BY C. A. SUSSMILCH AND H. I. JENSEN. 193 



which led to the greatest downthrow of the more westerly blocks. 

 The level of the original peneplain is probably represented by 

 the mesas averaging from 1,800 to 2,000 feet. The 3,000-feet 

 level is the effect of differential uplift and hypabyssal intrusions. 

 The 1,400-feet level is the result of downthrow, and exposure to 

 wiud-erosion in the absence of the protecting lavas. The strip on 

 which Narrabri is situated has undergone a much greater down- 

 throw. 



To summarise, the processes have been 



(a) Monoclinal folding of a peneplain, and intrusion of laccolites, 

 leading conjointly to the formation of a conoplain superimposed 

 on a gentle fold. 



(b) Expulsion of lavas. 



(c) Step-faulting with progressive downthrow to the west. 

 Further lava-flows. 



These processes cut up the original peneplain into three main 

 levels: (1) 2,700 to 3,000 feet; (2) 1,800 to 2,000 feet; (3) 1,400 

 feet. 



The original peneplain was probably coextensive with that on 

 which the WaiTumbungle lavas rest (elevation 2,000 feet). The 

 latter was also arched up by intrusions so that it reaches a 

 height of nearly 3,000 feet in the heart of the mountains. The 

 same peneplain extends to Dubbo, where it has an altitude of about 

 1,000 feet, and has alkaline lavas resting on it. Probably it is 

 this same peneplain we meet again at a level of 3,000 feet in the 

 Canobolas Mountains. 



To account for this, we have to imagine that — 1) After the 

 period of Cretaceous sedimentation over the area between Dubbo 

 and Bourke, all the country, for 250 miles or so from the 

 Cretaceous shore, had been reduced to a peneplain. (2) The Cre- 

 taceous sea withdrew, either as the result of a general lowering of 

 sea-level (Suess), or as the result of a general uplift of the whole ' 

 of the continent. (3) The area occupied by the Cretaceous trans- 

 gression continued to subside relatively to the rest of the con- 

 tinent, as a result of secular contraction; other surroundiji^;--...,,,^^^^ 



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