ae e 
a 
a a — 
- 
FELSITE AND QUARTZ-PORPHYRY. 
109 
Tabulation of the results of a microscopic study of the felsites and felsitic porphyries 
of the Keweenaw Series—Continued. 
Specimen num- 
ber, 
Place. 
853...| Same place as 852. 
876... 
Foot of north cliff of 
the Great Palisades; 
north shore Lake | 
Superior, Minne- 
sota. 
Macroscopic charac- 
ters. 
ents. This rock is 
thickly studded 
with brown and 
black, eurving, hair- 
like markings. SiO, 
77.12 per cent. 
Aphanitic ; dark pur- 
plish-red and light 
pinkish-red, in inter- 
twisted curving 
2 
So 
3S 
gm | nts 
Behe pa ie 
5 3 3 
GC |als] & 
SW. | 28 | 56/7 Ww. 
bands. 
NE. | 22 | 56/7 W. 
Matrix aphanitic; 
dark purplish-red 
closely banded with 
lighter tinted, non- 
continuous bands, 
and rows of lighter 
colored spots. 
White, kaolinized 
orthoclases one- 
tenth inch in length, 
are the most impor- 
tant porphyritic in- 
gredients. More 
minute quartzes are 
abundant. 
Microscopic descriptions of thin sec- 
tions. 
usually large particles, and strings of 
particles, of red and brown ferrite. 
As already indicated, these strings of 
particles are sufficiently large to at- 
tract attention in the hand specimen. 
See Figs. 15, 16, Plate XIII. 
Colorless matrix, saturated with net- 
worked secondary quartz, and thickly 
studded with particles of red and 
black ferrite; the great abundance of 
these particles in portions of the sec- 
tion, and their nearly complete ab- 
sence in others, produces a strong 
banding. 
Matrix very strongly banded. The 
mostabundant bands present acloudy, 
gtay appearance, and are seen in po- 
larized light to be largely composed 
of non-polarizing matter, with which 
are abundant polarizing particles,some 
of which, atleast, belong to secondary 
quartz, There are also present in 
these bands exceedingly minute par- 
ticles of brown ferrite. Other bands 
are nearly colorless and transparent, 
and these are made up chiefly of indi- 
Vidualized quartz. Still other bands 
present much of a brown, blotchy stain, 
and are thickly studded with long, 
black, ferrite needles. The needles 
are at times straight, but more often 
have a marked curvature; and, while 
they show a marked tendency to fol- 
low the general directions of the 
bands, they yet lie across one another 
in such a way as to suggest the ap- 
pearance of a brush-fence (compare 
Zirkel, in Fortieth Parallel Report, 
Vol VI). The narrow ones of these 
bands are notcontinuonus even through 
the width of athin section. Allthicken 
and thin suddenly, and all are inter- 
- twisted in various curving forms, mak- 
ing abrupt turns when coming intocon- 
tact with the abundant porphyritic 
quartzes and orthoclases. The bands 
containing ferrite needles are least 
continuous and are sometimes found 
making forms like the letter $ within | 
