OUTER SANDSTONES OF THE POROUPINES. 221 
which has been traced already from the vicinity of Portage Lake to the 
Ontonagon River. As further east, so also in the Porcupine region, this 
layer consists of a series of alternating dark-gray to black clayey shales 
and dark-gray sandstones ; all dark-colored because of their comparatively 
large content of basic detritus. Calcite is very often found in the matrix, 
and one phase consists almost exclusively of basic detritus with calcite 
cement. The total thickness averages about 600 feet, varying somewhat 
from this figure. On the map of Plate XIX the belt is marked as very 
irregular in the surface width, the irregularity arising from the numerous 
changes it undergoes in direction and amount of inclination. 
At the Nonesuch mine, 8. E. 4, Sec. 1, T. 51, R. 43 W., the shale is 
seen with a thickness of over 200 feet, trending N. 45°-50° E., and dip- 
ping S. E. 28°. Near the base of the shale is the sandstone seam, 4 feet 
thick, worked at the Nonesuch mine for its copper. This rock is a dark 
greenish-gray, fine-grained sandstone, which in the thin section is seen to 
be composed in some measure of basic detritus, along with porphyry 
detritus, and numerous single quartzes. The basic detritus appears in 
the shape of much decomposed red- and green-stained particles, showing 
both feldspathic and augitic ingredients. A good deal of magnetite is pres- 
ent, and native copper is very abundant, for the most part clustered around 
the magnetite particles. In a few places secondary calcite lies between 
the grains. The thin section of this rock is figured at F ig. 2, Plate XVI. 
Beneath the copper-bearing sandstone there come a few feet of shale, also 
carrying some copper, and beneath this some two and a half feet of a light- 
gray sandstone, harder than usual, beneath which again is the great sand- 
stone layer previously described. 
The thin seam just mentioned as occurring at this junction has been 
traced for a number of miles in the valley of Iron River, and is the one 
which has attracted so much attention in this region for the silver it con- 
tains. Under the microscope it is seen to differ from the Nonesuch copper 
sandstone, and, indeed, from the rock of the shale belt generally, in that it 
contains some water-deposited (secondary) quartz between the grains, 
which consist, as usual, of mingled porphyry and basic detritus and 
quartz particles. Calcite has also been deposited more or less plentifully 
