284 
Geology of Sydney. 
seen in the neighbourhood of the magnetite, evidently 
due to an incipient alteration of the magnetite. As a 
rule, magnetite is seen in crystals, but also forms 
irregularly-shaped patches. It was one of the first 
minerals crystallized out of the magma. 
Besides occurring in porphyritic forms, the 
minerals named are also found as granules embedded 
in the base. Microliths of apatite can also be found 
in most of the slices, as an inclusion in other minerals. 
Conclusion. 
The basalt occurring as a dyke at Bondi contains 
a high percentage of soda. The soda-bearing mineral 
of the basalt is isotropic and gelatinizes with cold 
hydrochloric acid. As there is chlorine in the rock 
and no sulphuric acid, the mineral is most probably 
sodalite. The hydrochloric acid solution of the rock 
gives abundant cubes of sodium chloride on slowly 
evaporating. Based on the chlorine present, we 
find that there is a surplus of soda, after allowing 
for sodalite and soda felspars. Probably some soda 
exists as a constituent of the residual basis. 
There are two igneous dykes at Bondi, one includ- 
ing the fissures running north and south, a second 
along a line coming seawards from the direction of 
Greenwich. The basalt contains traces of chromium. 
A needle of the basalt is susceptible of permanent 
magnetism, and when swung freely will act as a mag- 
netic needle. 1 
1 From a paper by the author, Ro t v. Soc. N.S.W., 1S94. 
