180 
Geology of Sydney. 
unconformities indicate an interval of time, during 
3 O 
which great changes may have taken place, of which 
the records are “ either entirely lost, or have to be 
sought elsewhere.” 
In figure 45 we have an example of an un- 
conformable succession, whore Permo-Carboniferous 
beds are shown resting on the upturned edges of 
Silurian strata. 1 hese last highly-inclined rocks were 
laid down as more or less horizontal beds one above 
the other. They hardened and compacted probably 
under pressure, or in part by the solution and re-pre- 
cipitation of some of their mineral constituents. They 
were then elevated above sea-level, for they are of 
marine origin, by a folding or puckering of the earth’s 
crust. The upper portions of the folds were sub- 
sequently worn down by denudation. Then they were 
once more depressed below the sea, and formed a floor 
on which succeeding marine deposits 1 were laid down. 
Lastly, the old beds and tiro newer deposits they now 
carry were lifted again from beneath tiro sea, and tire 
vast thickness of fresh-water deposits of the Permo- 
Carboniferous laid down during a protracted sub- 
sidence. Thus we see how it is, that an uncon- 
formity such as exists between the Narrabeen Shales 
and the Permo-Carboniferous means a long lapse of 
time with no record here to tell of the physical 
changes that were in progress or of the influences 
affecting the animals or plants that lived. 
1 Marine beds are not shown in this sketch section although such beds are kuowu 
in the Western districts. 
