walcott. MINNESOTA. 185 



stone or St. Croix sandstone, Prof. N. H. Winchell defines the Pots- 

 dam formation as follows: l 



Potsdam formation. — Tilted red sandstones, shales and conglomerates, changed by 

 igneous gabbros and dolerites locally to red qnartzites, felsites, qnartz-porpnyries 

 and to red granite. The Keweenian and Hnronian in part, in Wisconsin. 



He places the Taconic group second and defines it as follows: 



Taconic group. — Horizontal black slates and gray quartzites, with interbedded 

 limestones and diorites (the Animikie group), changed to tilted slates, quartzites, 

 iron ores, and silicious marble. The Gunflint beds, the Mesabi iron rocks, the 

 Ogishke Muncie conglomerate (?), the Thomson slates and quartzites, the Vermilion 

 iron rocks; the Huronian in part, in Wisconsin and Michigan. 



A description of the Sioux quartzite as it occurs at New Ulm is 

 given by Prof. Wiuchell in the secoud annual report. 2 In the first vol- 

 ume of the final report of the Miuuesota survey he describes the St. 

 Croix sandstone as it occurs in Houston, Winona, and Fillmore Counties. 3 

 The entire thickness of the foi mation in Winona varies from 488 to 558 

 feet. 4 Under the title of Potsdam quartzite a description is given in 

 this volume by Messrs. Wiuchell and Upham of the pre-Cambrian 

 quartzite of northwestern Minnesota. 6 



From the Sioux quartzite series at Pipestone, Minnesota, Prof. 

 Winchell described, in 1885, Lingula calumet and Paradoxides barberi, 

 referring them to the equivalent of the Potsdam sandstone of New 

 York. 6 



In a note on the revision of the stratigraphy of the Cambrian in 

 Minnesota, Prof. Wiuchell includes under the St Croix terrane, Jordan 

 sandstone, St. Lawrence limestone, shales, and Dresbach sandrock. 7 

 It is interesting to note that after the upper member of the St. Croix, 

 the Jordan sandstone, we find " Potsdam " (?) ; also after the Dresbach 

 sandstone at the base and (Potsdam) after formations placed as pre- 

 St. Croix. 



The geological age of the Sioux quartzite of the pipestone quarries 

 in Dakota and Minnesota is discussed by Prof. R. D. Irving, 8 who con- 

 cludes that their tilted position, great thickness, and lithologica 4 ! con- 

 trast with the Potsdam sandstone "make it evident that they are not 

 merely a downward continuation of that formation, but, on the con- 

 trary, form a great uuconformably underlying series." 



1 Notes on the age of the rocks of the Mesahi and Vermilion iron districts. Geol. and Nat. Hist., 

 Survey Minnesota, 11th Ann. Rep. for 1883, 1884, p. 170. 



2 The geology of the Minnesota Valley. Geol. and Nat. Hist. Sur v., Minn. 2d Ann. Rep. forl873, 1874, 

 p. 157. 



s The Geology of Minnesota. Vol. 1, of the Final Report, 1884, pp. 223-227, 257-259. 



«Op.cit.,p.258. 



8 Op. cit. pp. 422-424, 499-503. 



6 Foasils from the red quartzite at Pipestone. Geol. and Nat. Hist. Sarv. Minnesota, 13th Ann. Rep. 

 for 1884, 1885, pp. 65-72. 



7 Revision of the stratigraphy of the Cambrian in Minnesota. Geol. and Nat. Hist. Survey Minne- 

 sota, 14th Ann. Rep. for 1885, 1886, p. 337 



* Preliminary paper on an Investigation of the Archean formations of the Northwestern States. U. 

 S. GeoL Surv. 5th Ann. Rep. , 1883-84, 1885, pp. 201, 202. 



