286 The American Geologist. November, isoo 
ant, and to reddish gray when orthoclase predominates. The 
plagioclase crystals are conspicuous on fresh surfaces, and 
single cleavage faces may be even three to four centimeters in 
length. The augite and secondary hornblende are usually 
much less abundant, but occasionally they also form large 
crystals. Orthoclase is conspicuous from its red color, which 
is due to fine hematite dust scattered through it everywhere. 
Magnetite often forms masses whose black metallic luster is 
clearly determinable. Acicular, colorless apatite needles are 
usually abundant, and visible even without a magnifying 
glass. Less commonly the rock is of finer grain and then the 
color is darker and the pyroxene more abundant. On de- 
composition the rock usually becomes darker, and the ferro- 
magnesian minerals become more conspicuous, and can be 
seen to fill the areas between the decaying feldspars. 
Under the microscope the dominant texture is seen to be 
very coarsely granitic, but the plagioclase sometimes shows a 
tendency to enclose the augite in poikilitic fashion. The feld- 
spar is in very large crystals, which may even occupy the 
whole of the section. The augite, next older, sometimes 
shows good crystal outlines wholly surrounded by feldspar, 
which in some cases has protected the pyroxene from uraliti- 
zation. The earliest minerals formed were the magnetite, 
usually in small masses and not abundant, and the apatite, in 
large crystals. But the mineral especially characteristic of the 
rock is the orthoclase, which, is clearly younger than the pla- 
gioclase. 
Constituent minerals. Labradorltc is the dominant 
feldspar of the orthoclase gabbro; it is usually in 
\"erv large anhedra, in which twinning is usually 
rare. The mineral is positis'c with p y v about 
//g, the acute bisectrix. The extinction in S //g is about 
20°, and in T «p it is as high as 62° to 64^ ; perpendicular to an 
optic axis the extinction is 41°, while the maximum extinction 
in the zone of symmetry of the albite twinning exceeds 30°. 
The extinction in cleavage pieces parallel to /'(ooi) is — 45^° 
to 6°, and parallel to g\o\o), it is about — 18'. The composi- 
tion is therefore near AbiAni. 
The labradorite, in large anhedra, is often surrounded by a 
more recent crystallization of orthoclase, which sometimes 
