PROBLEM OF THE GREAT AUSTRALIAN ARTESIAN BASIN. 195 



the Cretaceous, they rest for the greater part of their 

 breadth upon a surface cut very obliquely across the out- 

 crop of the Jurassic intake beds. The later Tertiary tilting 

 movement has brought about a gentle inward dip of these 

 strata and their lava covering. Some of the volcanic foci 

 irom which these or perhaps the younger basalt flows were 

 erupted, have been observed in the neighbourhood of 

 Hughenden, quite a number in that of Clermont — the former 

 within, the latter just without the Basin — and. others 

 towards Herberton; doubtless many more of these centres 

 await discovery. 



In addition thereto, and perhaps of greater importance, 

 are the Miocene alkaline (trachytic) lavas and intrusive 

 dykes and plugs (rocks all rich in soda, but poor in lime) 

 that are found along a broad belt stretching with breaks in 

 a curving direction from Casterton in Victoria to Clermont 

 and Rockhampton in Queensland. 1 These were followed 

 by great basalt outpourings, the lavas issuing from cones, 

 and either forming great sheets or having flowed down 

 valleys eroded during mid-tertiary times. 



This eruptive phase near the close of the Pliocene was 

 accompanied or immediately followed by great continental 

 uplift with some block-faulting, while on the extreme 

 eastern edge of the continent the forces of compression 

 acted in a westerly direction. 



From analogy with other parts of the world, e.g., South 

 Africa, where similar structures occur and similar geo- 

 logical episodes have been experienced, it is to be expected 

 that bene&th the uncompressed and only feebly warped 

 Artesian Basin, the older strata will have been injected by 

 igneous matter, possibly on an extensive scale. The exist- 

 ence of tertiary sills, dykes and even batholithic intrusions 

 below this interior region, or at least its eastern part, is a 



1 T. W. E. David, Handbook of Australia, B.A.A.S., ch. vii, 1914.' 



