BY W. N. BENSON. 255 



clearly as if they were of recent origin, but there can be no doubt 

 that they are at least pre-Tertiary in age, and probably pre-Permo- 

 Carboniferous, resulting originally from the "Schuppen" faulting 

 of the whole region during the period of the intrusion of the 

 ultra-basic rocks. The present relief is due entirely to differ- 

 ential erosion. Where a mass of Tertiary basalt extends across a 

 well-marked fault-scarp (such as that west of Mount Elijah, near 

 Black Springs Railway Station) the basalt is quite undisturbed 

 by the fault; and where the fault-lines extend south or north of 

 the agglomerates into regions of uniformly soft rocks, they 

 cause no relief, indeed cannot be traced save in chance sections. 

 It is almost certain that some of these faults pass below the 

 basalts of the Nandewar Range, which are quite undisturbed by 

 them. 



South of Borah Creek, the geology is not very varied. The 

 long line of the Baldwin Range stands up a thousand feet above 

 the Manilla Valley, its eastern dip-slope and western scarp being- 

 broken only in the small fault-complex at its southern end, while 

 enveloping faults appear to cut off this range and that of Pyramid 

 Hill the other side of the Manilla, for both end sharply, and 

 the agglomerate does not cross the Namoi River. West of the 

 Baldwin Range, the area is a gently undulating region of Barraba 

 mudstones and tuffs, with, occasionally, argillaceous limestone. A 

 whaleback-hill of Baldwin rocks occurs west of Spring Creek, but 

 is cut off by a fault, and a great wall left by differential erosion 

 looking westward over the open valley of Wongo Creek. 



We now commence again from Barraba — To the east of that 

 town lies the Jump Up Range, a hard band of agglomerate, 

 which has been brought up into its present position by a con- 

 tinuation of the easternmost of the Black Mountain group of 

 faults (traced by exposures in the railway cutting and Barraba 

 Creek). East of the faulted agglomerate, the mudstones dip 

 steeply to the east, and are overlain by Tertiary basalt; west of 

 the fault, they dip much more gently. In this region of gently 

 dipping mudstones, a small plain extends northwards from the 

 Black Mountain Range, so flat as to suggest that it was a lake- 



