256 
Geology of Sydney. 
rocks which cooled at or near the surface are known 
as Volcanic . Both Volcanic and Plutonic rocks may be 
conveniently designated Eruptive . It will thus be seen 
that it is not always necessary to determine exactly 
whether a rock is Plutonic or Volcanic, some of the 
more important lessons that may be learned from its 
history being at once apparent if we can get so far as 
to conclude that the rock is Eruptive in character. 
All the Igneous or Eruptive rocks near Sydney are 
basalts, and occur in the shape of dykes, or, in a few 
instances, in the shape of bosses. According to our 
Table, therefore, they are Basic rocks of Volcanic origin. 
It may thus be asked : Did our basalts actually flow 
from a volcano ? The basaltic lavas that we are more 
accustomed to hear about are those that flowed in 
liquid or often as merely plastic streams from a 
volcanic vent. The basalts on Mount Hay and Mount 
Tomah, and also the basalts about Bathurst and 
Orange, answer exactly this description. The dyke- 
like walls of basalt that cut through the sandstones 
and shales about Sydney probably never reached the 
surface . 1 The liquid magma was forced up from below, 
and cooled and hardened just where we find it. 
Although these rocks did not actually flow over the 
surface of the earth, they correspond to the general 
characters of rocks that cooled from a molten condi- 
tion at or near the surface. For the most part, our 
Sydney basalts occur in the shape of dykes. In most 
instances the original blue-black basalt is altered, by 
decomposition, into a soapy cream-coloured clay not 
1 Evidence is not wanting that may support an opposite view. 
