Gabbroid Rocks of Minnesota, — Win die II. 283 
Constituent Minerals. Olivine is perhaps the most 
abundant mineral of the rock. The xenomorphic ha- 
bit prevents any distinct crystal outlines. The clea- 
vages parallel to ^'(010) and /(ooi) are often well de- 
veloped. The mineral is dark green in mass, but this is large- 
ly due to the decomposition which is fairly advanced. 
The beginnings of alteration to bowlingite are everywhere 
visible; the abundance of the magnetite which separates out 
at the same time is good evidence that the olivine is very rich 
in iron. 
Labradoritc is the feldspar of the rock. It shows the cus- 
tomary twinning according to the albite, pericline and Carls- 
bad laws. The refringence is always higher than that of Can- 
ada balsam; the birefringence is low. The extinction angle in 
S//g is 31 °-33°. The extinction on cleavage pieces parallel to 
/(ooi) varies from 7° to 11°; on pieces parallel to ^''(010) it is 
very near 25°. The maximum equal extinction in the zone 
perpendicular to ^'■'(010) is at least 38°. It is thus clear that 
this feldspar has a composition very near AbiAn^. 
More rarely the angle of extinction in .S//^, is as low as 22° 
or 23°, while the only two sections perpendicular to n^ which 
have been found have given extinctions of 67° to 69°. This is 
considered to be a labradorite near AbiAni. 
The augite shows the customary prismatic cleavages, -with 
rare parting parallel to //\ioo). Twinning parallel to //'(lOo) is 
not uncommon. 
The dispersion is weak; /:>>'' about //g, the acute bisec- 
trix. The maximum extinction angle in the zone of the verti- 
cal axis is slightly more than 40°. 
The brown lamellae characteristic of bronzite are onlv rare- 
ly seen. Liquid inclusions are also rare. 
Magnetite is very rare as a primary mineral, and very 
abundant as a sort of bye product in the alteration of the 
olivine. 
Bowlingite is abundant in the troctolyte. It is nearly al- 
ways fibrous and the fibers or lamellae arrange themselves so 
as to nearly compensate each others optical properties. 
Cleavages parallel to //Yioo) occur. The color is variable in 
the greenish and brownish-yellow tints with weak pleochro- 
ism: 
