Gabbroid Rocks of Minnesota. — Winchell. 371 
studied, with the possible exception of the quartz gabbro. It 
forms fully one-half of the silicoferrolyte, where it is associated 
with another spinellid, the iron-alumina variety, hercynite. It 
occurs as a secondary mineral in every type, with tne possible 
exception of the silicoferrolyte, and the cordieritc noryte. It 
is thus the most widely disseminated mineral occurring in 
these gabbros. 
Hercynite occurs in small amount in the silicoferrolyte. 
It is the result of an excess of alumina in an extremely basic 
(ferric) rock. 
Hematite is rather common, in thin flakes and particles in 
the plagioclasyte and orthoclase gabbro ; it is less abundant 
in the olivine gabbro, the diabases, and the silicoferrolyte. 
Gothite is known only in the silicoferrolyte, and there oc- 
curs only rarely. 
Ilmenite occurs in small quantity accompanying magne- 
tite in the olivine diabase, and perhaps in other types. 
Limofiite is only known in the plagioclasyte, when highly 
altered. 
Pyrite occurs as a primary mineral in the diabases, and in 
the cordierite noryte ; it probably has a secondary origin in 
the plagioclasyte, and perhaps in part also in the olivine dia- 
base. It is always sparsely distributed. 
Riitilc occurs only in the quartz gabbro, in the form of 
minute crystals deeply striated. 
Staiirolite is a mineral whose presence is not to be ex- 
pected in an eruptive rock; it occurs in well formed, though 
microscopic crystals, as inclusions in the cordierite of the 
noryte. It is usually oriented with the surrounding mineral, 
the crystallographic axes, and the elongation of the staurolite 
and cordierite being parallel. Staurolite contains extremely 
minute inclusions, which are believed to h(t grap/iite. 
Calcite occurs in minute cjuantity in ever) ly]x' studied 
except the cordierite noryte and silicoferrolyte; it is relatively 
very abundant in the quartz gabbro. 
Mesolite occurs in the plagioclasyte only, ii is derived 
directly from the decomposition of the labradorile. 
Scolecite is likewise confined in occurrence to the plagio- 
clasyte. Its direct derivation from the labradorile is unques- 
tionable. 
