president's address. — SECTION c. 173 



that appealed to Tate, in evidence of more humid conditions in South 

 Australia during Pleistocene times, are well founded and require 

 explanation. 



We think that the required explanation is ready to our hand 

 in the physiographical changes that have transpired in South Australia 

 during the later geologic periods. The cooler conditions of climate, 

 with its abundant rainfall and permanent rivers and prolific life, 

 must have antedated the development of the great central basin of 

 Lake Eyre. The features pertaining to the dead river systems of South 

 Australia suggest the former existence of a long slope, of moderate 

 grade, extending from the interior to the south coast, with its main 

 watershed verj^ far north^probably as far as the Musgrave and the 

 MacDonnell Eanges. This would be the period when the plateau of 

 the Desert Sandstone was subjected to extensive erosion, and from 

 being a level plain was converted into mesas and buttes. 



In the earth movements that brought about so great a change 

 in the physical conditions of the country the positive and negative 

 elements of such movements must have been, to some extent, corre- 

 lated. The sagging of the earth's crust at Lake Eyre found its com- 

 pensation in the uplifts of the marginal areas. Thus the Flinders 

 and Mount Lofty Ranges were elevated by isostatic compensations 

 around the southern rim of the basin ; while the MacDonnell and other 

 ranges had a similar modern uplift along the northern Hmits of the 

 great subsidence. 



One thing is certain, the MacDonnell Ranges show the same 

 juvenility of structure as is apparent in the Mount Lofty Ranges. 

 The MacDonnells, on their southern side, are truncated by nearly 

 vertical fault-scarps, 1,000 feet in height * ; the rivers force their way 

 through the ranges, in deep gorges, at right angles to the trend of the 

 mountain systems, and escape by " gaps " which are almost unique 

 in their physiographical features. The Finke River, which is the 

 principal line of drainage through the ranges, after penetrating the 

 MacDonnell, the Krichauff, and the James Ranges, dies away in its 

 sandy bed long before it reaches its southern outlet, which originally, 

 it is presumed, passed through Lake Eyre. 



It may be regarded as an open question whether the rejuvenation 

 of the drainage of the MacDonnell Ranges was caused by a local uplift 

 or by the sinking of the surrounding country. The general features 

 of the ranges, and especially the great fault scarps facing south would 

 suggest the subsidence theory as the m^ore probable. 



The point of interest, so far as the present discussion is concerned, 

 lies in the fact that we cannot find an explanation for the remains of 

 the numerous dead rivers, having a north and south direction, without 

 predicating a drainage that flowed south from a vast central watershed. 



* H. Y. L. Brown, " Journey from Adelaide to Hale River." By authority, 1889, p. 6. 



