196 ALEX. L. DU TOIT. 



logical deduction, and is strongly supported by the abnorm- 

 ally high temperature gradients found underfoot. Pittman's 

 testimony regarding the presence of intrusive dolerite in 

 some of the bores in New South Wales is welcome, also his 

 admission that the high temperatures in some of the wells 

 may be due to expiring volcanism. 



The Warrumbungle Mountains 1 overlooking the south- 

 eastern corner of the Basin are important in this connection, 

 because the numerous vents and plugs piercing the Jurassic 

 beds indicate the existence below the surface of an ex- 

 tensive laccolitic mass, from which through prolonged 

 differentiation, a suite of alkaline (soda-rich) rocks of great 

 variety, both effusive and hypabyssal, has been derived 

 and erupted. The same again is the case in the marginal 

 range of the Nandewars, 2 where in addition faulting has 

 played a part, while another region of alkaline rocks has 

 Dubbo at its centre. 



The effect of heated waters charged with silica and sili- 

 cates, derived by contact with or directly from the cooling 

 magma, is proved by the diatomite deposits interstratified 

 with the trachytes and tuffs, and by seams of opal, chal- 

 cedony and silicified breccias in the Warrumbungles, such 

 silicification having been noticed in the Nandewars also, 

 and in the basalts at Springsure near Clermont. 



Again, the Warrumbungles, and to a lesser degree the 

 Nandewars, are well known for their curious permanent 

 springs high up among the mountains, difficult to explain 

 by hydrostatic pressure alone. Jensen, 3 who favours the 

 view that 00 2 set free by rock decomposition may be 

 assisting in bringing the water to the surface, considers 

 that this action is probably inadequate, while Symmonds 4 



1 H. I. Jensen, Proc. Linn. Soc. N.S.W., xxxn, pp. 557-626, 1907. 



2 H. I. Jensen, ibid., xxxn, pp. 842-914, 1907. 



3 H. I. Jensen, ibid., xxxn, pp. 579-581, 1907. 



4 E. S. Symmonds, loc. cit., pp. 23 - 33; also App. A by H. I. Jensen. 



