Gabbroid Rocks of Mmnesota. — Wincliell. 173 
hidusions. The feldspar of the normal gabbro is nearly 
always in large grains of irregular outline, which often make 
up a large part of the rock. It is quite fresh and only rarely 
shows secondary weathering or metamorphic products. It 
nearly always contains black opaque dust-like particles scat- 
tered everywhere, and occurring, though more rarely, in the 
other minerals of the rock. These are frequently less abund- 
ant or altogether absent about masses and crystals of mag- 
netite, and it is therefore inferred that they are fine particles 
of that mineral. The black acicular inclusions* so often found 
in gabbros from all parts of the world are sometimes lacking, 
as they also often are in the plagioclasytes ; but they are more 
frequently present, without however being accompanied by the 
colored squares and plates, denominated "microplakites" and 
''microphyllites" by Schrauf, w"hich are probably the sole and 
efficient cause of the iridescence for which certain labradorites 
are famous. Liquid inclusions are comparatively uncommon. 
As noted before all the other original minerals occur more or 
less commonly as inclusions in the feldspar, though the olivine 
is very often not in direct contact with the plagioclase, on ac- 
count of intervening rims of augite. 
The pyroxene very often shows the inclusions so well 
known in bronzite, and probably identical in nature with those 
occurring in diallage and also in labradorite. They are most 
abundant along the secondary diallagic parting parallel to 
/(ooi), or rather parallel to this plane, as they sometimes occur 
when no trace of the parting can be found. Their direction 
taken as a whole is parallel to./'(ooi), though the individual 
inclusions are often perpendicular to this plane. They also 
occur parallel to h\\oo), and scattered irregularly throughout 
the pyroxene, in the latter case having no definite orientation. 
The origin of these inclusions has been frequently discussed ; 
Kosmann and Trippke believe that they are infiltration pro- 
ducts; Judd makes them the basis of his theory of schilleriza- 
tion, considering them to be due to the action of liquids 
(chiefiy water) at great depth and under pressure. He con- 
siders that certain planes under strain would act as planes of 
easiest alteration, or "solution planes," along which negative 
*A further study of these inclusions will be found in the chapter de- 
voted to the plagioclasyte. 
