. E..F. PITTMAN AND-T. W. E. DAVID. CVII. 
of these plains‘is in a westerly to north-westerly direction 
at an average rate of about 2 feet per mile. With there- 
fore this very small amount of fall in their channels the 
tributaries of the Darling in New South Wales where they 
cross the western plains are little more than canals in good 
seasons and a chain of waterholes in dry seasons. In times 
of flood, however, these rivers overflow their banks and 
inundate the surrounding plains for a great many miles. 
Il. WATER SuPPLY.—The water supply of the areas of the 
western foothills of the Dividing Range and of the western 
plains is to be obtained from (1) rivers, (2) dams, (3) wells, 
and (4) artesian bbres. As the subject of the use for irri- 
gation of the river water and that of the dams is being 
treated of by other authors in this series of papers on irri- 
gation, we will restrict our remarks to the occurrence of 
water in wells and bores. 
(3) Wells.—Well-water has been obtained on the Western 
Plains chiefly from two classes of rock (a) Palaeozoic rocks 
such as granites etc., and (b) rocks of much later age refer- 
able to various epochs in the Cainozoic Era, and largely of 
late Cainozoic Age. In the case of (a) it is only when there 
has been extensive decomposition of the rock to some depth 
that the conditions are favourable for the percolation and 
storage of rain water. The downward limit of the zone of 
saturation is in this case the upper surface of the undecom- 
posed rock. A good example of well-water obtained from 
such a source is the “Medway Well,’’ 5 miles S.W. of 
Byrock, sunk to a depth of 109 feet in decomposed granite. 
This well, even in dry seasons, yields about 5,000 gallons of 
water per diem, and the water rises to within 40 feet of 
the surface.’ As regards (b), well-water has been obtained 
in deposits of late Cainozoic age in various parts of the 
Riverina in the plains bordering the Darling River and its 
+ Ann. Rep. Mines Depart., N. 8. Wales, 1885, p. 138. 
