FELSITE AND QUARTZ-PORPHYRY. 
111 
Tabulation of the results of a microscopic study of the felsites and felsitic porphyries 
of the Keweenaw Series—Continued. 
imen num- 
ber. 
Spec’ 
Place. 
1540..| North shore Lake Sa- 
1728 b 
perior, Minnesota, 
200 paces east 1539. 
1728..| North shore Lake Su- 
perior; north side 
Bead Island, mouth 
of Nipigon Straits, 
Ontario, Canada. 
A few hundred yards 
east of 1828, the 
same rock-mass. 
Quarter-section. 
About. 
Macroscopic charac- 
ters. 
| Section. 
| Township. 
| Range. 
35 | 63 | 5 E. Aphanitic; dark pur- 
plish-red. Very 
abundant, much al- 
tered red feldspars, 
up to one-half inch 
in length; also 
quartzes one-twen- 
tieth inchin di- 
ameter. Comes out 
in thin tabular frag- 
ments. 
Matrix dark pur- 
plish-red, aphanitic; 
porphyritic, flesh- 
colored feldspars, 
one-quarter to one- 
half inch long, ex- 
traordinarily abun- 
dant; quartz also 
very abundant,rare- 
ly exceeding one- 
tenth inch in diam- 
eter. Resembles 
1838, 1846d, 1970, 
and Michipicoten11. 
Matrix brick-red, 
banded with vaguely 
defined bands of 
lighter and darker 
red. The feldspars 
are less abundant 
than in 1728, and 
are whitish and 
porcellaneous from 
alteration. 
Microscopic descriptions of thin sec- 
tions. 
ground shows only the usual excess- 
ively fine quartz network, while the 
green blotches remain in large meas- 
ure dark thronghout an entire revolu- 
tion. Porphyritic orthoclases are of 
very large size, red-stained and deeply 
eaten into by the matrix. The por- 
phyritic quartzes present no unusual 
characters. 
In the ordinary light a matrix much 
like the lighter portions of 1539, ex- 
cept that red and brown ferrites are 
thickly clustered in some portions. 
In the polarized light the distinction 
between the quartz network and other 
independently polarizing particles is 
plainly seen. 
In the ordinary light the matrix pre- 
sents throughout a deep reddish- 
brown stain, produced by thickly- 
crowded ferrite particles, which, in 
the immediate vicinity of the porphy- 
ritic ingredients, present some indi- 
cations of a fiuidal texture; but, for 
the most part, the matrix is without 
such an appearance. In the polar- 
ized light the matrix is for the most 
part dark, presenting only very mi- 
nute, feebly-polarizing particles. The 
usual quartz network appears to be 
entirely wanting. The quartzes are 
much eaten, which is also the case 
with the porphyritic feldspars. See 
Figs. 4,7, Plate XIII. 
Close to 1728, but in the polarized light 
the background presents more indi- 
cations of individualization and is pe- 
culiar for its curvilinear clusters of 
ferrite particles. The quartzes are 
extraordinarily large and abundant, 
and are eaten by the matrix into many 
peculiar forms. See Figs. 1 2, 3, 
Plate XIII. 
