walcott.j SUMMARY LAKE SUPERIOR SANDSTONE. 335 



LAKE SUPEKIOR SANDSTONE. 



The earlier geologists speak of the Keweenawan series and the hon 

 zontal Lake Superior sandstones as formiug part of the same terrant, 

 and this view has been sustained by some more recent writers. Many- 

 others, however, have held the view that the horizontal sandstones 

 were the equivalents of the fossiliferous sandstones beneath the Mag- 

 nesian limestone of central Wisconsin, and that the sandstones of the 

 Keweenawan series were unconformably subjacent to the horizontal 

 sandstones. Prof. R. D. Irving, taking the latter view, says : 



The constant horizontality of the sandstone series, its restriction to low levels, 

 the actual unconformity visible in Douglas County, the proximity of the horizontal 

 sandstones to the enormously thick perpendicular fragmental beds of the Montreal 

 River, and the relations of the two series on the St. Louis appear to amount to dem- 

 onstration. In the Thunder Bay region the Canada geologists have proved a similar 

 nonconformity, and Sir William Logan's arguments would seem to show that the 

 same is true for the southeastern shore of the lake. That it may at times be difficult 

 to tell whether we have to do with the Keweenawan or the newer sandstones is un- 

 doubtedly true in some of those cases where the Keweenawan beds have a low incli- 

 nation, but such cases of difficulty are rare. 1 



Of the relation of the horizontal sandstones on the north side of the 

 Wisconsin pre-Gambrian area and the fossiliferous sandstone on the 

 southern side he says : 2 



An absolute proof of their exact relations is wanting, because nowhere as yet have 

 the two been traced into each other. Judging from the low altitude above the lake 

 to which the Lake Superior sandstone has been seen reaching, it appears improbable 

 that any connection ever existed between the two formations in the Wisconsin-Min- 

 nesota region. * * * It appears, on the whole, that the evidence is all in favor of 

 an approximate equivalency of the Lake Superior and Mississippi Valley Potsdam 

 sandstones. It seems probable to me, with my present knowledge of the facts, that 

 they were deposited in always disconnected basins, and that their sharply contrasted 

 lithological peculiarities are to be explained by corresponding differences in the an- 

 cient rocks from whose ruins they are built. 



Under the designations of the " Eastern sandstone w and " Western 

 sandstone" Prof. Irving describes the horizontal sandstones of the 

 I south shore of Lake Superior. By the " Eastern sandstone" he means 

 sandstone that fills the valley between the Keweenaw or main trap 

 range of Michigan and the so-called south range. The eastern end of 

 this depression is occupied by the waters of Keweenaw Bay. The sand- 

 stones skirt the shores in a band varying in breadth from a few rods to 

 1 or 2 miles, the older crystalline rocks occasionally reaching down 

 to the lake. These conditions prevail as far eastward as Marquette, 

 beyond which point to the eastward sandstone forms all of the shore 

 cliffs as far as the Sault. 3 



1 Geological Structure of Northern Wisconsin. rGeneral geology of the Lake Superior Region. 

 Geology of Wis., Survey of 1873-79, vol. 3, 1880, p. 23. 



* Op. cit., pp. 24, 25. 



• The Copper-hearing Rocks of Lake Superior, U. S, Geol. Surv., Monograph, vol. 5, 1883, p. 351. 



