158 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
in the above 25,000 feet, and the downward continuation of the series. The 
amount to be added for this downward continuation can only be guessed at. 
Eastward from the eastern part of Keweenaw Point to the Ontonagon 
River the total thickness of the Lower Division seen is from 12,000 to 17,000 
feet, but in all this distance only the upper limit of this is in sight, the 
same fault line and newer sandstone as met with on Keweenaw Point 
bounding the exposures on the south, and rendering uncertain the total 
thickness. 
West of Lake Agogebic the Eastern Sandstone does not extend far, 
and the two ranges of Keweenawan rocks, which to the eastward bounded 
it on the north and south respectively, come together; so that we have, for the 
first time, the whole thickness of the Lower Division at surface, with its lower 
limit well defined. On the Montreal River, taking the surface width and 
dip angles together, the apparent thickness is as much as 33,000 to 35,000 
feet, but how much of this may be due to the continuation westward of the 
Keweenaw fault, or whether this fault extends so far as this, it is impossible 
to say. It certainly does not extend much farther, and, from its evident 
rapid decrease in throw from the Ontonagon River westward, it seems prob- 
able that its influence on the Montreal cannot be great. 
On Bad River, 18 miles southwest of the Montreal, the Lower Division 
has a surface width, from the Huronian slates below to the sandstones of 
the Upper Division, of only 17,000 feet. Since the dip here is perpendic- 
ular, or nearly so, the thickness is not much less than this. As shown in 
another place,! this extraordinary thinning is connected with the presence 
below of a great belt of the coarse gabbro described in a preceding para- 
graph of this chapter. This coarse gabbro—whether with or without 
interbedded fine-grained beds is not now known—usurps most of the thick- 
ness, leaving only some 5,000 feet for the usual thin-bedded flows of the 
Lower Division. The explanation perhaps lies in the view that, while early 
in the history of the series there was poured out here an immense thickness 
of a rock which solidified into the coarse gabbro, later in its growth the 
vents were removed from here to either side. The coarse gabbro mass 
must have stood up to a great height, and the later flows terminated against 
1 Chapter VI. 
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