

338 THE CAMBRIAN. Ibull.81. 



quette, or east of Marquette, occupies the exact stratigraphic position of 

 the fossiliferous St. Groix or " Potsdam " sandstone of Wisconsin. The 

 description of the character and geologic equivalency of the sandstones 

 of the south shore of Lake Superior as given by Prof. Ghamberlin ac- 

 cords with the view I had formed from the study of the literature per- 

 taining to the subject. Although not considering it proved that the two 

 sandstones are exactly contemporaneous, I think that for all practical 

 geological classification they may be considered equivalent deposits. 

 This view is so well expressed by Prof. Ohamberlin that it is here 

 quoted : 



Lake Superior sandstone. — The sandstones on the southern side of the Archean Isl- 

 and are light-colored, being mainly yellow or white, varying locally to pink, brown, 

 and green. They are nowhere bodily dark. An easy explanation of this is found in 

 the fact that they were derived from the light-colored quartzose and granitic rocks 

 of the southern face of the land. But passing around to the Lake Superior Basin on 

 the northern side of the island, where erosion preyed upon the iron-bearing members 

 of the Hurouian series, and more especially upon the traps, sandstones, aud shales of 

 the copper-bearing series, the resulting beds are not only reddish brown in color, but 

 contain a notable ingredient of iron and of 6haly material derived from those forma- 

 tions. Indeed tbe deposit bears a very close external resemblance to the sandstones 

 of the Keweenawan series, and the two have been considered as identical by able 

 geologists. 



Concerning the distinctness of the two formations we entertain no doubtful opin- 

 ion. The Potsdam sandstone is habitually horizontal, while the Keweenawan is 

 tilted, indicating that the latter partook of a general stratigrapical movement which 

 did not affect the former, which, considering the attendant circumstances, is equiva- 

 lent to saying that the horizontal sandstones were not then in existence. The obser- 

 vations of Mr. Sweet in Douglas County are further proof of this. 1 The Potsdam 

 sandstone there abuts unconformably against the Keweenawan traps and contains 

 pebbles derived from them, showing not only that they were earlier formed but that 

 they were tilted and extensively eroded at the time the horizontal sandstones were 

 formed. The local disturbance of the latter does not vitiate the force of the evidence 

 when critically considered. Similar phenomena are presented along the southeastern 

 face of the promontory of Keweenaw Point. 2 



Besides these stratigraphical evidences of distinctness, the microscopical observa- 

 tions of Prof. Irving and the chemical analyses of Mr. Sweet show an important con- 

 stitutional distinction between them. 3 Whereas the Keweenawan sandstones are 

 largely nonquartzose, the horizontal beds are highly quartziferous. 



As the latter are traced eastward along the south shore of Lake Superior they are 

 found, according to Dr. Rominger, to be interstratified with and graduated into the 

 light-colored sandstones which prevail in the eastern portion of the upper peninsula 

 of Michigan, and which are traceable into direct continuity with the light-colored 

 sandstones of Wisconsin. The same geologist (as also Foster and Whitney) is author- 

 ity for the statement that in the Keweenaw Valley Silurian limestone overlies this 

 sandstone. 



The southern light-colored sandstones, like their northern equivalents, abut un- 



' Geol. Surv. Wis., vol. 3, pp. 340-347 ; also 350-352. 



2 This has recently been denied by Dr. M. E. Wadsworth, but more recent examination by experi- 

 enced observers shows his discussion of the subject to be more pronunciative than trustworthy. Bui. 

 letin of the Museum of Comparative Zoology, at Harvard College, Geol. Series, vol. 1. The facts in 

 detail as determined by the later observations will be found in Prof. living's forthcoming report on 

 the copper-bearing series. 



9 Geol Wis., vol. 3, pp. i5 and 350, and microscopical PL xrx 4. 



