266 
Geology of Sydney. 
houses may happen to be built over these old volcanic 
vents, need not allow their midnight rest to be disturbed 
by misgivings as to the security of their peaceful dwell- 
ings.” 
Some years ago basalt used to be quarried on 
Humphrey's property at Burwood. Fine undecom- 
posed basalt was found, and worked to a depth of one 
hundred feet, evidently from a dyke, probably a con- 
tinuation of the Rookwood basalt. 
In order to study the intimate structure of basalt, 
a thin section must be prepared, and mounted upon a 
glass slip, with Canada balsam. The preparation 
of these slices is a matter altogether of patience and 
practice ; but it is not nearly so difficult as it first 
appears. The simplest way to prepare this thin slice 
is as follows : — 
1. Select a sound undecomposed portion of rock; 
flake off, with a hammer, a chip about the 
size of a half-penny, and grind down one 
surface, with emery and water, on an iron 
plate ; these plates are conveniently about 
12x12 inches. Towards the close of this 
portion of the operation the emery will have 
been ground into a very fine paste, so that 
the face on the rock will be fine and smooth. 
Should coarse emery remain, the rock will 
not have that smoothed surface required, 
and in this case it will be better to finish 
this part of the operation with some very 
fine flour-emery and water on a square 
plate of glass. 
