1 68 The America?i Geologist. September, i90o 
have been made. The extinction angle in cleavage fragments 
parallel io p{ooi) is ii° to 13°, and in pieces parallel to^^ (010) 
it is about 26°. The maximum equal extinction measured on 
the albite twinning in the zone perpendicular to ^^(010) varies 
from 35^° to about 40° in different specimens*. 
All the analyses which have been made show that the 
mineral is a labradorite whose composition varies between that 
expressed by Abs Am and Ab2 Ans. The specific gravity varies 
from 2.700 to 2.712, but its average is only 2.704*. The analy- 
ses follow : 
SiO. 
TiOg 
AUO3 
Fe203 
FeO 
MgO 
CaO 
Na20 
K=0 
H2O 
100.38 100.28 100.69 
I. Labradorite from gabbro from N. W. 34. S. E. %, Sec. 23, T. 
62, N. R. ID W. Minn.; by W. F. Hillebrand; see Bull. 148, U. S. Geol. 
Survey, p. 112. 
II. Labradorite from gabbro from Sec. 25, T. 64 N., R. 8 W. 
Minn. Ibidem. 
III. Labradorite from gabbro from Dulutli and Iron Range R. R., 
Minn. Ibidem. 
The analysis under I represents a feldspar very near the 
composition expressed by Aba An^; the other two are of the 
composition Ab^ Ans. 
The pyroxenic element of the rock ordinarily belongs to 
the group of diopside being very poor in alumina ; hypers- 
thene only occurs as one approaches the contacts with other 
rocks. The pyroxene is occasionally distinctly automorphic; 
the faces m{iio) and g\oio) have been observed. Twinning 
parallel to h^ (100) is not uncommon ; the characteristic cleav- 
ages are usually well developed forming an angle of about 87°. 
Parting is often developed parallel to /i\ioo)\ more rarely 
*Cf. Bayley: Jour. Geol. 1893, I, p. 700. 
1 
II 
III 
5345 
52.61 
52.50 
trace 
trace 
trace 
29.77 
29.80 
30- 15 
•33 
•57 
•47 
•15 
•23 
•15 
.11 
.20 
.10 
11-33 
12.25 
12.82 
4-33 
3.80 
372 
.68 
•53 
•53 
•23 
.29 
.25 
