192 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
northwest, for at the Atlantic mill, on the south side of Portage Lake—N. 
W. 4 Sec. 34—sandstone is observed dipping northwest 28° to 30°, while 
the shales in the ravines on section 22, near the railroad, dip at 26° only, so 
that the entire rock thickness in this space cannot be more than 2,500 to 
2,600 feet. 
Within this thickness must be crowded the equivalents of the Great 
Conglomerate, of the outer or Lake Shore Trap, and of the Outer Con- 
glomerate of the district east of Eagle River, a total thickness of some 
4,700 to 4,800 feet. Within this space must come also the dividing 
line between the Upper and Lower Divisions of the series. The only expo- 
sures in this interval, however, are those of the sandstone near the Atlantic 
mill, on the south shore of Portage Lake, and one conglomerate near the 
southeastern side of the gap. Between this sandstone and the base of con- 
glomerate 22 is a thickness of some 1,400 feet, and it would appear prob- 
able that the sandstone belongs to the top of the Great Conglomerate. This 
sandstone is a reddish kind, much of the usual character, and is in its lower 
layers very conglomeratic, becoming even a moderately coarse conglome- 
rate. This character may be very well seen on the road from the Atlantic 
mill to the Atlantic mine. The finer-grained sandstone was examined under 
the microscope, and showed a fine reddish matrix of subangular to angular 
fragments of the usual porphyry detritus, but with an abundance of angular 
quartz particles. Scattered through this finer matrix are large particles, 
many of which are single quartzes. There are also among them single 
feldspars, chiefly orthoclase, particles of porphyry matrix and here and 
there diabase fragments. The abundance of angular quartz fragments, 
both in the finer matrix and in the larger particles, is a matter of interest, 
since these fragments are unusually full of large cavities, filled with a 
bubble-bearing fluid, in many of which there is to be seen a little cube of 
salt. These characters seem to render it improbable that these quartz frag- 
ments should have come from a quartziferous porphyry. Their origin is 
very doubtful. 
Of the Upper Division in the Portage Lake region the exposures are 
not plenty. The shales of the ravine near the Mineral Range Railway; 
the same shales with sandstones on the wagon-road west from Hancock; the 
