RELATIONS OF THE KEWEENAWAN AND HURONIAN ROCKS. 407 
he and Pumpelly had been led by the strong appearance of conformity 
between the Keweenawan and unfolded Huronian of the Penokee region. 
Working myself a few years later, I was led to acquiesce in this view.’ 
To Brooks’s later view I again in a measure acceded, in my discussion 
of the structure of Northern Wisconsin for the third volume of the Geol- 
ogy of Wisconsin; though still maintaining an intervening erosion without 
true unconformity for the western half of the Lake Superior basin. The 
whole history of this discussion is but a new illustration of the danger of 
generalizing as to the structural relations of a system of rocks before the 
whole ground has been looked over. 
So far as Brooks’s argument is based on lithological grounds, it 
has been sufficiently considered in foregoing paragraphs. The structural 
grounds for his conclusions are (1) the unfolded condition of the Kewee- 
nawan beds as contrasted with the frequently folded Huronian, and (2) the 
absence of patches of the Keweenawan rocks in the numerous deep syn- 
clinals of the Huronian in the Marquette region. The first of these points 
merits no consideration, for the very point at issue is the time of folding of 
the Huronian. With regard to the second point it may be said that it is 
_ plainly to be seen that the absence of patches of the Keweenawan rocks 
overlying the Huronian in the Marquette region is quite as difficult of 
explanation, or even more difficult, on an hypothesis of complete uncon- 
formity as on one of complete conformity. On Keweenaw Point the Ke- 
weenawan rocks have a thickness measured by miles, ending to the south- 
ward in a bold escarpment beyond which no patch of these rocks is to 
be found. The absence of such outliers, especially of such hard rocks, 
would be incredible and contrary to all experience on any hypothesis of 
former extension southward. It has already been shown that the escarp- 
ment on the south side of Keweenaw Point must be a fault line, as Fos- 
ter and Whitney long since urged; and their view is strongly confirmed 
by the very absence of outliers to the southward. The Keweenawan rocks 
are, then, not to be found in the Marquette region for the simple reason 
that they never extended so far; and but for some features in the geology 
1“ On the Age of the Copper-Bearing Rocks of Lake Superior,” Amer, Jour. Sci., June, 1874. 
