Gabbroid Rocks of Minnesota. — Wifichell. 1 8 1 
by Dr. H. N. Stokes of the laboratory of the United States 
Geological Survey on material from the same general region: 
I II 
Si02 47-70 46.06 
TIO2 1.80 1.05 
AhOs 19.04 18.87 
Fe.Os 87 .74 
FeO 8.84 11.73 
MnO trace trace 
MgO 8.65 Q.73 
CaO 8.96 8.53 
NaaO 2.53 2.14 
K.O... 53 -37 
H2O 1.33 1.03 - 
100.30 100.38 
Sp. Gr. 2.89 3.00 
I. Olivine gabbro (1136) from Birch lake. No appreciable BaO 
nor SrO; PaOs not determined. 
II. Average of two analyses by Dr. H. N. Stokes: First from 
S. E. J4. Sec. 19, 63 — 9 (near Snowbank lake), and second from Sec. 
35, 61 — 12, near Birch lake. Includes P2O5 ^.03; NiO =.io; see 
Bayley: Jour. Geol. 1893, I, p. 714. 
Titanic acid seems to be a constant element in the normal 
gabbro in very notable amount; the alumina is very high and 
the iron is nearly all in the protoxide form. The amount of 
magnesia is a true index of the relative abundance of augite 
and olivine. Phosphoric acid is in extremely small amount. 
Chapter II. Normal Diabase. 
The term diabase is employed in this article to designate a 
rock identical with a gabbro in its mineralogical composition, 
but possessing a texture distinctly ophitic instead of the grani- 
tic texture of the gabbros. Like the olivine-free gabbros the 
normal diabases (containing no olivine), are rare in Minnesota; 
the rock to be studied here contains a small amount of chryso- 
lite. Strictly speaking, it is therefore an olivine diabase, but 
in its chemical composition it differs less from the diabases 
absolutely free from olivine than from those containing the 
ordinary quantity of chrysolite, such as the rock forming the 
