174 president's address. — section c. 



The dry beds of such watercourses can be traced for hundreds of miles, 

 and it is only on the assumption that such a continental drainage 

 existed in Central Australia that we can explain its sub-fossil flora and 

 fauna. The persistence of a flora which is not characteristically desert 

 in its character, and that still holds its ground along the courses of the 

 present flood rivers of Central Australia, may be regarded as survivals 

 from a time when the coiuitry was more abundantly clothed with vege- 

 tation than it is at present. 



The present arid condition of Central Australia has had a gradual 

 evolution, and is the result of various causes that have operated to pro- 

 duce cumulative eSects. Of these the following may be suggested : — 



(1) The development of an extensive senkungsfeld in south- 

 central Australia, which, by lowering the altitude of the land, had the 

 effect of raising the mean annual temperature. 



(2) A lessening of rainfall consequent on the changed meteorologi- 

 cal conditions. With a more heated interior the cyclonic storms of 

 the southern coast would draw in the warm and dry currents from the 

 north, Avhich, combined with the low altitude of the land, would have 

 the effect of absorbing the moisture of the advancing storm rather than 

 of condensing it. This would prevent rain falling on the advancing side 

 of a barometric depression, and therefore limit the precipitation of rain 

 to the departing quadrant of the atmospheric disturbance, when the 

 cooler air drawn in from the south-west would promote the conden- 

 sation of the aqueous vapour. These are the typical conditions of the 

 rainfall of South Australia at the present time, and restricts the southern 

 rains chiefly to the coastal regions. 



(3) By the development of corrugations of the surface in an east- 

 and-west direction — transverse to the original drainage — the river 

 systems were dismembered, and thereby the natural irrigation of the 

 country was largely destroyed. 



(4) The slow movement, leading to the reversal of the drainage, 

 produced over a long period a gradual reduction of grade and a process 

 of base-levelling and aggradation of load that choked the river valleys 

 and rendered them increasingly absorbent. 



(5) As the result of these altered conditions the surface waters were 

 correspondingly lessened — reduced in volume and intermittent in their 

 flow. 



(6) When the warp of the region was sufficiently advanced to create 

 a lip-like watershed, the rivers — or what was left of them — became 

 obsequent and flowed inwards. But, with an inland drainage, the waste 

 could not be got rid of, and, as a consequence, the surface of the land 

 became covered with sand and the waters more or less saline. 



(7) The in'^reasing desiccation of the land, as well as the salinity 

 of the waters, proved unfavorable to plant life — bare ground took the 

 pla'^e of vegetation, which still further intensified the aridity. 



