98 
Olivine Gabbro. 
Plate xvii. , fig. 1. 
Locality. — Mount Davies, Tomkinson Ranges. 
MacroscojJically. — Dark green, coarse-grained, heavy 
rock, apparently composed essentially of a pyroxene. Frac- 
ture very rough. 
Mici'oscopically. — Texture hypidiomorphic to allotrio- 
morphic granular, of medium-sized grain : composed princi- 
pally of diallage, olivine, and plagioclase. The diallage 
varies in colour from very faint green to colourless, and shows 
the basal striation to perfection. Well-defined, irregular 
cross-partings are prominent. Alteration to serpentine is 
seen in different stages of progress. 
Olivine greenish to colourless, darkened by p^ranular iron 
ores by decomposition. Crystal boundary rounded, and the 
cleavage (010) is distinct in a few examples. The crystals of 
olivine are altered to serpentine, sometimes completely, with 
deposition of a ferruginous ''dust." 
The plagioclase (labradorite) is scanty, and occurs chiefly 
in aggregates. The albite twin lamellae frequently 'wedge 
out." Undulatory extinction, produced by pressure, com- 
mon. The scarcity of this mineral gives a decided basic char- 
acter. 
Ores of chromium were not observed in the rocks ex- 
amined, although the Murru Yilyah outcrop, adjoining the 
gabbro, contains a secondary siliceous infiltration which is 
stained by chromium. 
DiORITE. 
Plate xvii., fig. .3. 
Locality. — Indulkana. 
Macroscoyically — Heavy, compact, dark-coloured, finely 
crystalline rock, coated on the surface with a rusty brown 
product of weathering. 
Microscopically. — Fine-textured, holocrystalline rock. In 
the sections e^.amined quartz is absent. 
A slight amount of orthoclase occurs as irregularly 
bounded individuals, often squeezed in between idiomorphic 
crystals of plagioclase. The plagioclase felspar is twinned 
according to the Albite and Karlsbad laws, the former being 
often accompanied by Pericline. From determinations on 
sections from the zone at right angles to (010) the felspar 
appears a slightly basic Andesine. Zoning comparatively 
scarce. The felspar is clouded (more so along the central por- 
tions) by kaolin and possibly calcite. 
Hornblende light brown, enlarged in certain directions 
by an outgrowth of secondary, often fibrous, green mineral 
