Gabbroid Rocks of Minnesota. — Wi?tckell. 287 
even penetrates far into the plagioclase (see plate XI, Fig. 2). 
The line of contact between the labradorite and orthoclase is 
often difBcult to find. 
The labradorite contains large crystals, often well formed, 
of augite, and others of apatite. Magnetite is practically ab- 
sent, both in the acicular form and also as the fine dust which 
is so common elsewhere. But the labradorite has not escaped 
from the alteration processes which have so changed the rock; 
it contains many decomposition products, especially fiakes of 
hornblende and clinochlore; sometimes the hematite particles 
have penetrated along fractures and around diallage inclu- 
sions. Further the cleavages have sometimes opened, giving 
the appearance of fine twinning. 
Andesinc is rather common, but clearly subordinate in 
abundance. The optic angle is very large, and the mineral 
sometimes negative. The dispersion about n, is /J<t^ diffei- 
ing from labradorite. The extinction in sections perpendicu- 
lar to lu^ is about 10°, perpendicular to //p it is 66°-68°. An- 
desine shows the same inclusions and alterations as the labra- 
dorite. 
Orthoclase occurs surrounding and even penetrating the 
plagioclases (see plate XI, Fig. 2). Carlsbad twinning is re- 
markably rare. This feldspar, seen in mass, has a dark pink 
tint due to the innumerable inclusions of hematite. The min- 
eral is optically negative, and from the measure of 2d, the 
value of 2E is 71° 4' or, if N^i.523, 2\=w° 52'. It has been 
possible .to determine that the extinction in sections perpen- 
dicular to //p is 90°, while in those perpendicular to n^ it 
does not seem greater than +4°. This orthoclase does not be- 
long to the deformed variety, since the plane of the optic axes 
is parallel to ^'(010). 
The inclusions are extremely numerous, and nearly all of 
them seem to be of recent origin. Besides the hematite parti- 
cles, flakes and crystals of hornblende are very common, while 
chlorite, epidote and titanite are more uncommon. The last 
two occur in highly altered areas, sometimes showing evi- 
dence of pressure and now changed to what may be called 
saussurite. Sometimes the orthoclase is optically broken up 
into many parts, while physically still intact; at other times 
it has been actually broken up into a fine granular aggregate 
