236 NOTES ON THE GEOLOGY OF THE INTERIOR OF N.S.W. 



pi'esent ones) not only at different levels or on different horizons, 

 but also over separate areas. 



If my explanation be the right one, a glance at Diagram TI., 

 will show what becomes of the floods that " never come down the 

 river," and will show, too, how all the wells on the plains are 

 down to the river drift, otherwise there is no water. On the 

 plains north of Nyngan I have seen the water, after rain, flow 

 from the banks of the creeks towards a depression, a few hundred 

 yards away, and disappear at such a rate as would make the creek 

 " run," yet the latter was perfectly dry. A vast amount of the 

 rainfall is absorbed in this way that would otherwise reach the 

 surface creeks, and so eventually help to swell the river that 

 wovild then be flowing by Bourke. At Mullungudgeree and other 

 places the water rises in the wells on the old river drift. After 

 the reference to the " Macquarie Cataract," further explanation of 

 this is unnecessary. 



I may observe that I have examined a great number of wells ol 

 quite another class from any referred to in this paper. They give 

 good water for a time only, and have been called " surface wells." 

 They are sunk in pans or beds of sand that occupy depressions in 

 the clay. The water is merely surface water absorbed by the sand. 



Giralaynhone Country. — The Cobar Giralambone country consists 

 of inclined silarian slates and schists, with patches of devonian 

 lime and sand-stones. The whole area (100 to 150 Q) miles square) 

 is considerably elevated above the plains. Bare and jagged peaks 

 •stand out over the general level. Nowhere, as far as I have seen, 

 is the bed-rock at any great distance from the surface. The 

 conditions under which we should expect to find under-ground 

 waters here and on the plains, evidently difler as widely as the 

 geology of the two districts. 



Warrego Country. — Of the Cretaceous or Warrego country I 

 cannot at present speak from experience. But it is clear to 

 every geologist, from what is known of Cretaceous rocks, that as 

 regards underground waters we are justified in separating the 

 areas where these rocks occur, from both the Great Tertiary 

 Plains and the Cobar Giralambone country. 



