Gabbroid Rocks of Minnesota. — Wifichell. 2 1 3 
and labradorite, and also by exogenesis of pyroxene, horn- 
blende, and occasionally olivine. It occurs both brown and 
green, and both varieties are intensely pleochroic, with: 
«g = brown reddish brown green green 
«m = brown reddish brown grass green green slightly 
brownish 
«p = pale yellow very pale yellow brownish yellow pale green 
to colorless 
The absorption is usually «m > n^ > «p, but occasionally 
it is sensibly ?ig= nm. > np. In the normal diabase (954) the 
optic angle in biotite is unusually large; from the measure of 
2d, 2E^26° about. Both green and brown biotite seem to 
form directly from the augite; both contain pleochroic halos 
about apatite, allanite and titanite. 
The chemical composition is given in the first column fol- 
lowing. It contains a little less iron and somewhat more 
magnesia than the normal diabase. The titanium is doubtless, 
in part at least, contained in the magnetite. 
I II 
SiOg 49.18 49.88 
TiOg 1.09 1. 19 
AI2O3 19.01 18.55 
FcgOa 89 2.06 
FeO 7.79 8.37 
MnO 51 .09 
MgO 6.42 5.77 
CaO 9.12 9.70 
NagO 3.32 2.59 
K3O 82 .68 
HgO 2.06 1.04 
100.21 100.21 
Specific gravity 2.84 2.92 
2.97 
I. Olivine diabase from Pigeon point, Minnesota (1843). No ap- 
preciable BaO nor SrO; Pi-Oc not determined. By W. F. Hillebrand; 
published by W. S. Bayley: op. cit., p. yj. 
II. Olivine diabase from Pigeon point, Minn. Includes RaO — ng 
and P,Os=.i6. By F. W. Hillebrand. Ibid. 
