406 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
In the next chapter I shall attempt to show that the whole of the Lake 
Superior basin is a synclinal trough in the Keweenawan beds. From the 
relations of the Huronian and Keweenawan in the western half of the basin 
it is plain that they also partake in this synclinal—a fact in itself strongly 
indicating that they here underlie the Keweenawan without true uncon- 
formity. ‘ 
Thus far, however, I have considered the structural relations of the 
Keweenawan rocks to the unfolded Huronian only. In the case of the 
folded Huronian the problem to solve is the time when this folding took 
place. Did it entirely precede the Keweenawan depositions and outflows? 
was it contemporaneous with them? or was it wholly subsequent? The 
solution of this problem is not easy to reach without more extended knowl- 
edge of the Huronian than we now possess. 
On the South Shore, the folded Huronian beds and the Keweenawan 
are never in close proximity. They are nearest to each other in the Ke- 
weenaw Point and Marquette districts. On the east shore of the lake the 
Keweenawan beds are found near to folded Huronian, but unfortunately 
our knowledge of that coast is too meager to base any conclusions upon as 
to the relations of the two systems of rocks. The only other portion of the 
Lake Superior basin in which the Keweenawan and folded Huronian ap- 
proach each other is that district lying northward from Thunder and Nipi- 
gon bays. Here again, unfortunately, our ignorance as to the Huronian 
leaves us in a good deal of doubt; for while it seems certain that folded 
Huronian schists exist in this region, it is also, as already indicated, a matter 
for grave doubt as to how far the schists called by Bell Huronian, belong 
in fact to an older formation. 
The relations subsisting between the Keweenawan rocks of Keweenaw 
Point and the folded Huronian to the southeast form the subject of an 
article by T. B. Brooks, published in 1876, in which he urges the exist- 
ence of a true unconformity between the two rock systems.’ The same 
writer, along with Pumpelly, had only a few years previously held to a 
complete conformity between the two systems.” To this last-named view 
1Amer. Jour. Sci., March, 1876, ‘‘On the Youngest Huronian Rocks South of Lake Superior.” 
3Geol. Surv. of Mich., Vol. I, Part II, pp. 1-6. 
