waixottJ LAKE SUPERIOR SANDSTONE. 193 



In their report on the geology of the Lake Superior laud district, 

 Messrs. Foster and Whitney describe, under the heading of " Potsdam 

 sandstone," 1 a sandstone on the north shore, occurring in insulated 

 patches on the isthmus between Thunder and Black Bays. This sand- 

 stone attains, according to Logan, a thickness of at least 200 feet. 2 On 

 the south shore the sandstones of the northwestern side of Keweenaw 

 Point are considered to be of the same age as those on the south side 

 of the point, extending along the lake shore from Keweenaw Bay to St. 

 Mary's River. The sandstone extending southward, subparallel to the 

 bay, is also cor related with the sandstone mentioned ; and all of these 

 are correlated with the Potsdam sandstone of the New York series. 

 They quote from the manuscript of Prof. James Hall in relation to the 

 westward extension of the sandstone in Wisconsin, in the Upper Mis- 

 sissippi Valley. 3 



In some observations on the age of the sandstones of the United 

 States Prof. 0. T. Jackson concludes that the Lake Superior sandstone 

 is of Upper Silurian age. 4 



Messrs. Foster and Whitney returned to the discussion of the age of 

 the sandstone of Lake Superior in 1851 and conclude there can no longer 

 be any doubt that this sandstone lies below the lowest fossiliferous 

 members of the Silurian in the position of the Potsdam sandstone of 

 Eew York. 5 They describe with considerable detail the mode of occur- 

 rence of the horizontally bedded sandstone at the southeastern end of 

 the lake along the Sault Ste. Marie, with its extension westward along 

 the coast to Keweenaw Bay. They show it to be one continuous for- 

 mation around the great central granitic and azoic nucleus; that it is 

 a granular, quartzose material, mostly friable, and containing little 

 iron ; that the thickness of the whole formation did not exceed 100 feet 

 at Sault Ste. Marie, and that at the Pictured Rocks it was probably 

 more than 300 or 350 feet, gradually inci easing from the east toward 

 the west. 6 They then describe a sandstone on the western side of the 

 Keweenaw Point and Isle Royale which is now referred to the Algon- 

 kian. This is the best resum6 of the information relating to the Lake 

 Superior sandstone published up to its date. 



• In his tabulation of the lowest Protozoic sandstones Dr. D. D. Owen 

 places the Lake Superior ferruginous and argillaceous sandstones, 

 shales, and conglomerates at the base of the section, assigning it a 

 thickness of 5,000 feet. 7 On the accompanying map the Lake Superior 



•Lower Silurian System. Potsdam and Calciferous Sandstones. Kept, on Geol. of Lake Superior 

 Land Distr., Pt. 2, 1851, pp. 113-134. 

 2 Op. cit., p. 115. 



3 Op. cit., pp. 133, 134. 



4 [Some observations on the ago of the sandstones of the United States.] Bost. Soc. Nat. Hist. Proc, 

 vol. 3, 1851, pp. 335, 336. 



6 On the age of the sandstone of Lake Superior, with a description of the phenomena of igneous 

 rocksi. Am. Assoc. Proc, vol. 5, 1851, p. 25. 



6 Op. cit., p. 29. 



7 Geol. Surv. of Wis., Iowa, and Minn., and, incidentally, of a portion of Nobraska Territory. 

 Philadelphia, 1852, p. 53. 



Bull. 81 13 



