430 E. C. ANDREWS. 



bases are now filled with alluvium. The Darling system 

 flows over this almost iVat un-I.-ss surface, and 1,000 miles 

 from the sea is only from 300 to 400 feet above sea level. 

 The plateau is composed of Pre-Palseozoic, Palaeozoic, 

 Jurassic, Lower and Upper Cretaceous rocks indifferently. 

 The area between the Murray and Murrumbidgee Rivers 

 is composed of low lying black soil plains named Riverina. 

 Immense areas of "black soil " plain also extend along the 

 branches of the Darling River system, and partly drown 

 the plateau just described. The rainfall over this area 

 varies from 20 to 30 inches on the eastern margin of the 

 interior to less than 10 inches on the western boundary of 

 New South Wales. Many of the large streams which rise 

 in the Great Divide fail either to reach the Darling or the 

 sea, but die away in marshes, in salt lakes, or "clay pans." 

 A characteristic of the large streams as they traverse the 

 "black soil" is the wealth of ana-branches and of lagoons 

 or billabongs possessed by them. It may here be pointed 

 out that in later Tertiary time the land of the interior was 

 moderately raised and that the streams such as the Darling 

 and Murray occupy broad valleys in its surface. Sub- 

 sidence and alluviating processes followed, while at present 

 the streams are excavating and destroying the plains in 

 turn, because material is not now being carried down from 

 the highlands in such quantities as in recent geological 



(b) The Highlands ami the Great Divide— The Main 

 Divide of Eastern Australia lies in the Highland region, 

 but the latter is not confined to the Main Divide. 1 As will 

 be shown later, the Highlands of Eastern Australia consist 

 of a peneplain which was raised, warped and faulted at the 

 close of the Tertiary Period and the Main Divide is therefore 

 of extreme youth. Frequently the highest portions of the 



1 See also Gregory, J. W., (a) pp. 93, 91. 



