Gabbroid Rocks of Minnesota. — Winchell. 1 99 
is ng and the optic angle is large. The maximum equal ex- 
tinction in the zone perpendicular to ^^(010) exceeds 34°; the 
extinction in sections perpendicular to «g varies between ^ti" 
and 38°; while in sections perpendicular to wp it varies from 
57° to 59°; the extinction in cleavage pieces parallel to p{poi) 
is usually over 10°, varying to 15°, while parallel to ^'(010) it 
varies from 23° to 29°. The feldspar therefore varies in com- 
position between Ab3 An* and Abi An^. 
The analysis of this feldspar by Bayley gave the following 
results: 
f I 
Si02 53-73 
AlsOs 30.39 
Fe203 1.26 
CaO 10.84 
NasO 3.76* 
100.00 
Sp. Gr. 2.699 
I. Labradorite from olivine diabase from Pigeon point, Minn. 
W. S. Bayley. Op. cit., p. 34. 
The pyroxene of the Pigeon point diabase presents certain 
very exceptional peculiarities. It shows the ordinary cleavage 
at 93°, parting parallel to /(ooi) and more rarely parallel to 
JC{\oo), twinning parallel to the same planes and high refring- 
ence. The color is variable, sometimes being absent, and at 
other times distinct. The absorption formula is not determin- 
able in thiri section. In mass the mineral is black; in thin 
section it is greenish brown with weak pleochroism : 
«g --=^ pale green 
«m = pale yellowish green 
?ip ^ pink (or flesh colored) 
Bayleyf found that the colorless variety gave a maximum 
extinction angle in the vertical zone of only 7° 30', and had a 
rather low birefringence. In sections of the olivine diabase 
(1843) ^ pink pyroxene showing the characteristic cleavage 
at 93°, and extinction bisecting the cleavage angle in trans- 
verse sections, alters quite readily to brown and green horn- 
*By difference. 
tW. S. Bayley: Op. cit., p. 36. 
