354 H. I. JENSEN. 



mixture of windblown detritus and silt deposited in a kind 

 of marsh by floods from the Central Tableland in the period 

 of increasing rainfall before integrated drainage was 

 re-established. Evidence of a greater rainfall at the present 

 time is afforded here by two facts, (1) a slight amount of 

 undulation has been produced in the windblown detritus by 

 rainwaters in the present cycle, (2) the Bogan River 

 flows in a slight depression and reaches the Darling in flood 

 time so that the drainage has become integrated. 



Under both the red and black soils of the Bogan we get 

 hundreds of feet (from 300 to 1500 feet) of Tertiary gravels 

 and clays which were deposited under wet meteorological 

 conditions, possibly in a lake. The coarseness of the sands 

 and gravels interbedded in this series shows that big streams 

 deposited here the material carried from the central parts 

 of the Cobar massive. The great differences in depth to 

 bed rock in the various bore boles around Nyngan show 

 that the depression in which this heavy aliuviation took 

 place was probably formed by downfaulting in a period 

 when the Cobar massive was a rugged mountain group 

 widely different from the smooth and weatherworn aspect it 

 presents at the present time. In fact mesas of Triassic 

 rock occur under the Bogan silts, though to-day no Triassic 

 (Trias Jura) rocks remain on the Cobar massive, proving 

 that a block of country around Nyngan was depressed or 

 downfaulted in early Tertiary period. 



Two points of interest might here be briefly touched on. 



Firstly — The difference in climatic conditions between 

 the period of aridity and the present is not great, and it 

 might be held that the restoration of integrated drainage 

 is the effect of an uplift of the Central Plains rather than 

 of increased rainfall. Whether this is the case or not I 

 cannot say, but I think that recent conditions are less arid 

 than those of the period of disintegrated drainage. 



