170 • The American Geologist. September, 1900 
Most of the iron is in the form of protoxide ; the alkalies 
and water are in negligible quantity. 
The olivine * of the normal gabbro is very rarely twin- 
ned; its cleavages parallel g^ipio) and /'(ooi) can only be 
seen when the mineral has begun to alter. They then be- 
come very distinct, appearing as fine lines very straight and 
fairly continuous. The cleavage parallel to ^^010) is much 
better than the other ; both are often accentuated by black, 
opaque inclusions probably of secondary magnetite. In mass 
the mineral is orange to olive-green; in thin section it is color- 
less. The crystal form of olivine is not uncommon, though the 
outline is usually more or less irregular, and the angles 
rounded. 
The chemical composition of the olivine occurring not far 
from Birch lake shows it to be a hyalosiderite with the ratio 
Mg : Fe about 1.43 :i. The olivine analyzed was almost wholly 
unaltered, and gave the following results : 
I 
Si02 35-58 
TiOs 1.22 
AI2O3 ,. .. , -92 
F2O3 trace 
FeO 33.91 
MnO .35 
CoO .20 
NiO ? 
MgO 26.86 
CaO .90 
H^O .31 
100.25 
I. Olivine from gabbro from S. E. J4. S. E. ^, Sec. 19, T. 63 N., 
R. 9 W., Minn., by W. F. Hillebrand. See Bayley: Jour. Geol., 1893, I, 
P- 703- 
The high per cent of the titanium is interesting as showing 
that this might perhaps be termed titanolivine ; it is at least 
an intermediate stage. 
At the time of the consolidation of the gabbro all the super- 
abundant iron oxide crystalized in the form of magnetite. In 
spite of its early crystallization, crystal outline is rare, except 
*rhe olivine is frequently replaced in this rock by fayalite, accord- 
ing to N. H. Winchell. 
