Gabbroid Rocks of Miimesota. — W'lncJiell. 361 
the diabase becomes porphyritic, or fine grained, as frequently 
happens. 
Second type of variation. The normal type of the family, 
i. e., the olivine gabbro, is composed of nearly equal parts of 
leucokratic elements (labradorite) and melanokratic elements 
(pyroxene and olivine). It presents variations of two catego- 
ries : 
1. Variations without loss of equilibrium between the 
leucokratic and melanokratic elements. 
Olivine ) I P}'roxene =0 — Troctol\te 
> Labradorite= ■: 
Gabbro ) / Olivine =0 — Normal Gabbro 
2. Variations with loss of equilibrium. 
Olivine ) Labradorite=o — Peridotyte(melanokratic type) 
Ga bro ) Pyroxene, Oli\iner=o-Plagioclasyte (leucokratic 
type) 
The decrease of the pyroxene and the increase of the oliv- 
ine produce a rock which dififers from the olivine gabbro in ex- 
terior characters in that it is heavier and usually darker in 
color, being very dark green. Its alteration produces a rusty 
green or a dark brown color. .Sometimes the pyroxene persists 
in the form of narrow borders about other minerals, and may 
even present a more perfect development occasionally. It 
then encloses the olivine grains, and thus presents the poiki- 
litic texture. But the granitic texture is much more abundant, 
and the rock is often a little finer grained than the normal 
type. Magnetite is rare as a primary mineral ; a large part of 
the iron content of the rock is contained in the olivine, whose 
richness in iron is attested by the abundance of secondary 
magnetite. Apatite seems to have disappeared from the rock, 
but this absence may be merely accidental, or only local. This 
type is the troctolyte. 
The reduction of the olivine and the increase of the pyrox- 
ene produce a rock which is less common than the troctolyte 
in northern Minnesota, whereas in other gabbro areas the re- 
verse is usually the case. The resultant rock is a granular ag- 
gregate of labradorite and pyroxene with more or less mag- 
netite and apatite. It is the normal gabbro. 
The decrease of the labradorite and the increase of the 
