The Keweeiiazvan i?i Minnesota. — Elftman. 137 
The ophitic (lustre-mottling) texture is found only rarely. 
This consists of large grains of diallage enclosing numerous 
feldspars similar to the ophitic texture common in the 
diabases. 
The pegmatitic texture is not known as an original texture 
in the gabbro. It is common in certain altered phases of the 
rock, and consists of the alteration of the feldspars and a sub- 
sequent replacement of the altered parts by quartz which is 
similarly oriented throughout each feldspar grain. This 
process, when completed, gives the appearance of two minerals 
originally crystallized within each other. The fibrous inter- 
growth of minerals, common in the gabbro, may be a species 
of original pegmatitic texture. It consists of minute fibers of 
pyroxene and feldspar extending from the crystalline grains 
of both minerals, and lying side by side, forming a fibrous ag- 
gregate. Under a high power these fibers are shown to be 
forms of incipient crystallization branching out from the larger 
grains and restricted in their growth by a mutual contact. 
Minerals. The primary constituents of the gabbro are, 
plagioclase, orthoclase, diallage. hypersthene. augite, olivine, 
magnetite, garnet, apatite, pyrite. native copper, chalcopyrite 
and graphite. The secondary minerals are hornblende, bio- 
tite, chlorite, epidote, quartz, hematite, limonite, leucoxene, 
kaolin, serpentine, calcite, malachite, azurite. 
Plagioclase is the most abundant component. The min- 
eral occurs in broad grains with a fairly regular outline. 
Nearly all of the grains possess broad twinning lamellae easily 
seen upon a freshly broken specimen. The color varies from . 
the clear, colorless fresh mineral to the milky white color of 
the highly altered mineral. The cleavages are always promi- 
nent and shovk' the striated surface due to the twinning. Un- 
der the microscope, twinning, according to the albite law. is 
common, and according to the pericline and Carlsbad laws 
rarer. The extinction angle varies in different individuals 
from 20 to 40 degrees, indicating the presence of labradorite, 
bytownite and anorthite. The acicular inclusions character- 
istic of gabbroitic feldspar are nearly always present. They 
are sometimes parallel with the longer axis of the crystals. 
Each feldspar usually has several sets of parallel inclusions 
which cross each other at various angles. In manv sections 
