258 
Geology of Sydney. 
noted that when sandstones are used for the bottoms 
of furnaces they become hardened, and develop a 
prismatic structure. A fine example of prismatic 
sandstone could at one time be seen at a quarry 
near the outlet of the main sewer, Bondi (Fig. 
32). This has long since been quarried away, but 
some columnar sandstone and masses of basalt can 
still be seen in the cliffs, although much of the last- 
named rock is altered to the soapy clay already 
referred to. 
There is another locality where the basalt is to 
this day hard and undecomposed. This is the Moore- 
fields dyke, Canterbury (Fig. 15). Here we see one 
of the few basaltic dykes around Sydney in which at 
some depth the rock is still well preserved, showing very 
little signs of decomposition. It is mined for road 
purposes by sinking shafts, at various points, to a depth 
of 60 to 100 feet, connecting these shafts by drives, 
and breaking down the rock pretty much in the 
same way that a quartz reef is worked. The basalt is 
10 to 15 feet in width. This interesting example is 
well worth a visit when stone is being quarried. 
When work is not being carried on, the shafts are 
usually full of water. 
At Rookwood, just south of the Rookwood Park, 
a dyke is worked for road metal, and where the work- 
ings are deepest some undecomposed basalt is still 
procurable. The dyke is, in places, 10 feet in width, 
with smooth walls of shale on each side. It runs in 
an easterly and westerly direction. As may be seen 
