vtalcott.) MINNESOTA. 181 



accumulated along a cliff of the copper-bearing rocks and that the age 

 of the conglomerate must be late " Potsdam.' 71 



In an extended paper on the classification of early Cambrian and 

 pre-Cambrian formations Prof. R. D. Irving illustrates the unconformity 

 between the " Potsdam " sandstone and the subjacent Huroniau and 

 KeweenaWan rocks. He also discusses the similar stratigraphic rela- 

 tions of the Cambrian and pie-Cambrian rocks of the Grand Canon 

 Region. 2 



MINNESOTA. 



The presence of the lower sandstone along the Mississippi River 

 below the Falls of St. Anthony to the Iowa line was proved by the 

 Survey of Dr. D. D. Owen in Wisconsin, Iowa, and Minuesota. Alter- 

 nating exposures of the strata occur along the river and some of the 

 tributaries flowing into it from the west. 3 On the accompanying map 

 there is also a narrow area colored Potsdam along the valley of the 

 Minnesota or St. Peter's River. 



Little additional information in regard to Minnesota was published 

 until Prof. N. H. Winchell, in the first annual report of the geological 

 survey of Minnesota, discussed, under ths title of" Potsdam Sandstone," 

 the Sioux quartzite of northwestern Minnesota, and " Tbe St. Croix 

 Sandstone," the fossiliferous sandstone of the southeastern portion of 

 the State, referred to the Potsdam by Dr. Owen. 4 Of the term " Pots- 

 dam," he says : 



This term is strictly applicable only to the sandstones of New York State, to which 

 the name was first given, and to the equivalents of those strata in their extension 

 through the West. It has been abundantly proved that the red sandstones of Lake 

 Superior, however disturbed and changed locally, or however much increased in 

 thickness by the ageucy of volcanic outbursts, are the exact equivalents of the New 

 York Potsdam. Thev occupy the first position over the luetamorphie slates of the Bu- 

 . ronian rocks on which they lie uncouformably, and from which they differ in being 

 but slightly and only locally metamorphosed. They retain usually their evidently 

 sedimentary characters, and have not well preserved fossil remains. 6 



He states further that in Wisconsin the upper sandstones are found 

 to lie uncouformably upon the red sandstones where they have been 

 tilted by volcanic agency, thus referring the quartzite of Sauk County 

 to the Potsdam, and the superjacent fossiliferous, Upper Cambrian 

 sandstones that have been correlated by authors with the Potsdam of 

 New York, to his St. Croix sandstone. Tbe red sandstone or Sioux 

 quartzite is said to be both older than and unconformable with the 



1 Transition from the copper-bearing series to the Potsdam ±.m. Jour. Sci., 3d ser., vol. 27, 1884, pp. 

 463-465. 



2 On the classification of the early Cambrian an«J pre-C ambrian formations. U. S. Geol. Surv., 7th Ann 

 Rep. for 1885-'86, 1888, pp. 365-454. 



3 Geol. Surv. of Wis., Iowa, and Minn., and incidentally of a portion of Nebraska Territory, Phila- 

 delphia, 1852, pp. 48, 49. 



4 General sketch of the serology of Minnesota. GeoL and Nat Hist. Survey Minnesota, 1st Ann. Rep. 

 for 1872-*73, 1st ed , pp. 68-80. 



'Op.cit., p. 68. 



