318 
Geology of Sydney. 
scooped out and washed down from above. Now, 
could we look down from a considerable height, and, 
as it were, take a bird’s-eye view of the Blue Mountains, 
exactly similar features would be presented to us as 
we observed in the earth embankment. Wherever 
the soft clay strata of the Goal Measures exist, there 
are sloping surfaces ; where the harder Haivkesbury 
rocks appear, there are cliffs, while the easily dis- 
integrated clay-beds have been washed away, even to 
undermining the over-lying sandstone. The latter 
has resisted, to a great extent, the denuding agencies, 
and been left in projecting masses, which, at last, 
having the supporting soft strata removed, break away 
and roll in great bosses down the sides of the hill 
perhaps as far as into the bed of the stream below, a 
perpendicular cliff being left where they broke away, 
from the main mass of rock. It may be objected that 
the effects of the present streams are not sufficiently 
powerful to have worked such changes ; but then it 
must be considered that these forces have been 
operating through an immense period of time ; and, 
moreover, that in the later Tertiary epoch we have 
geological data indicating that this part of the 
Continent was subject to a much heavier rainfall. 
Again, in the earlier Tertiary period a great portion of 
the Continent was covered by the sea. It was 
probably during this later epoch that these, valleys 
first began to be marked out as the sea- water receded ; 
and the subsequent draining off from the land of the 
rainwater gradually scooped out and deepened the 
