Dykes near Sydney. 
285 
The most astonishing feature of these igneous 
dykes is the great depth from which the molten matter 
must have been forced. We usually associate the 
outflow of molten rock with a volcanic orifice or pipe. 
In the case of these dykes, it is not reasonable to 
suppose that they were filled with basalt from a pipe 
or a number of pipes. Everything points to the fact 
that “the fissures were rent so profoundly in the crust 
of the earth as to reach down to a reservoir of molten 
rock which straight away rose in them / 5 
It is not easy to decide the total thickness of the 
geological formations that the basalt about Sydney 
welled up through, but approximately we may count 
on the following : — 
Strata Cut by the Dykbs near Sydney. 
Feet. 
300 
... 1,000 
... 2,000 
. . . 2,000 
. . . 2,500 
. . . 2,000 
... 5,000 
and 
. . . 5,000 
Wianamatta Shales 
Hawkesbury Sandstone 
Narrabeen Shales 
Upper Coal Measures ... 
Upper Marine Series 
Lower Marine Series 
Carboniferous 
Older Palaeozoic Rocks, Serpentines, 
Granites 
19,800 
The dykes have probably risen through nearly 
four miles of stratified rocks and granite, and the 
original reservoir of molten matter was no doubt much 
deeper down in the crust of the earth. 
