walcott.] MINNESOTA. 183 



antiqua (Con.) and TAngula prima (Con.), a Discina (or Orbicula), and 

 uncertain impressions supposed to be of a Pleurotomaria and of crin- 

 oidal remains. A species of Theca has also been described from Keese- 

 ville. In comparison to this he cites a large number of species de- 

 scribed by Owen and Hall from Wisconsin. 1 



(8) In this it is shown that the number of species of the Lake Superior 

 district and from the New York district, in the sandstone referred • by 

 him to the Potsdam sandstone, are the same, namely three. 



Notwithstanding these considerations, it has not been thought best to attempt the 

 delineation of the areas of these sandstones separately on the preliminary geological 

 map accompanying this report. 2 



. A detailed description of the lithological characters of the St. Croix 

 and Potsdam sandstone is also given. 3 



The preceding observations by Prof. Winchell have been quoted in 

 detail as they are the foundation for the theoretical views subsequently 

 advanced by him correlating the Potsdam sandstone of New York with 

 various other sandstones of pre-Silurian age. 



On the map accompanying the report (opposite p. 45) the Sioux quartz- 

 ite area of northwestern Minnesota is colored the same as the St. Croix 

 sandstone of the southeastern portion of the State. In the legend the 

 color is placed under the heading of Potsdam and St. Croix. 



In the fourth annual report Prof. Winchell describes the St. Croix sand- 

 stone as it occurs in Fillmore County and illustrates the geographic 

 distribution on an accompanying map. 4 In some notes on a deep 

 well drilled at Minneapolis, Minnesota, the same writer states that Nos. 

 11, 12, and 13 represent the St. Croix sandstone with a total thickness 

 of 217 feet. Beneath this there is a red marl and red sandstone which he 

 refers to the same horizon as the catlinite beds of the Sioux quartzite 

 series. Of No. 14 it is said: "No. 14 may represent the Lingula flags 

 or the upper portion of the Potsdam, so called." No. 15 is correlated 

 as undoubtedly the upper portion of the great series of marls and sands 

 which characterize this horizon in Minnesota. u It is the same forma- 

 tion as the rock that embraces the well known 'pipestone' or catlinite 

 of Minnesota." 5 



The Sioux quartzite is referred by Prof. Winchell to the Potsdam 

 sandstone in a description of the geology of rocks in Pipestone County 

 in the sixth annual report. It includes the famous pipestone quarry near 

 the center of Pipestone County. 6 



A more extended. account of the strata referred to the "Potsdam" 

 sandstone is published by Prof. Winchell in the tenth annual report 



1 Op. cit., p. 71. 2 Op. cit„ p. 73. 3 Op. cit., pp. 75-80. 



•Report on the geology of Fillmore County. Geol. and Hat. Hist. Surv., Minnesota,- 4th Ann. Rep. 

 for 1875, 1876, pp. 31-32. 



8 Notes od the deep well drilled at East Minneapolis, Minnesota, in 1874-75. Minn. Acad. Sci., 

 Bull., vol. 1, 1876, pp. 188-189. 



6 The geology of Rock and Pipestone Counties. Geol. and Nat. Hist. Survey, Minnesota, 6th Ann. 

 Rep., for 1877, 1878, p. 97. 



