178 president's address. — section c. 



climatic changes, in geologically recent times, that have so 

 profoundly influenced the destinies of the country. At 

 the same time it explains the remarkable absence of 

 rivers from the southern seaboard. 



We have in the later geological history of South 

 Australia an illustration of the momentous consequences 

 that may turn on what might appear as an unimportant 

 physiographical movement. A simple tilt of the earths^ 

 surface — a sag to the north and a ridge to the south — 

 had the effect of obliterating a bounteous hydrographic 

 system that had taken a full geographical cycle to bring 

 to perfection. Under such circumstances — contrasting 

 the past with the present — South Australians may be 

 excused if they indulge in regretful reflections on the 

 " might-have-beens " in the geological history of their 

 country. 



DESCRIPTIONS OF PLATES. 



Plate III. 



Map showing the broad watershed which divides the inland drainage from 

 the coastal drainage in South Australia. The true line of water parting is 

 indefinite, as some streams, near the summit, flow either one way or the other, 

 according to circumstances. Note the broken and fragmentary drainage, 

 especially on the eastern side, which, hydrographically, belongs to the Murray 

 basin. 



Plate IV. 



Sketch-sections of the step-faulting of the western side of the Mount Lofty 

 Ranges. The old peneplain of the ranges has been block-faulted. Fotir seg- 

 ments have slipped down to\\ ards the rif^-valley of Gulf St. Vincent. Thi« move- 

 ment has destroyed the north and south drainage of th.e country and compelled 

 the rivers to take a transverse, or east and « est. course. 



