Gabbroid Rocks of Minnesota. — Winchell. 30 1 
Inclusions occur rarely, but they are so small as to escape 
accurate identification. They are gray, black, and earthy; of 
wholy irregular outlines, and are believed to be graphite. 
Staurolite has not been found except as inclusions in the 
cordierite in which it nearly always has a definite orientation. 
The elongation of the two minerals is parallel, but of contrary 
sigh i. e., //g of staurolite is parallel to wp of cordierite. While 
the optic planes of both minerals are parallel to the elongation, 
they are mutually perpendicular, and//p of staurolite is parallel 
to nm of cordierite. Thus sections in the vertical zone show- 
ing the maximum birefringence (//g — -'?p )of the staurolite. 
show only a medium birefringence in the cordierite, and vice 
versa. This relation is shown in Plate XIX, Fig. 19. 
Nevertheless the staurolite prisms occasionally occur with 
an orientation wholly arbitrary. 
Zircon occurs in well defined rectangular crystals. Zircon 
causes intense greenish halos in the anomite, and much feebler 
ones in anthophyllite. It has not been found in cordierite. 
Apatite occurs in very small acicular crystals terminated 
by the basal plane, or a steep pyramid. The mineral is clearly 
biaxial, though no measure of the optic angle (extremely 
small) has been possible. 
'The apatite sometimes contains liquid inclusions. It 
causes greenish halos in biotite, not as intense as those due 
to zircon. 
Epidote is very rare; it occurs in irregular crystals, with 
marked cleavages, and formless grains. The birefringence is 
variable even in a single crystal but the maximum is 
(wg - «p )= .036at least. 
Spi?iel has only been observed as a very thin coating on 
magnetite, especially when the latter is enclosed in cordierite. 
It is very dark green, often opaque, and wholly isotropic. It is 
probably pleonaste. 
A?ithophyllite occurs as a fibrous or lamellar alteration 
product of the bronzite. All stages of this change can be seen 
from the compact granular bronzite to fibrous amphibole. 
The alteration usually begins on the borders and follows the 
rough cleavage lines, developing chiefly in one direction paral- 
lel to the fibers. The two minerals usually have parallel 
orientation and simultaneous extinction. The anthophyllite. 
