250 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
thousands of feet’ of sandstone underlain by a much greater thickness of 
fine-grained, basic eruptive rocks (diabases and melaphyrs), with included 
felsitic porphyries and porphyry-conglomerates; in other words, that we 
have the same Upper and Lower Divisions of the series recognized farther 
east. The upper sandstones are the same as those seen on White and Bad 
rivers, in Ashland County, Wisconsin, dipping southward, as was first 
shown by Mr. E. T. Sweet, who was, however, in error in supposing that 
the sandstones exposed on the Kettle River are also a continuation of 
these* The latter, as already shown, belong with the light-colored Potsdam 
sandstone of the Mississippi Valley. The Keweenawan upper sandstones 
do not extend so far to the west. 
Douglas County Copper Range-—For 12 or 14 miles northward from 
the exposures in the valley of the Upper Saint Croix the country is heavily 
drift-covered, and no exposures have ever been observed. On the northern 
side of this area, however, is again a belt, 2 or 3 miles in width, in which 
there are frequent exposures of bedded Keweenawan diabases, dipping 
southward. This belt forms a bold range, or series of ranges, north of 
which, and extending to the shores of Lake Superior, is a lowland under- 
lain by horizontal sandstone. This district has been examined in some de- 
tail by Mr. E. T. Sweet, from whose description’ I cull the following facts, 
quoting his own words whenever practicable : 
The line of junction on the north* is somewhat curved, but in the main pursues an 
E. N. E. course, nearly parallel with the strike of the crystalline strata. * * * * 
Most of the northward-flowing streams in Douglas county leave the crystalline rocks 
and enter upon the sandstone district through deep gorges and in wild and precipitous 
falls. In the walls of these gorges both formations are usually beautifully exposed, but 
the sandstone, for a distance of from twenty to three or four hundred feet from where 
we would expect to find the point of contact, has evidently been affected by some great 
lateral pressure, for we find the layers broken into short lengths, and tilted at various 
angles, generally to the northwest or from the line of strike of the erystalline rocks. 
In following down the stream the sandstone layers in the walls of the gorge gradually 
show the effects of less disturbing influences, and finally assume horizontality and 
1Mr. Sweet’s estimate, Geology of Wisconsin, Vol. III, p. 350, is excessive. 
2“‘Notes on the Geology of Northern Wisconsin.” Transactions of the Wisconsin Academy of 
Science, Arts, and Letters, Vol. III, p. 48, 1876. 
3 Geology of Wisconsin, Vol. III, p. 336, et seq. 
4 With the horizontal sandstone just mentioned. 
