PROBLEM OF THE GREAT AUSTRALIAN ARTESIAN BASIN. 149 



These layers, varying in nature from extremely indurated 

 types to incoherent "sand-rock," and consisting for the 

 most part of quartz grains with a certain proportion of 

 kaolinised felspar, exhibit great differences in their thick- 

 nesses at different points. The Conference's map (I. pi. 14) 

 showing their value over part of New South Wales, gives 

 figures ranging from 100 to over 700 feet, the lower values 

 being coincident with the margin of this section of the 

 basin. In Queensland there are similar changes from point 

 to point, but the data published are not sufficient to estab- 

 lish any definite relationship of the above kind. 



The water-bearing section of the strata is some 500 — 

 1000 feet or thereabouts, below the base of the marine blue 

 shales, which may attain a thickness of several thousands 

 of feet, and which form a practically impervious covering ; 

 in this connection it is unfortunate that so few sections 

 of boreholes exceeding 2,000 feet have been published. 



Towards the west and north-west, these porous beds seem 

 on the whole to come closer to, if not actually to lie at, the 

 very top of the Jurassic Series, and it is not at all unlikely 

 that, with the development of an arenaceous marginal 

 phase in the Cretaceous, such as would occur along the 

 shelving floor of impervious rocks, the water-bearing beds 

 of the east and centre, would by means of this overlap, be 

 put into partial or complete communication with the Lower 

 Cretaceous beds, and that the latter would then become 

 the actual Artesian member. According to a remark of 

 Carne, 1 this obtains in the north-western corner of New 

 South Wales. 



This relationship apparently holds in South Australia 

 too, where perhaps the whole of the strata passed through 

 in the deep boreholes is of Cretaceous age, though the 

 Jurassic may be represented in certain localities and prob- 



1 Brit. Assoc. Handbook, N.S.W., p. 607, 1914. 



