The South Australian Naturalist. 



43 



kiioAvn as "peneplain." The word means, of eourse, almost a 

 l)iain. This refers to the fact that such river-worn surfaces are 

 never so thoroughly level as Nature's "built-up" plains, such 

 as the mallee plains of A'ictoria and South Australia, the Dar- 

 ling Downs, the basalt plains of Western Victoria, or the lime- 

 stone plains of the Nullarbor. A peneplain, however far ad- 

 vanced towards perfect levelling, always retains indications of 

 its origin in broad valleys and low intervening rises. To return 

 to our story. AVe had pictured the hard, ancient rocks of the 

 Darling and Mount Loft}" Ranges thus worn down to a pene- 



Figuie 1. — Diagiaiu to show the lelatiouship between the ancient 

 rocks (Cambrian) of the Adelaide hills and the more recent (Tertiary) 

 rocks of the plains. The ancient rocks have been broken into 

 a series of huge blocks, that are in part sunken, step-like, towards the 

 west. The hidden blocks are covered by the limestones, sands, and 

 gravels that form the plains. These Ijeds are in the west covered by the 

 w-aters of Gulf St. Vincent. 



plain surface. Really, the rocks of the Darling Ranges (W.A.) 

 are much older than those of Mount Lofty (S.A.), although the 

 latter belong to the oldest fossil-bearing series in the world. 

 Indeed, a distinguished student of the puzzling rock complex of 

 the Darling Ranges once expressed to me his belief that those 

 rocks were portion of the original "crust of the earth." How- 

 ever that may be, both series count their ages in millions of 

 years, and both were subjected to many changes of structure 

 and elevation during the long period prior to the time when 

 they became penei)lain surfaces. At a time when the present 

 structure and outline of Australia Avere being largely deter- 

 mined, these peneplain surfaces were broken up by a series of 

 huge earth-fractures running north and south. Great crustal 

 movements took place, leading to the slow elevation of some of 

 the blocks, forming plateaus. At the same time other blocks 

 sank, down, stepwise, towards the west. 



