108 
Tabulation of the results of a nricroscopie studg of the felsites and felsitic porphyries 
| 
} 
Specimen num- 
ber. 
820... 
852... 
COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
Place. 
--| Island on north side 
ot Beaver Bay, Min- 
nesota, 
Cedar Island, north 
shore Lake Super- 
ior, Minnesota. 
North shore Lake Su- 
perior, Minnesota. 
of the Keweenaw Series—Continued. 
Quarter-section. 
Section. 
Township. 
Range. 
Macroscopic charac- 
ters. 
NE. 
Sw. 
55 
55 
8 Ww. 
8W. 
7W. 
Aphanitic; conchoi- 
dal; dark pur- 
plish-red, banded by 
indefinite waving 
bands of light-red. 
Porphyritic ingredi- 
ents: quartz, very 
abundantin crystals 
one-tenth to one- 
twentieth inch in 
diameter, and pink 
feldspars one-twelfth 
inchinlength. The 
feldspars tend to 
have their longer 
axes in the direction 
of the banding. 
SiOz, 76.83 per cent. 
Matrix  aphanitic, 
dark purplish-gray, 
blotched and banded 
with red; very 
abundant porphy- 
ritic pink feldspars 
one-tenth inch in 
length, and more 
minute quartzes. 
The arrangement of 
feldspars, and the 
fine red banding, in- 
dicate flowage. 
Aphanitic, pinkish- 
violet; highly con- 
choidal fracture ; no 
porphyritic ingredi- 
Microscopic descriptions of thin sec- 
tions. 
part secondary quartz, but are not all 
evidently so; these bands also hold 
abundant minute opaque ferrites. In 
the transparent bands are quite large 
irregular patches of a yellowish-green 
material which might be altered au- 
gite or epidote, but which all remain 
dark between the crossed nicols 
throughout an entire revolution. 
Matrix nearly colorless, faintly tinted 
pinkish-gray, cloudy. In the polar- 
ized light this matrix is seen to be 
made up of individualized material, 
in large proportion, saturated with 
secondary quartz in an arborescent 
tracery suggestive of the most deli- 
cate frosting; the filaments of this 
quartz network polarize together in 
relatively large areas. Excessively 
minute, opaque ferrite particles abun- 
dant. Rare non-continnous bands 
composed of quartz particles, as in 
the last-described section, occur, and 
here again occurs the greenish-gray 
non-polarizing substance above de- 
scribed. Large-sized, doubly termin- 
ated, rounded quartzes are the chief 
porphyritic ingredients. 
Matrix very close to that of 818. The 
patches of the peculiar greenish-gray 
substance there described are here 
very plenty; they are often drawn out 
into long strings, and also occur in 
small particles dotted over the section 
so exactly in the manner and with 
the shapes of the usual ferrite par- 
ticles, as to suggest the formation of 
the ferrites from them by alteration. 
Porphyritic ingredients: quartzes, 
with the usual characters and large 
and size; oligoclase. Thisrock and the 
two preceding are plainly nearer to 
the glassy condition than uny others 
described in this list. 
Colorless, cloudy matrix, completely 
saturated with secondary quartz in a 
network coarser than usual; thickly 
scattered through this matrix are un- 
