BY W. N. fiEJJSON. 259 



upper surface is broken by intrusions of tuff and breccia, just as 

 described in the Nundle and Tamvvorth regions(l, Pts. iv., v.). 



The western edge of the agglomerate is probably down-faulted. 

 The fault is quite definite some miles to the north, but is un- 

 fortunately obscured where it should cross Cobbadah Creek. 

 Hence, the upper surface of the agglomerate is not clearly ex- 

 posed here. 



As shown in Plate xx., fig. 4, Cobbadah Creek lies in a syncline 

 in the Baldwin rocks. To the north of this creek, the agglomer- 

 ate dips to the south, as seen in the ranges east of Tin Hut 

 Creek; while, to the south, the northerly dip is very clearly 

 shown in the cuttings along the zigzag road coming down from 

 Cobbadah to Horton. 



The Blue Knob Laccolite. — The most remarkable feature south 

 of Cobbadah Creek is the Blue Knob dolerite-laccolite. This is 

 roughly saucer-shaped. East of it, at the head of Sheep Station 

 Creek, the dip is south-west; but, tracing round the northern 

 face, the dip changes to south, then to south-east, and finally to 

 east, which we have seen to be the dip of its long continuation 

 south of the Nandewar Range. It commences south of Dunee 

 homestead, crosses Oakey Creek, which cuts a great gorge 

 through it, crosses the front of Blue Knob, where it forms the 

 steep northern face, and exposes a thickness of 700 or 800 feet 

 of igneous rock. It appears to transgress the bedding-planes a 

 good deal, or to have been intruded subsequently to some fault- 

 ing, for the rocks adjacent to the north-western end of the 

 intrusion are largely agglomeratic and quite different from those 

 to the east. This, however, has not been investigated in detail. 

 Just as in the Manilla River Gorge, there was found to be a zone 

 of strong crumpling some distance below the dolerite, so, north of 

 Blue Knob, at about 300 feet stratigraphically below the intru- 

 sion, there is a patch of very disturbed rock. 



The southern boundary of the intrusion is no less interesting 

 than the northern. To the east and west, it is overlapped by 

 the Nandewar Range basalt, but, in the central portion, the 

 Barraba rocks occur, and are eroded by the head- waters of 



