30 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
varieties has been noted on the North or Minnesota Shore only. The other 
has been observed on both the South and North Shores, and is often hard to 
distinguish from a kind in which the red shaly material is most confusedly 
mingled with the vesicular amygdaloidal diabase, which at times seems to 
grade into the detrital matrix, and again to be separated from it in more or 
less distinctly defined balls; an appearance suggesting the deposition of 
detrital material upon and within the extremely scoriaceous upper portion 
of a lava flow. 
Sandstones make up much the greater portion of the detrital members 
of the series, reddish sandstones prevailing. These run from earthy and 
shaly to quite coarse granular, but are always aluminous from the presence 
of a more or less decomposed feldspathic constituent. They vary from 
brick-red to quite dark-red in color, and are made up in large measure of 
the detritus of the same acid rocks that have supplied the pebbles of the 
conglomerates, as was first shown by Pumpelly. In the darker kinds more 
or less basaltic detritus is included. Quartz is never an exclusive, nor 
often even a very prominent, ingredient in any of the sandstones belonging 
without dispute to this group. Certain sandstones forming the eastern side 
of Keweenaw Point, and again the Wisconsin shore as far west as Fond 
du Lac in Minnesota, are highly quartzose in their uppermost portion, but 
these do not belong to the Keweenaw Series. Many of the red sandstones 
are highly charged with secondary calcite. Those kinds of sandstone 
which are dark-gray to nearly black in color are made up of basic detritus, 
usually mingled with more or less of the common porphyry detritus, and 
cemented by secondary calcite. These sandstones often contain only a 
rare quartz grain, having then not over fifty per cent. of silica. They grade 
into finer varieties, which at times pass into an earthy black shale or slate. 
These gray sandstones and accompanying black shales, with a thickness of 
several hundred feet, have been recognized in a single belt, running from 
the neighborhood of the Gratiot River, on Keweenaw Point, to Bad River, 
in Wisconsin—a total distance of 150 miles. 
The source of the materials which make up the porphyry conglom- 
erates and red sandstones has been a matter of speculation to all writers 
