Gabbroid Rocks of Minnesota. — Winchell. 29 1 
Hornblende is by far the commonest and most important 
mineral formed since the final consolidation of the rock. It 
shows only the outlines of the pyroxene with its own prismat- 
ic cleavages. It is compact, or, more rarely, fibrous. When 
the pyroxene was twinned, the hornblende is twinned similar- 
ly. The color is dark green, both in mass and in thin sections, 
with strong, but variable, pleochroism: 
«g = green dark bluish green 
iixn = yellowish green green 
//p = yellow greenish vellow 
The absorption is n,^ >/'m>''/p - 
The birefringence is notable with: 
n^ - «in = .008 at most. 
tim - «p = .015 at most. 
n^ -Hp — .023 at most. 
Hornblende is optically negative, and the value of the acute 
optic angle derived from the measure of 2d is: 2E=74° about. 
The maximum extinction angle in the vertical zone is at 
least 18°. 
Hornblende contains inclusions of mituite apatite crystals 
surrounded by marked green halos. It alters rarely to chlor- 
ite. 
Actinolitc is quite rare in very fine fibers or lamellje. Like 
hornblende it is a decomposition product, but it is formed at 
some distance from the augite masses. It has a pale green 
color with weak pleochroism. 
4u^ --= pale green 
«m =^ yellowish green 
Wp =- (greenish) yellow 
The absorption is ;/g >/?m>//p. 
It has a lower refringence than the hornblende as shown 
by the fact that: 
Actinolitc //g<//g apatite (while apatite //g<//in horn- 
blende). 
The birefringence is very nearly the same as that of horn- 
blende and /'/g — /*!p ^=.022 at least. 
The elongation of the fibers is positive and the ma.ximum 
extinction angle in the vertical zone is 10" at least. 
Clinochlorc is the chlorite of the rock, and it is the final re- 
