Gabbroid Rocks of Minnesota. — Winchell. 349 
lar masses surrounding or cementing the elements. Oc- 
casionally the andesine penetrates the quartz. 
The liquid and gaseous inclusions are usually irregularly 
distributed. Movable bubbles have been noted. 
Secondary quartz is not uncommon. It usually produces 
"quartz enlargements" by orienting itself exactly on the near- 
est primary quartz. (See plate XII, figure 2.) It also occurs 
m minute veins, and small masses of arbitrary orientation. 
A?idcsine is the only feldspar which has been found. It 
occurs in the usual large xenomorphic forms with the albite 
twinning nearly universal, and the pericline and Carlsbad 
twinning occasionally added. 
In mass the andesine is green or red from numberless 
alteration products; in section it is usually very cloudy from 
the same cause. The mineral seems to be positive with a very 
large optic angle. The maximum extinction in the zone per- 
pendicular to ^'(010) exceeds 20°; extinction in Sng does not 
seem to exceed 5°; perpendicular to ?/p it isabout65°; cleavage 
pieces parallel to p{ooi) give an extinction of about 2°; parallel 
to ,^'(010), it is about 10°. The composition of the plagioclase 
is therefore very near Abs An^^. 
The alteration products of the feldspar are calcite and a 
sericitic mineral; more rarely epidote, penninite, quartz, and 
hematite appear within the andesine. 
Apatite is usually well formed and always automorphic. 
It is sparsely distributed, but occurs in rather large acicular 
crystals terminated by the basal plane or a steep pyramid, or 
both. The' faces definitely identified are: p(pooi), w(ioio), 
b\ioii), and /zXii2o). 
Liquid and gaseous inclusions are sometimes abundant, 
and are usually arranged along planes, either crystallographic 
or curving. 
Zircon occurs in very small, well formed, rectangular 
crystals, which are practically colorless in thin section. The 
mineral is rather rare, and perhaps would be overlooked if it 
did not cause intense halos in the penninite and biotite, where 
it is generally found. In these halos the birefringence of the 
chlorite is increased about .002 ; and the refringence is in- 
creased also. (See plate XIX, figure 20.) 
Secondary Minerals. Biotite is notabundant.lt 
