302 
COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
one, which is exposed on the strike for several hundred feet at the 
foot of a cliff, on the first point above the mouth of Split Rock 
River, shows a weathered, greenish, earthy matrix, and rather 
sparsely scattered amygdules of laumontite. The other amygda- 
loid, seen on the face of the first point below Split Rock River, has 
a brownish, aphanitic matrix, and is so highly vesicular that the 
vesicles touch one another. These vesicles are small, very smooth- 
walled, elongated in a common direction, partly empty, but for the 
most part filled with amygdules of chalcedonic quartz, laumontite, 
or chlorite. These layers occupy a surface width, measured from 
west to east, of over half a mile, and must have a thickness, there- 
fore, of abileast S00 teetaes teem eer-eeee aia ae ere eet eee 
UI. Dark-gray to black gabbro.—This layer or series of layers forms the coast 
line for about a mile in the S. E. 4 of Sec. 6 and S. W. 4 of See. 5, 
T.54,R.8 W. The lower layers are rather fine-grained to medium- 
grained, dark-gray to nearly black, rough-textured, and marked by 
strong luster-mottlings. The thin section shows labradorites ar- 
ranged in curving lines, which are evidently the result of flowage. 
The augites are relatively large, and inclose each countless minute 
feldspars. The magnetite is unusually abundant, being so thickly 
crowded between the augites as to render these interspaces nearly 
black. Quite large fragments of the rock are raised by the magnet. 
Though olivines are generally present in these luster-mottled rocks, 
none could be detected here. This section is figured on Plate VIII, 
Fig. 4. Higher layers become coarser in grain, and of a rougher 
texture, with less marked luster-mottlings. In the thin section of 
this coarser rock the augites are strongly diallagic and still larger 
than the plagioclases, but not nearly so much so as in the previously 
described rock. They are usually fresh, though occasionally altered 
to chlorite. Good-sized olivines, partly fresh, but generally traversed 
by broad brown and green bands of alteration, are here abundantly 
present. Still higher up, near the top of these layers, the grain 
becomes quite fine again, but no thin sections have been examined. 
Near the middle of the 8S. E. 4 of See. 5, T. 54, R. 8 E., this gabbro 
is interrupted by a vertically placed mass of excessively coarse- 
grained anorthite-rock. The cutting mass is from 50 to 75 feet 
wide, and bears north and south. It shows on both sides of a little 
square-angled, rock-walled bay, on the south point of which it rises 
as much as a hundred feet above the lake. On both sides of the 
cutting mass the black gabbro is filled with large angular masses 
of the same coarse anorthite-rock. The included masses reach 
sometimes many tons in weight, and in some places predominate 
over the including gabbro, which then appears as if veining the 
coarser rock. At the west angle of the bay the included masses 
are nearly absent, and the gabbro resumes its usual vertically col- 
umnar appearance. At the north angle of the bay the anorthite- 
800 
