Tlie Keweenazvafi in Mvmesota. — Elftman. 1 39 
the crystal. Twinning has not been noticed in any of the 
sections. Pleochroism is strong as follows: c = light green, 
a = brownish red, b= greenish yellow or pink to red. 
Polarization colors are low and the extinction is parallel on 
every axis. The tabular microlitic interpositions usually 
found in hypersthene are almost wanting in the mineral in 
this rock. These, like the tabular inclusions in the diallage, 
are doubtless due to alteration. 
Augite occurs in subordinate quantity as irregular anhe- 
drons. The color is usually light brown, varying on the one 
hand to dark brown in which each grain is uniformly colored 
throughout, and on the other to almost colorless. The min- 
eral is everywhere traversed by parallel cleavage lines and 
numerous irregular fractures with no definite direction re- 
ferred to the cleavage. There is no perceptible pleochroism. 
Polarization colors are high and the maximum extinction on 
the brachypinacoid is about 54 degrees. 
Olivine is present as irregular grains having frequently 
partial crystal outlines. It is surrounded by the pyroxenes, 
and when these minerals are present in the rock the olivine 
seldom comes into contact with the feldspar. The rim of 
pyroxene is sometimes scarcely noticeable and from this nar- 
row extent increases until it forms large plates. Inclusions 
of magnetite are nearly always present, and locally are so 
numerous as to render the olivine opaque. In some sections 
the magnetite assumes a vermicular form, which, on account 
of its being found in the perfectly fresh mineral, is probably 
primary. A dendritic form of the magnetite is sometimes as- 
sociated with the alteration products of the olivine, and is 
probably secondary. The color of the olivine is greenish to 
yellow. The mineral may be recognized in the hand speci- 
men by its yellow, wax-like color when fresh, and by the 
brown, earthy appearance when altered. Cleavage lines are 
absent, but numerous irregular fracture lines are prominent. 
In thin section the surface of the mineral appears rough on 
account of the high index of refraction. Pleochroism is ab- 
sent, polarization colors are high and extinction is parallel. 
The mineral is usually quite fresh in all phases of the gabbro. 
In the forellenstein it is more or less altered and the usual 
lattice structure accompanying alteration is very prominent. 
