282 
Geology of Sydney. 
Microscopic Structure. 
The general structure of the rock under the 
microscope may be described as micro-porphyritic, 
with practically no traces of flow in the disposition of 
the felspars. The amount of base present is somewhat 
in excess. Much of the latter is isotropic, or so feebly 
double refracting as to be undeterminable. The 
minerals which occur micro-porphyritically in the 
base are, olivine, augite, plagioclase, with aggregates of 
magnetite and sodalite. The olivine crystals are 
abundant on every slide, and only occasionally show 
alteration into serpentine. The large individuals 
are idiomorphic, but rarely perfect; they are for 
he most part broken, and the parts separated. 
Cubes of magnetite are common as inclusions. The 
way wedge-shaped masses penetrate the peculiar 
olivine crystals is really characteristic of all the slices 
made. 
Large olivines are often seen penetrated in this 
way. The larger olivines show a disposition to 
develop in lath-shaped forms rather than in forms 
familiar in basalts. Some of the crystals will occa- 
sionally measure one-twelfth of an inch. 
The augite in the slides is abundant and charac- 
teristic. Idiomorphic crystals can be seen, measuring 
from the one-twenty -fifth to the one-twelfth of an inch. 
They show a warm brown border which fades towards 
the centre of the crystals. A faint pleochroism is 
noticeable, but not in every augite on a slide. 
Plagioclase is not so well defined and distinct as 
