234 NOTES ON THE GEOLOGY OF THE INTERIOR OF N.S.W., 



I. The Great Tertiary Plains or Merri INFerri Country, 



II. Siluro-Devonian, or Cobar Giralanibone Colmtry. 



III. Cretaceous or Warrego Country. 



The Plains. — The tertiaiy plains extend from the Bogan to the 

 northern boundary of the colony and are represented to a smaller 

 extent towards the south-west. They ai^e one great alluvial deposit 

 which show the vast amount of mateiial removed by denuding 

 influences from the western slopes noi-th of the 33rd parallel. 



The efiicacy of denudation for a work of such magnitude may be 

 doubted ; but as we find the plains at every known depth composed 

 of materials similar in character to deposits now in course of forma- 

 tion, it would be rash to say what might not have been accomplished 

 by erosion in a counti-y which has been above the sea since mesozoic 

 times at the very least. Writing of Le Puy, Dr. Geikie says that 

 his first impression was one of utter bewilderment, and upset all 

 previous estimates of the power of rain and rivers. There is 

 almost no amount of waste and erosion that may not be brought 

 about in time by the influence of frost, springs, rain and rivers. 

 To understand this in our colony, one has only to travel over 

 some of the hundreds of miles of plains in the interior. 



If we imagine the present surface of the interior covered with a 

 deposit of impervious clay, to the depth of 200 feet, the plains would 

 beginabout Wellington insteadof Dubbo, so far would they encroach 

 on the high lands. The present or Bell River would of course flow 

 on to the plains about the same place and cut a bed for itself on 

 the new surface. Now if a well were sunk through this 200 feet 

 of clay a good supply of water would be found in the old river bed, 

 and further, if this well happened to be sunk on that part of the old 

 river which we know as the "Cataract of the Macquarie" the 

 water would rise to the surface we imagine to exist 200 feet above. 

 Diagram II., will make this clear. There is a bar of rock on the 

 river at this i>oint which would efi'ectually stay the flow, while 

 the impervious clay above would retain the water in that direction. 

 The supply must be practically inexhaustible, for at the place 

 where the river comes on to the new surface, there is the old 

 river bed dipping away, and as capable of absorbing water as if 

 the new deposit never covered it. 



