216 IDA A. BROWN. 
The extinction proved the symmetry of the mineral to 
be monoclinic. Simple twinning parallel to (010) was 
noticed in a few grains. The double refraction was indeter- 
minate owing to the strong natural colour of the mineral. 
With convergent light the mineral gave a biaxial negative — 
interference figure. The optic axial plane is parallel to 
(010). The value of the optic axial angle was calculated 
by the graphical methods of Prof. Becke, but the result 
is only an approximation as the brushes obtained were hazy, 
possibly owing to dispersion, The strong colour of the 
mineral also interferes with the measurement. 2V was 
calculated to be equal to 56°. 
Little can be said of the relation between the chemical 
composition and the optical properties of this amphibole. 
In a paper’ entitled ‘A Contribution to the Optical Study 
of the Amphiboles,’ W. EK. Ford has worked on the minerals 
analysed by? Dr. KF. C. Stanley and has drawn graphs 
showing the relations between the common oxides and the 
mean refractive index. Taking the mean refractive index 
to be 1°66 the hornblende under consideration agrees fairly 
well on the whole with these graphs, which show it to be 
a member between the tremolite-actinolite and hornblende 
groups. The following table shows the relation between 
the actual chemical analysis and the theoretical position 
on the graphs which have been drawn through the mean 
positions of the oxides for Dr. Stanley’s analyses. 
ih i 
SiO, 44-27 46-0 
AlO\ 9:56 8+5 
Fe,0, 0-11 5:0 
WO 2 HesOe 9-67 12-0 
FeO ieeuh os Shkets 
MgO 10°83. 140 
Cad 11-30) Wiles 
K,0 4 Na,0 1:94 3-0 

1 Amer. Journ. Sci., Vol. 37,1914.” Amer. Jour. Sci., Vol: 23, 1907 

