Origin of the Mountains. 
297 
be met with, in every attempt to piece together the 
movements that took place in their actual sequence. 
To the writer’s mind, the following interpretation is 
most in keeping with observed facts, although there 
are alternative views which future observation may 
render more acceptable. 
During the earlier Tertiary the whole basin was 
elevated to the height of about 1,500 feet. We had 
then a great plateau, watered with rivers of consider- 
able size ; there was a heavy rainfall, and the period 
probably corresponded to the Pluvial period referred 
to by Mr. Wilkinson. It was during this condition of 
things that the great valleys of the mountains were 
carved out by running water. Three or four points 
of volcanic eruption were in activity. A great 
volcano poured out its sheets of lava, remnants of 
which remain on Mounts Hay, King George, Tomah, 
and Wilson. Volcanic fires were also aglow on the 
Canoblas — and at the same time, probably, the rend- 
ing and the Assuring of the strata took place, that 
allowed the ascent of molten rock to form the dykes 
around Sydney. Subsequently another earth move- 
ment began, something in the nature of a fold ; this 
resulted in the depression of the coastal region allow- 
ing the sea to enter the valley of the Hawlcesbury 
and the system of mountain gorges that now form the 
harbour of Port Jackson. This movement resulted 
also in a further uplifting of the western points of the 
region west of Penrith. Thus, while one half of the 
basin was depressed, the other half was elevated, the 
