120 
COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
Tabulation of the results of a microscopic examination of augite-syenites, &c.—Continued. 
Specimen number, 
3055. ..! 
Place. 
Mouth of Tischer’s 
Creek, north 
shore of Lake 
Superior, Min- 
nesota, 
| Quarter-section. 
SE. 
| Section. 
13 
| Township. 
50 
| Range. 
14 W. 
Macroscopic char- 
Red; earthy; 
shows occasional 
feldspar facets, 
but is for the 
most part with- 
out crystalline 
structure; mi- 
nute nests of 
quartz abund- 
ant; very rough 
fracture. 
Microscopic descriptions. 
is a quartz in smaller areas, 
which is plainly secondary. 
Since large and small quartzes 
frequently polarize together, 
it is plain that all must be 
taken as secondary. <A very 
few of the feldspars show 
banding in the polarized light. 
There are also present good 
sized pale-grecnish, strongly 
dichroic, much shattered 
hornblendes, which often pre- 
sent much the appearence of 
uralite. Titaniferous iron 
and apatite are not uncommon 
ingredients, and the whole 
aspect of the rock suggests 
the possibility of its being an 
orthoclase-gabbro, in which 
the uralitic alteration of the 
hornblende and the deposition 
of secondary quartz have been 
pushed to anextreme. (Com- 
pare the description of pp. 
50-56. See also Fig. 1, Plate 
XV.) 
In the ordinary light the thin 
section presents a red-stained 
background, which is com- 
pletely riddled by very coarse 
secondary quartz in such a 
manner as to suggest for parts 
of the section a complete shat- 
tering before the deposition of 
the quartz. The uncrystal- 
line appearance of the red 
matrix in the ordinary light, 
and the networked arrange- 
ment of much of the quartz, 
suggest that this rock might 
more properly be included 
among the felsitic porphyries; 
but in the polarized light a 
considerable proportion of the 
background resolves itself 
Angle between 
maximum ex- 
tinetions of 
adjacent hemi- 
tropic bands of 
the plagioclase 
in sections cut 
at random in 
the zone O: it. 
Angles on| 2° . 
osite oe 
sides of| 4g 
cross-hair.| & © 
° ° ° 
