110 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
Tabulation of the results of a microscopic study of the felsites and felsitic porphyries 
of the Keweenaw Series—Continued. 
; d ie 
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Ag 2 = Macroscopic charac- | Microscopic descriptions of thin sec- 
oe Place. Dy ala : TS. tions 
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& E 
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North shore Lake Su- 
perior, one mile be- 
low mouth of Bap- 
tism River, Minne- 
sota. 
-| North shore Lake Su- 
perior; bay below 
Grand Marais, Min- 
nesota. 
Red Rock Bay, Indian 
Reservation, north 
shore Lake Super- 
ior, Minnesota (not 
Close to 876. 
porphyritic white 
kaolinized feldspars 
are very abundant. 
The arrangement of 
these feldspars and 
of the lighter ma- 
terial in the matrix 
tends to produce 
curving lines. 
Aphanitic; red, 
blotched with yel- 
lowish-white; no por- 
phyritic ingredients, 
but minute flashing 
points, due to the 
secondary quartz, 
may be seen in @& 
bright light. The 
rockis much altered 
and softened and 
comes out in sharp- 
edged tabular frag- 
ments. 
Aphanitic ; flesh-red ; 
porphyritic red fela- 
spars and black quar- 
tzes. 
other bands. The whole appearance 
of this banded matrix plainly indi- 
cates movement while ina fluid condi- 
tion, and is much the same as com- 
monly observed in the modern rhyo- 
lites. The porphyritic quartzes and 
feldspars present the usual characters, 
the quartzes carrying often the usual 
embayments of the matrix, as also 
well-marked, partially devitrified, 
doubly terminated glass inclusions, 
See Figs. 3, 4, Plate XII; also Figs. 
9,10, 11, 12, Plate XII. 
The | In the thin section the faint white 
banding noted macroscopically is seen 
to be produced, as usual, by the pres- 
ence in these bands of relatively 
coarse quartz and their comparative 
freedom from the red stain which af- 
fects the rest of the rock. The usual 
excessively fine quartz network af- 
fects the whole rock, and in all por- 
tions, except the lightest-colored 
bands, the usual brown, opaque fer- 
rites are abundant. The porphyritic 
quartzes and orthoclases present no 
unusual characters ; the former show 
very large, doubly terminated glass in- 
clusions (negative crystals). See Figs. 
6 and 8, Plate XII. 
In the thin section this rock is seen to be 
completely saturated with the usual 
quartz network, but shows also num- 
erous polarizing particles, apparently 
sndependent of the secondary quartz. 
Red stain and ferrite particles rather 
less abundant than usual. 
In the ordinary light the matrix of this 
rock is only faintly stained, and is pe- 
culiar from being strewn with irregu- 
lar greenish blotches. In the polar- 
surveyed). ized light the nearly colorless back- | 
