. These 

 three faunal breaks establish at least three faunas and three cor- 

 responding time divisions between the Algonkian and the St. 

 Peter (Chazy). 



3. Below the St. Lawrence, and extending downwards 

 to Algonkian rocks, lies a sandstone, generally a very pure quartz 

 sand but locally of a varying composition, which is recognized 

 by paleontologists as upper Cambrian. Above the upper Cam- 

 brian lies the Lower Calciferous, carrying the St. Lawrence 

 sandstone, shale .and dolomite; the Jordan sandstone and 

 the Oneota dolomite : then follows the upper Calciferous con- 

 sisting lithologically of the New Richmond sandstone and Shak- 

 opee dolomite. 



4. The clean, purely quartzose condition of the sandstone 

 formations associated with the dolomites, together with the semi- 



*The Pleistocene History of Northeastern Iowa, by W J McGee, Elev- 

 enth Ann. Rep. Director U. S. Geol. Survey, 1890, part. I, p. 332. 



tThe Geological Formations of Iowa, by Charles Rollin Keyes. Iowa Ge- 

 ological Survey, Vol. I, First Ann. Rep. for 1892. Des Moines, 1893, p. 23. 



