RELATIONS OF THE KEWEENAWAN AND HURONIAN ROCKS. 405 
towards the west, already noted as affecting the latter rocks, is observed 
also in the underlying Huronian, so far as can be traced westward.” 
So far as this region is concerned, the unconformity is only one, at most, 
of intervening erosion, without intervening disturbance. 
On the North Shore, again, from the Saint Louis to Thunder Bay, a 
distance of some two hundred miles, there are just the same relations be- 
tween the Animikie Huronian and the overlying Keweenawan as in the 
Penokee region. Both formations are exposed here on so large a scale, 
and both lie for the most part so flat, that there can be no doubt at all as 
to their relations. The parallelism in bedding between the two is complete. 
At the eastern end of the Minnesota coast, for instance, both incline lake- 
ward at an angle of 10°, the contact of the two being in sight, with some 
hundreds of feet in thickness of each. The same perfect parallelism at a 
still flatter angle is to be observed on the east side of Thunder Bay, where 
the overlying Keweenawan has at its base several hundred feet of sand- 
stone. 
Indicative of an intervening erosion on the North Shore is the absence 
of the sandstone just mentioned only a few miles to the southwest, where, 
at Grand Portage Bay, the two formations come together; while the rela- 
tion of these sandstones to the underlying slates at Thunder Cape amounts 
to a demonstration of this intervening erosion. Along the east side of 
Thunder Bay the Animikie slates may be seen for miles lying beneath 
the Keweenawan sandstones, but when Thunder Cape is reached the slate 
suddenly rises entirely across the horizon of some six hundred to seven hun- 
dred feet of the sandstone. Logan explains this peculiar behavior by sup- 
posing the Thunder Cape rocks to be separated from those around them by 
a fault. But, as I have shown on a previous page, a much simpler and 
more satisfactory explanation is reached by supposing an intervening 
erosion between the slate and sandstone.’ The truth of this view is further 
confirmed by the occurrence of ledges of the overlying sandstone on the 
flanks of the Thunder Cape elevation, and by the occurrence in the sand- 
stone of pebbles derived from the underlying slates.” 
1As first suggested by T. S. Hunt, Second Geological Survey of Pennsylvania, Report E, Part 
I, p. 239. 
27. Macfarlane, Canadian Naturalist, New Series, Vol. IV, p. 459. 
