32 



l)y fhe action o£ fresli water. To allow of such an ''accumula- 

 tion the water must have been impounded by that escarpment. 

 The surplus water would escape at the lowest level, and follow 

 the trend of the ground — deflected at intervals by obstructions 

 — and would discharge itself over the littoral escarpment near 

 "Wellington. Thus while the river was reducing its bed to an 

 uniform slope throughout its length from Overland Corner, it 

 would at its effluent end be cutting its way back. Probably, 

 because of the soft nature of the material forming the channel, 

 the rate of excavation by the moving water has been greater 

 than that at the falls ; however, by their united action there 

 has been formed the mighty gorge. As the level of the im- 

 pounded waters fell below the upper level of their sediments, 

 so they would be eroded by the current thus established, and 

 initiate the gorge of the upper section of the river. Pinally, 

 the whole gorge is excavated to its present depth, and a uniform 

 slope of the river bed being formed erosion has ceased. As the 

 volume of water in the bed decreased, so its shallow parts be- 

 came silted up and finally dry, except when floods occurred and 

 deposited fresh sediments upon the flats. 



My theory may seem at first sight so startling as to place it 

 beyond the pale of acceptance, as it involves the existence of 

 a vast lacustrine area and a river of far greater volume than 

 is at present, and this implies a correspondingly increased rain- 

 fall if the drainage area were the same then as now, which may 

 be taken for granted. There are many independent evidences 

 existing that this continent has passed through a period when 

 its rainfall was greatly in excess, and was perhaps augmented 

 by glacial conditions. The storage capacity for water of our 

 lake basins in the dry zone of Central Australia is vastly 

 superior to their present condition. Lake Eyre, for instance, 

 its margin is 40 feet below sea-level, whereas if filled its depth 

 would be increased by not less than a hundred feet, and its 

 area enlarged many times. And yet there is evidence that at 

 one time it was a vast expanse of water, and the vegetation in 

 its vicinity capable to sustain life in such huge creatures as the 

 Diprotodons, whose remains are scattered widely in and around 

 its basin ; a country in its present state abandoned, even by 

 the kangaroo. As I have elsewhere* pointed out, the existence 

 of those gigantic herbivorous marsupials in such localities de- 

 mands climatic conditions favourable to the growth of a vege- 

 tation capable of supporting them, and that their extinction is 

 attributable to those climatic changes which brought about 

 dessication, involving a reduction in volume of the waters of 

 inland lakes, and finally converting their basins into salt-pans. 



* Trans. Boy. Soc, S. Aust., vol. ii., p. IxYii., 1879. 



