Gabbroid Rocks of Minnesota. — Winchell. 211 
rarely green, but either one may apparently be formed directly 
from the pyroxene. The pleochroism is very marked with: 
Halo about allanite. 
«g = dark brown dark brown bottle green (bluish) 
«m = brown dark brown pale yellowish green 
«p = brownish yellow pale yellow pale yellow 
Absorption ng > ;/m > n^ . 
These colors often mingle as the brown hornblende passes 
into the green. 
The birefringence is strong, being .030, at least, in the 
brown hornblende and somewhat less in the green hornblende. 
The extinction angle in sections of the vertical zone varies 
from 0° to at least 17°. 
Hornblende often contains pleochroic halos about in- 
clusions of apatite, allanite, etc. These halos are usually of 
a greenish brown color, and thus the brown color is conspicu- 
ous in green hornblende, while the greenish tint is very evident 
in brown hornblende. 
Hornblende has been observed in small patches sur- 
rounded by labradorite, but it is believed to be secondary here 
also, though it is not fibrous. 
Magnetite and pyrite occur as secondary as well as primary 
minerals; the latter sometimes being derived from the former. 
Hematite is decidedly uncommon; its blood red color pre- 
vents its being overlooked. 
Boivlingite is the commoner decomposition product of the 
olivine and is always present. It has a lainellar cleavage 
parallel to A^i^o), and perpendicular to the optic plane.- The 
color of bowlingite is variable, probably according to the state 
of oxidation of the iron, from green to brown; it is always 
pleochroic with: 
«g = green greenish brown dark brown dark reddish brown, 
«m -= y^ll^"^'^^^ brown brown reddish brown 
'" green 
pale light yellowish 
)i^ = greenish greenish brown reddish yellow 
yellow yellow 
The absorption is always //g > //m > //p . 
The pleochroism is very variable in intensity, and even in- 
distinguishable when the mineral is only feebly birefringent. 
