Gabbroid Rocks of Minnesota. — Winchell. 355 
however, is common. The composition of the rock shows 
that the magnetite is non-titaniferous. 
IJcrcynitc can not be distinguished in exterior or micro- 
scopic characters from magnetite. Its presence was first sus- 
pected from the fact that an excess of ahmiina exists in the 
rock. Its presence . was estabhshed as follows : the coarse 
powder of the rock was digested in hot hydrochloric acid, 
changed now and then, and a little iodide of potassium added 
each time. This was continued until the attack by the acid 
had ceased. After filtering and drying, the quartz was elimi- 
nated by means of a heavy liquid, and an examination with the 
microscope showed that the residue was composed of augite 
and a spinel, resembling magnetite in outward characters. 
The augite was next eliminated by a rapid treatment with 
hydrofluoric acid, and the spinel, thus purified, was tested for 
magnesia, which was found only in traces. It must, therefore, 
be hercynite; it is in small amount in the rock, but evidently 
absorbs the excess of alumina. 
Fayalite has heavier, but more irregular, cleavages than 
chrysolite. Cleavage parallel to g'ioxo) is especially good; 
more irregular cleavages parallel to /(ooi) and h\\o6) are 
common. Crystal outline does not occur. Fayalite is trans- 
parent and colorless in thin section; in mass it is greenish 
yellow, but readily colored darker green or brown by de- 
composition. The refringence is inferior to that of augite and 
superior to that of tremolite. The birefringence is stronger 
than that of chrysolite, with: 
«g— ;7m =^ -009 at least. 
nm. — «p — -029 at least. 
n^ — «p = -038 at least. 
Fayalite is negative with an optic angle considerably 
smaller than in chrysolite. The dispersion about the acute 
bisectrix ?«p is p>t'. The chemical composition of the rock 
shows that the fayalite is non-manganiferous. 
The pyroxc7ic of the silicoferrolyte shows the usual pris- 
matic cleavages very distinctly. Parting and fine twnning 
parallel to h^{\oo) also occur. Its birefringence is: 
«g — ??ni -016 at least. 
//,n — «p .007 at least. *" 
n^ — »p ---=- .023 at least. 
