310 W. J. CLUNIES ROSS. 
a geological map and partly described it.' He shows several out- 
crops of basalt, and mentions that a creek at Swallow’s Nest has 
excavated a channel two hundred feet deep in solid basalt. He 
was of opinion that the Bathurst basalt was poured out as lava 
stream in that district. Swatchfield itself is the name of a large 
parish, about thirty to thirty-five miles from Bathurst. It is 
some miles south of the township of Oberon and not far from the 
sources of the Fish and Campbell Rivers, which rise near together 
but afterwards separate widely, finally uniting to form the Mac- 
quarie. Swatchfield is thus on the river system that drains the 
Bathurst area, and, assuming that the drifts below the basalt were 
brought down by a river which followed approximately the course 
of the present Macquarie, there can be little doubt that we have 
in this district the source of our Bathurst basalts. It is highly 
desirable, however, to obtain confirmation of the assumption, if 
possible. A specimen of basalt from the neighbourhood of Oberon 
was therefore very welcome, and it has been cut and analysed. 
(Slide and micro-photograph, No. 6). The rock is much weathered 
but its structure may be made out. It isa distinctly porphyritic 
rock and carries large crystals of olivine, much corroded at the 
edges and crossed by cracks filled with serpentinous matter. Apart 
from these, the crystals are clear and shew little or no included 
magnetite. It appears probable that they were brought up by 
the lava in a solid form and were not formed in the rock itself. 
The ground mass is much decomposed, but small crystals of felspar 
similar to those in the ground mass of the Bald Hills basalt may 
be seen, and there are indications of flow structure. When 
analysed, the silica was found to be from 46-75 to 47-75 per cent. 
The alumina 19-2 per cent.; ferric oxide 14:5 per cent.; lime 7°84 
per cent.; magnesia 8-2 per cent. The specific gravity 2°83. 
This similarity of character is a strong point in favour of the 
view held by Mr. Wilkinson. The township of Oberon is about 
3,500 feet above sea level, so that there is a fall of about eight 
1 Report of Department of Mines, 1877 (Sydney 1878) pp. 200 to 205, 
and map. 
