Basalt. 
277 
Macroscopic Examination. 
In hand specimens, the Bondi rock is a blue-black 
dense rock that rings under the hammer. The texture 
is compact, and, to the unaided eye, a fine glistening- 
structure is the only indication of its varied consti- 
tuents. A hand-lens will show an occasional porphy- 
ritic mineral not more than half a millimetre long. On 
slicing the rock these prove to be olivines or augites. 
It decomposes to a light grey crust as seen in the 
specimens exhibited. This, by the further removal of 
its more soluble constituents, becomes the soapy stiff 
rock, in appearance not unlike Fuller’s earth, that fills 
the fissures already referred to. The light grey colour 
of this decomposed crust is somewhat remarkable, as 
the rock contains 1 3% of ferrous and ferric oxide — 
quite as high a percentage as many basalts that weather 
to a bright red clay. A polished slab of the rock is 
etched with more than ordinary rapidity by hydro- 
chloric acid. The polished slab is almost black. When 
acted on by the acid it shows a light grey ground, with 
the unattacked minerals standing out as black granules. 
Its specific gravity is 2*94, a density somewhat above 
typical basalt. 
Magnetic Properties. 
1 he appearance of the etched surface suggested 
trying the effect of a magnet on the rock. A thin bar of 
the basalt, five millimetres long, was cut, and I found 
that a magnet lifted the bar readily. A second bar was 
cut and mounted as a magnetic needle. Permanent 
