188 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
the region southwest of T. 57, R. 31 W., all save the very uppermost of 
the rock belts of the Bohemian Range, using that term in the wide sense 
of the summary of page 187, are beneath the Eastern Sandstone. (4) On the 
other hand, west of Eagle River the rock belts soon diverge in strike from 
the lake coast, striking more to the southward, and, as a result, a rapidly 
increasing amount of the Upper Division of the series is brought between 
the trap belt, or Lower Division, and the lake coast. At Portage Lake 
there must be upwards of 8,000 feet of this Upper Dives between the 
trap belt and the west coast of the point. 
Southwest of the Gratiot River, the outcrops and openings are chiefly 
on those belts whose equivalents in the eastern extension of Keweenaw 
Point are the ill-exposed and comparatively little known belts of the median 
valley. Beginning on the south side of Portage Lake, mining openings 
extend nearly continuously with the course of the layers some seven or 
eight miles to the northeast, and in the vicinity of that lake are pretty well 
distributed across the formation, so that Pumpelly’s descriptive sections 
cover nearly all of the Lower Division here at surface. Northeastward from 
the Albany and Boston mine, as far as the Allouez, the openings are much 
rarer—one being the famous Calumet and Hecla mine—and the exposures 
very sparse, the only ones of consequence being at or near the contact with 
the Eastern Sandstone in the tributary streams of the west side of Torch 
Lake. This gap Marvine has bridged in his masterly correlation of the 
rocks of Houghton and Keweenaw Counties.’ 
The Portage Lake section will, then, be best next described, after 
which the rarer exposures to the northeast may be more briefly alluded 
to. This section, between the fault line and the conglomerate num- 
bered by him 22, Pumpelly has worked out in great detail. The total 
length of his measurement is about 14,400 feet, corresponding, with the dip 
of 55°, to a thickness of about 12,000 feet. With the exception of a few 
relatively thin conglomerates, this section is made up of beds of diabase and 
amygdaloid, with some melaphyr, in no respect different from the prevail- 
ing beds of the Kagle River section. The lower layers are heavier than the 
1 Geological Survey of Michigan, Vol. I., Part II, Cap. IV. 
