Gabbroid Rocks of Minnesota. — WincJiell. 169 
parallel to g'ipid), and the mineral thus passes to diallage. In 
thin sections the mineral is very light colored and yet a dis- 
tinct pleochroism can often be observed with : 
Thickness Thickness 
.025 to .03 mm. .i±mm 
«g --= pale greenish green 
«m = pale yellowish green yellowish green 
np = pink brownish yellow 
The refringence is higher than that of olivine as shown 
by the Becke method; the birefringence* is at least .022; the 
mineral is biaxial and positive with 2E very large. The dis-, 
persion about ;/g is p>v. 
The extinction in sections of the zone perpendicular to 
//'(lOo) bisects the angle of the cleavages; in sections of the 
zone parallel to the vertical axis the maximum extinction 
angle is found in sections parallel to the optic plane; this angle 
is higher than 30°. 
The chemical composition of this mineral shows a notable 
amount of titanium ; an analysis from a similar rock, but at 
some distance from northeastern Minnesota follows :f 
I 
SiOa 49-8o 
TiOg 1.29 
AI2O2 2.86 
Fe.O, 2.48 
FeO 10.82 
MnO .37 
MgO 15-33 
CaO 16.50 
Na^O .51 
KoO .12 
H.O .33 
100,41 
I. Diallage from gabbro from Ashland Co., Wis.; includes also 
traces of P.O. and Li.O (?); but no SrO nor BaO; by W. F. Hille- 
brand. See Bull. 148, U. S. Geol. Survey, 1897, P- 105. 
*Throughout this article the value of the birefringence is estimated. 
unless otherwise stated, by means of the Tableau des Birefringences 
of Michel Levy and Lacroix (Mineraux des Roches, 1888), only such 
sections being used for this purpose as have a thickness not less than 
.025 mm. and not greater than .03 mm. 
fNo analysis of the Minnesota material has yet been made on ac- 
'count of the difhculty experienced in trying to obtain pure material. 
