200 The American Geologist. October, i9oo 
blende, and also to biotite. The refringence is very high as 
in all pyroxenes. 
The maximum birefringence is very near the normal pro- 
ducing colors as high as a clear green with: 
«g — «m = -022 to .020 
«m — «p = .001 to .003 
;7g — «p =.023 to .023 
As indicated by these values the angle of the optic axes 
is extremely small; indeed the hyperbolas of the interference 
figure scarcely separate, whereas in all known pyroxenes they 
should, not only promptly separate, but entirely leave the field 
of the ordinary microscope. So far as known to the writer 
no pyroxene* has ever been described having an optic angle 
(2E) less than 65°, and in all the common varieties! this angle 
is very constant and exceeds 100°. There exists normally a 
variation of one or two degrees in the value of the angle in 
common monoclinic pyroxenes, and the studies of Wiilfing, 
Doelter, and others seem to show that the angle in the non- 
aluminous pyroxenes increases directly with the amount of 
F^O. In the aluminous pyroxenes the angle varies very 
slightly and no certain relation has been established. In the 
sodic augites the angle increases with the amount of iron and 
sodium. 
Thin sections of the olivine diabase (1843) from Pigeon 
point present the remarkable anomaly of a pyroxene whose 
angle even in a single section is distinctly variable, and some- 
times becomes so small as to be nearly uniaxial. Thus in one 
section an anhedron directly perpendicular to the bisectrix 
gives from the measure of 2d with the axial goniometer of 
Lacroix, the valuej: 2E^i3° 16'. This small angle seems 
to be nearly constant in the one thin section, but in others 
from the same rock sample the angle varies from 2E=22° 55' 
to 2E^48° 33' and 2E^49° 3', while a number of grains from 
*Since this was written the writer has been informed that the same 
anomaly was discovered by Prof. N. H. Winchell about a year ago, 
while studying rocks from the same locality; see Vol. V, Final Rep. 
Geol. Nat. Hist. Surv. Minn. 
tThat is: augite, diopside, diallage. enstatite and hypersthene. 
J Allowing for the maximum of error due to the fact that the hyper- 
bolas are not sharply defined, the section being very thin, the angle 
2E certainly does not exceed 15°. 
