198 W. R. BROWNE. 
porphyry, and may possibly be the unaltered equivalent of 
this latter. In general appearance and mineral constitution 
it also bears a striking resemblance to a quartz-porphyry 
from Yass district, with which indeed it may be genetically 
connected. 
Closely associated with this Rosebrook porphyry is a. 
dark blue exceedingly compact felsitic rock, with relatively 
small development of phenocrysts, which are mainly quartz. 
By weathering and bleaching this rock assumes the appear- 
ance of a quartzite, as at Toll Bar Bridge. From here to 
Rosebrook it forms a considerable part of the eastern 
boundary of the limestone; itisentirely free from shattering. 
A ridge of the same rock is to be seen near Rock Flat, 
proving for it a meridional extension of at least 16 miles. 
Another variety of porphyry, which is found to the east 
of Cooma and north as far as ‘* Rosebrook,”’ is a dark rock 
with much smaller and very abundant phenocrysts of quartz 
and felspar; at first sight the rock appears to be tuffaceous 
but this is not so. It is in all cases highly and irregularly 
jointed, and very often shows a rude cleavage. On the 
appearance of the rock in the field one would pronounce it 
to be of an entirely distinct type from the Bushy Hill and 
Rosebrook porphyries, the chief differences being the smaller 
grainsize of the phenocrysts and the greater abundance of 
felspar. 
The remaining type of porphyry has evidently been 
intruded last of all, and has been subjected to very much 
less crushing than the other varieties. It is to be found 
on the Greenhill Road about two miles out from Cooma. 
Though just a little east of the line of the Bushy Hill out- 
crop, and west of some of the small-grained porphyry, the 
rock is here quite massive and free from shattering, proving 
its relative youth. The same porphyry also occurs farther 
to the east along the road to “‘Nitholme”’ and on the Kydra 
