OLIGOCLASE FELSPAR—VICTORIA. 375 
No. 13.—ON OLIGOCLASE FELSPAR FROM MOUNT 
ANAKIES, IN VICTORIA. 
By A. W. Howirt, F.G.S. 
(Read Tuesday, January 11, 1898.) 
In studying the voleanic rocks of Victoria under the microscope, 
I have felt that the use of convergent polarised light might be 
made to give more conclusive results were it possible, in the first 
instance, to subject isolated crystals—for instance, of the triclinic 
felspars found in these rocks—to examination, and thus obtain 
data for reference. 
Unfortunately, it is most difficult to obtain such crystals, and 
the only locality whence I have obtained them is Lake Purrumbete, 
in the western district ; Mount Frankin, in the northern district ; 
Mount Anakies, about 20 miles northwards from Geelong ; and 
the Dargo High Plains, in Gippsland. 
In order to ascertain what results might be expected from such 
an examination, I selected several isolated crystals of a triclinic 
felspar from a number which had been collected in the volcanic 
detritus at Mount Anakies. These crystals were in good perserva- 
tion, and, although rounded on the edges and corners, the crystal- 
line planes were readily distinguishable. The crystals were 
compounded of the planes P (001) OP., M (010) «2 P ~,—1 (110) 
cob’ EF and. T (110) co BP: 
From each of four of these crystals three slices were prepared 
according to the planes P, M, 1, and T, and also in a direction 
normal to P and M. 
The samples selected for this paper are numbered I and II. 
In the three slices I determined the obscuration angles in P M and 
in the direction normal to P M with the following results :— 
I. Seen in this slice according to P with crossed nicols there are 
numerous alternations of very narrow and of wider lamella, 
arranged according to the Albite law. In the slice normal to P 
and M there is not only the same synthetic structure—hbeing, 
indeed, the continuation of the twin plates seen on P—but also 
a twining, as numerous and similarly arranged, according to the 
Perecline law, thus forming together a well-marked grating-like 
figure. Upon M this structure is not apparent. The observations 
made in slices of No. II were identically the same. 
