862 THE GEOLOGY OF THE NANDEWAR MOUNTAINS, 



not very reliable, at any rate the time I spent in the vicinity was 

 not sufficient to work them out properly. The confusion is due 

 to (a) the extraordinary conditions under which the conglomerates 

 were formed, (b) subsequent quartz porphyry intrusions, 

 (c) mountain-building movements. However, speaking generally, 

 the rocks S.S.E. of Upper Maule's Creek and its branch, 

 Oakey Creek, dip in an easterly direction, whilst those 

 west of them dip N.W., so that these two creeks occupy the 

 position of the crest of an anticline, or perhaps the line of 

 junction of Devonian and Carboniferous rocks. The latter 

 hypothesis is the less probable, as the rocks on both sides of the 

 line are very similar. 



Between Coolah Station and Boggabri the country is studded 

 with mesas and mesa-like ridges of barren sandstone. Near 

 Boggabri buttes of rhyolitic lava and tuff become abundant. To 

 the south of Boggabri, between it and Gunnedah, the formation is 

 Permo-Carboniferous sandstone capped with alluvial in places. 

 West of Boggabri lies the Pilliga Scrub, in Upper Coal Measure 

 country, composed of very barren sandstones. Between Narrabri 

 and Boggabri the same formations occur. On the road, about 

 half-way between the towns, some large basaltic sheets are 

 crossed; further north, at Tipperina, the road passes through an 

 edge of the Pilliga Scrub (sand). 



Seen from Maule's Creek plain or from Boggabri, the Nandewar 

 Range south of Coolah presents the remarkable appearance of 

 being capped with peaks which all have a steep slope to the south 

 and a gentle slope to the north. The only explanation 

 which I can suggest to account for the phenomenon is that there 

 may have been step-faulting with the downthrow of the southern 

 wall in each case. 



The Manilla Range west of Boggabri I did not find time to 

 visit, but judging by its rugged appearance it is probably 

 composed of volcanic rocks, in all probability rhyolite. 



4. Geomorphogeny. 

 (a) General Discussion. — From the foregoing description we 

 see that the Nandewars, like the Warrumbungles, may be looked 



