292 
Geology of Sydney. 
ment of resemblances and 
reader : — 
BASALT. 
Black and blue-black in color. 
A basic rock. 
Bulk for bulk, heavier than 
diorite. 
In origin Volcanic. 
Basalt, although occurring in 
dykes, is most commonly 
found in sheets which 
have flowed from a vol- 
canic vent. 
A hemi-crystalline rock, 
usually with a paste ; 
often a glassy base. 
Essential Minerals : — Plagio- 
clase felspar + augite 
olivine. 
Magnetite mostly present. 
differences may help the 
DIORITE. 
Greenish black to black in 
colour. 
An intermediate rock 
(mostly). 
Bulk for bulk lighter than 
basalt. 
In origin Plutonic. 
Diorite, although an Igneous 
rock, shows no signs of 
“flow,” and did not cool 
on the surface of the 
earth. 
A holo -crystalline rock, no 
paste. 
Essential Minerals : — Plagio- 
clase felspar + horn- 
blende. 
Magnetite mostly present. 
