218 IDA A. BROWN. 
The chemical analysis shows that the felspar is a remark- 
ably pure member of the plagioclase group, which may be 
called bytownite, approximating to the composition Aboez 
Anz. 
(b) Optical Properties. 
The mineral occurs as allotriomorphic grains, which show 
no trace of decomposition. Cleavage is well developed in 
two directions parallel to (010) and (001). 
(001)4(010) = 89°. 
The mean refractive indices as measured by immersion: 
of cleavage chips in liquids are 
On (010) cleavage 1°571 
On (001) cleavage 1°562 
Zoning is absent. In sections perpendicular to (010) and 
(001) extinction is at 39° from the plane of the albite lamellae 
and, in sections parallel to (010), at 28° from (001) cleavage.. 
Twinning is well developed after Carlsbad, albite and 
pericline laws, the last making an angle of 11° with (001) 
cleavage, on (010) section. The optic axial angle was. 
determined by the graphical methods* of Professor Becke. 
ZNO 
In conclusion the writer wishes to thank sincerely Dr. 
W. R. Browne of the University of Sydney and Dr. C. 
Anderson of the Australian Museum, Sydney, for their kind’ 
help and advice during the preparation of this paper. 
1 Johannsen, Petrographical Methods, p. 480. 

