200 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
Point and the Porcupine Mountains. All of these rocks trend N. 30° E., 
and dip 40° to the northwest, or at a lower angle than the beds of the 
Trap Range just to the southward. The exposures in the streams on sections 
4, 5, and 7 of T. 50, R. 39 W. are closely similar to the last described, but 
much less extensive. In the NE. 4 Sec. 4 the shale dips 48° NW. and trends 
N. 65° E., while in sections 5 and 7 the corresponding figures are 35° NW. 
and N. 50° E. 
The shale seen at these several points is dark purplish-gray to nearly 
black, and from excessively fine-grained to aphanitic, the finest kinds being 
quite soft and highly argillaceous, the interbedded sandstone also varying 
in color from dark-gray to heavy black. Thin sections of it were ex- 
amined both from the exposures on Sec. 21, T. 51, R. 38 (8009-3011), 
and from those on See. 4, T. 50, R. 39 (3008). All of the sections show 
the usual porphyry detritus, viz, matrix and single quartzes. With these, 
in all the sections, is mingled more or less basic detritus. This seems to 
be most abundant in the blackest kinds, in which there is also an abundant 
calcite cement, filling all interstices between the fragments. The basic 
detritus appears in the shape of particles of the basic rocks, showing more 
or less plainly the several ingredients, always much altered, and of par- 
ticles of the single minerals, viz: augite, almost wholly altered to a greenish 
substance, triclinic feldspar and magnetite. 
Everywhere north of the line of the black shale, between Portage 
Lake and the Ontonagon, the country is underlain by the sandstones of the 
Upper Division, which are now and then exposed in the stream beds, and form 
large cliffs on the lake shore. The inclination in these beds is everywhere 
lakeward, but the amount of dip lessens rapidly northward from the Trap 
Range, and on the lake shore becomes very slight. 
From the Portage to the point between the Fire-Steel and Flint-Steel 
rivers the general trend of the layers is more to the south than that of the 
coast, which thus constantly ascends in the series, the sandstones on the 
point alluded to belonging near the top of the Upper Division. Further 
west the reverse is the case, the coast trending more to the south than the 
rock layers, and from the Flint-Steel to fhe Union River there is a steady 
