94 
COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
Tabulation of the results of a microscopic study of the quartzless porphyries of the 
Keweenaw Series. 
imen num- 
ber. 
Spec: 
Place. 
Quarter-section. 
Macroscopic charac- 
ters. 
Microscopic characters. 
= 
a) 
o 
So 
1781 A 
-| Stannard’s 
2616. -. 
Rock, 
Lake Superior. 
Pebble from Albany 
and Boston Con- 
glomerate, Kewee- 
naw Point, Michi- 
gan. 
Pebble from the con- 
glomerate at the 
National Mine, 
Rockland, Mich. 
INIWieee 
Center 
& 
ic 
af 
a a 
8 55 
Aphanitic; dark red- 
dish-brown; shows 
no visible porphy- 
ritic ingredients. 
SiO,, 65.81 per cent. 
33W.| Aphanitic; dark 
chocolate-brown; 
conchoidal fracture; 
holds abundant mi- 
nute, pinkish, por- 
phyritic feldspars. 
SiO,, 65.35 per cent. 
16 50 |39 W.} Matrix aphanitic; dark 
reddish-brown; car- 
ries minute porphy- 
Titie tabular feld- 
spars. 
In the ordinary light, the matrix of this 
rock presents a general red background 
thickly studded with opaque brown 
ferrites. The only porphyritie ingredi- 
ents are augites, which are now repre- 
sented by patches of red translucent, or 
opaque black ferrite, within which are 
little remnants of unaltered augite. 
These ferrite patches often show the crys- 
talline outlines of the original augites. 
Elongated holes, worn in the section by 
grinding, were probably occupied by mi- 
nute decomposed porphyritic feldspars. 
In the polarized light, the groundmass 
presents a considerable proportion of iso- 
trope matter, in which are occasionally 
recognizable minute tabular feldspars. 
Brightly-polarizing, irregularly-outlined 
particles and clusters of particles in the 
ground-mass appear to belong to quartz, 
possibly also to orthoclase. Some of 
these clusters plainly belong to second- 
ary quartz. Thelow percentage of silica 
(65.81), the abundance of altered augite 
and of ferrite particles, and the scarcity 
of secondary quartz, all serve to separate 
this rock from the more acid felsites; 
while it is separated from the diabase- 
porphyrites by its higher percentage of 
silica and rarity of tabular feldspars. 
Both in the ordinary and polarized lights the 
section of this pebble resembles closely 
that of the rock from Stannard’s Rock; 
the large ferrite patches, representing 
altered augite, are, however, much less 
abundant in this rock, which also holds 
not unfrequent small porphyritic oligo- 
clases. 
The groundmass of this rock is much like 
that of 1781 A, containing, however, 
rather more secondary quartz. The por- 
phyritic feldspars are oligoclase. 
