60 
COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
Tabulation of observations on anorthite-rock—Continued. 
Minnesota. 
g | | 
2 Place. g | SESE char- 
5 = ; 
FI | |S 
5 By) SEH 
a 6 |a\|a] & 
750....| North shore Lake | SW.| 5 | 54 | 8W.| Coarse-grained; 
Superior; 13 nearly white, 
miles below with faint green- 
mouth of Split ish tinge. 
Rock River. 
759....| North shore Lake | NE.| 5 | 55] 8W.| Very coarse- 
Superior; 2 grained; nearly 
miles below colorless, with 
mouth of Split faint greenish 
Rock River. tinge. 
792....| Falls of Beaver | SW.| 12 | 55| 8W.| Very coarse- 
River, Minne- grained; trans- 
sota. lucent; nearly 
colorless, but 
with faint green- 
ish tinge. 
795..-.| Beaver River, | SW.| 12 | 55 | 8 W.! Coarse-grained. 
Anorthite, in large crystals, 
Constituents as determined by 
Angle between 
maximum ex- 
tinctions of 
adjacent hemi- 
tropic bands of 
the plagioclase 
in sections cut 
microscope. at random in 
the zone O: i. 
Angles on) 9 
opposite| o S 
sides of|4¢ 
cross-hair.| B © 
| 
° ° ° 
Pure anorthite .....-.----------- 39 34 73 
42 40 82 
35 33 68 
Anorthite, in large crystals, 
makes up most of the section. 
The large crystals are often 
crushed into fragments at 
their extremities. Among 
the fragments are smaller an- 
orthite crystals, and numer- 
ous small rounded grains of 
augite. 
Pure anorthite, in very large, 
unbroken crystals. 
makes up most of the section. 
A few small grains of olivine, 
altered to a brown ocherous 
substance, occur in the feld- 
spars. A little augite lies be- 
tween the feldspar grains. 
Bands of viridite cross the 
feldspars. The anorthite car- 
ries long rows of minute, 
rounded inclusions, readily 
seen with a low power (70 to 
80 diameters). These rows 
run for short distances paral- 
lel to the cleavage, but are 
mostly in rows crossing the 
cleavages. A higher power 
shows numerous bubbles in 
these inclusions. The bub- 
bles will not disappear or 
move at a temperature of 112° 
C. Black microliths also oc- 
cur in the particles. 
