GEOLOGY OF THE COOMA DISTRICT, N.S.W. 221 
the present Monaro peneplain. It was thought that they 
might have been centres of eruption, but no evidence has 
been found to confirm such a view. Now if theseare really 
residuals, it follows that the extrusion of basalt which 
formed them must have occurred prior to the evolution of 
the present topography. 
Again the Murrumbidgee near the mouth of Bridle Creek 
is seen to flow between banks which are of slate capped by 
basalt, as if an old valley had existed which had been filled 
with lava, and the rejuvenated river had cut through this 
and down considerably below the level of the former 
stream-bed. 
On the other hand the basalt existing in the Mittagang 
road valley and that of Pilot Creek must have been extruded 
subsequent to the formation of these valleys, that is to say, 
subsequent to the uplift of the Berridale fault-block. In 
addition to this, the basalt hills are often terraced, denoting 
a succession of flows, and in various places one finds a flow 
of perfectly fresh and recent-looking basalt on ioe of an 
earlier and much decomposed one. 
While therefore there has been no definite field evidence 
made available, it seems at least possible from the above 
considerations that the extrusions of lava, which were 
doubtless connected with the earth-movements, were pro- 
longed over a considerable period or may belong to two 
widely separated epochs; they may even have lasted into 
recent geological times. Petrological work on the basalts 
may do something towards the elucidation of this question, 
and further field-work may also help to settle the matter. 
No decisive evidence as to foci of eruption was discovered. 
It is thought that one focus may have been the head of 
Pilot Creek, where the valley is abruptly terminated by 
the scarp of a basaltic platform raised toa height of about 
250 feet above the level of the valley. Of course the out- 
