366 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
quartzose character, being at times just like the darker colored portions of 
the quartzose sandstone of the central part of Wisconsin. 
The phenomena of the contact in the Douglas County Copper Range 
have been described in some detail on a previous page, where I have also 
shown that the Western Sandstone in all probability sustains the same rela- 
tions to the Keweenawan diabases against which it rests as does the Eastern 
Sandstone of Keweenaw Point to the north-dipping beds of that typical 
region, this similarity of relation being carried out even to the faulting that 
I have shown to obtain in the latter district. 
This similarity of structural relations, taken together with similarity in 
lithological character, renders it very probable that the Eastern and West- 
ern Sandstones are geologically equivalent. But they are nowhere con- 
nected, and the Western Sandstone has not been traced to any point where 
its relation to any of the Mississippi Valley fossiliferous formations can with 
certainty be made out, although the appearances in northwestern Wisconsin 
and northeastern Minnesota are decidedly in favor of its being the down- 
ward continuation of the Mississippi Valley Cambrian Sandstone. 
THE MISSISSIPPI VALLEY CAMBRIAN OR POTSDAM SANDSTONE. 
I have already shown that the Keweenawan diabases and interbedded 
conglomerates are traceable, mile by mile, from the typical region of Ke- 
weenaw Point to the Saint Croix River on the west side of Wisconsin ; and 
that here they underlie the fossiliferous Cambrian Sandstone of the Missis- 
sippi Valley, in such a manner as to render certain the tilting and great 
erosion of the Keweenawan beds before the deposition of the sandstone; 
the latter for fifty miles in an N. E.-S. W. direction, with interruptions due 
to denudation, lying athwart the course of the tilted Keweenawn beds, 
which are here disposed in synclinal form. Whatever difficulties may hang 
about the structural relation of the Eastern and Western Sandstone of Lake 
Superior, there are here none; so unmistakable are their structural relations 
in this region, that any geologist still doubting the separation of the Ke- 
weenawan rocks from the overlying Cambrian Sandstones by an interven- 
ing disturbance and erosion should feel himself debarred from denial until 
he has thoroughly examined the facts in the field. 
