254 
Geology of Sydney. 
In structure , an Igneous rock may be : ITolo- 
crystalline, such as Granite (see Fig. 10); partly 
crystalline, such as Basalt (see Fig. 0); or glassy, 
such as Rhyolite, 
Fig. 72. Granite Rocks near Tarana, X. S. Wales, showing the 
weathering of Granite into boulders of disintegration, t 
According to chem ical, composition , Igneous rocks 
are grouped into three classes, depending on the per- 
1 Travelling from Sydney by mil. these rocks may be seen from the train, to the 
left of the railway line, a few miles beyond Taraua. 
