156 ALEX. L. DU TOIT. 



below 1° F. in 30 feet, and often as steep as 1° F. in 20, 

 since this abnormality may in part be due to effect of the 

 advance of the waters upon the shelving floor towards the 

 points of efflux constituting the Mound Springs. 



Nevertheless, there are considerable discrepancies from 

 such an a priori reasoning, as in the case of the steep 

 gradients about Richmond and Saxby Downs, and the patch 

 along the presumed "intake" due east of Muttaburra. 

 Several abnormal tracts are also found in the south, one of 

 steep gradient at Warren, and particularly one of uncom- 

 monly flat gradient (1° F. in 70 feet) due west of Walgett; 

 the way in which they run in belts (extending roughly N. - 

 W.) would be rather puzzling to explain upon the meteoric 

 hypothesis, after comparing the three diagrams and notic- 

 ing the rather regular character of the underground flow 

 hereabouts. 



In seeking an explanation for the steep gradients obser- 

 able, several possible factors can be discarded as ineffective 

 — for example radio-activity — while the existence of reser- 

 voirs of petroleum can hardly be invoked. To some extent 

 the low thermal conductivity of the great cover of over- 

 lying Cretaceous shales, and generally dry sandy drifts 

 might have been responsible, aided by the presence of 

 occasional coal-seams, and by the fact that the water- 

 horizons would possess a higher thermal conductivity, 

 while, had the basin formerly possessed a thick capping of 

 Tertiary beds, a blanketing effect would thereby have been 

 introduced, that, even after subsequent denudation of the 

 region, might not yet have vanished. 



Since the basement rocks are no different to those with- 

 out the basin, and are no more conductive therefore, and 

 since the strata are not situated in a region of extensive 

 crustal folding, we are forced back to the only reasonable 

 and at the same time adequate hypothesis, namely that of 



