204 W. R. BROWNE. 
prismatically jointed over a space of two or three acres. 
The columns are remarkably regular, mainly hexagonal in 
section and up to 18 inches in diameter, and no columns 
were observed more than 4 feet long. In texture the rock 
is exceedingly fine, and olivine occurs in nodules or segre- 
gations, in much greater proportion than in the normal 
basalt. In places there has been a tendency to subsidiary 
jointing, producing a kind of pisolitic effect on the weathered 
surface. Only the summit of the North Brother is com- 
posed of this very compact basalt; below it the hill is 
terraced and the basalt is of the ordinary fine-grained 
type. 
A noteworthy example of variation in the texture may 
be observed west of the Myalla Road 4 miles south of 
Cooma. Here there is a flat-topped terraced ridge, the 
topmost terrace being of coarse-grained basalt, doleritic in 
aspect and microscopically seen to have ophitic fabric. The 
basalt of the terrace immediately underneath is much finer 
in grain and of granulitic fabric. Two possible explanations 
suggest themselves. What we now see as the upper terrace 
may really represent the bottom part of a thick flow, whose 
top has been denuded; the bottom part would have cooled 
comparatively slowly and so have become somewhat 
coarsely crystalline. Or else the coarse-grained rock might 
have been intruded as a kind of dolerite sill between two 
pre-existing flows, one of which is now denuded away. 
To the N.W. and east of Cooma the basalt has in some 
instances filled old pre-Tertiary valleys; for example, the 
road from Cooma to Murrumbucca via Mittagang Bridge 
runs mostly along such a valley for 11 miles, and the former 
course of the Upper Murrumbidgee from McCarty’s Cross- 
ing to the junction of the Dalgety and Berridale Roads is 
now marked by flows of basalt. No tuffis have been any- 
where found associated with any of the flows. 
