188 
Geology oe Sydney. 
and fresh-water shells are known to have existed. 
Fossil shells allied to the fresh-water mussel are found 
from the Narrabeen up to the latest beds above the 
Hawkesbury Sandstone. There were doubtless other 
forms of animal life, of the existence of which we have 
no direct proof. 
There were few active volcanoes immediately 
around, and these were probably restricted to the 
earlier part of the Trias. Much of the scoriae* ashes 
and lavas of the preceding period were worn away by 
such agencies as rain, running water, wind, and the 
alternating effects of heat and cold. The sediments 
thus produced were swept into the Triassic lake. The 
land was undergoing a slow but steady subsidence. 
Changes were being wrought apace over the country 
that was being denuded. Through alterations of level 
in the land, a granitic area to the south-west was 
brought within the watershed of our great lake. The 
swollen rivers brought down ever-increasing loads of 
granitic sand. Gradually the lake became shallow 
through the accumulation of these sands, and islands 
appeared here and there, covered with a thick vegeta- 
tion. Great tracts of shallow water grew into a 
swamp, rich in plant life. Turbid waters filtering 
through reed-beds supplied the finely-comminuted 
material that made our shales. Some beds of shale 
were formed in depressions in the sandstones, with 
dry land all around. The dry land referred to, con- 
sisted for the most part of unconsolidated sand, and 
much of this was tossed about and redistributed by 
