
210 IDA A. BROWN. 
NOTES ON HORNBLENDE AND BYTOWNITE FROM 
HYPERSTHENE GABBRO, BLACK BLUFF, 
NEAR BROKEN HILL. 
By Iba A. BROWN, B.Sc.. 
Deas-Thomson Scholar in Mineralogy and Demonstrator in Geology, 
The University of Sydney. 
(Communicated by W. R. Browns, D.Sc.) 
[Read before the Royal Society of N. S. Wales, December 6, 1922.] 
— 
Introduction 
. Mineral Separation. — 
. Hornblende 
(a) Chemical Composition. 
(6) Optical Properties. 
4. Felspar 
Co kb 
(a) Chemical Composition. 
(6) Optical Properties. 
Introduction. 
The rock in which these minerals occur is the gabbro 
(A 186) from Black Bluff near Broken Hill, described in the 
Memoir of the Geological Survey of New South Wales, No. 
8, 1922, App. 1, pp. 295, 353. 
Under the microscope it is seen to be holocrystalline, 
medium-grained, with typical gabbroic fabric, and consists 
of plagioclase, light grey and colourless pyroxene (mono- 
clinic and rhombic), and brown hornblende, with magnetite, | 
apatite, and pyrites as accessories. | | 
A Rosiwal analysis shows that the minerals are present 
in approximately the following proportions of percentage 
weights:— Plagioclase 51°28 
Pyroxene 30°51 
Hornblende 11°43 
Magnetite, etc. 1°81 

100°03 

