348 C. A. SUSSMILCH. 



Age of the Peneplains. — Beyond the occurrence of 

 Tertiary fossil plants in the alluvium under the basalt flows 

 on the surface of the Monaro peneplain, such as at Dalton 

 near Gunning, no evidence has been obtained which supplies 

 any information as to the geological age of these peneplains. 

 Nor is it at all likely under the circumstances that any 

 satisfactory evidence will be forthcoming until the pene- 

 plains in the marine tertiaries of Victoria, if such exist, 

 are studied and correlated with those under discussion. As 

 the river valleys which have been cut into the tablelands 

 since the uplift of the youngest peneplain, are still for the 

 most part mere gorges, it seems improbable that the uplift 

 can have taken place earlier than the Middle Tertiary. 



Use of the Term Peneplain. — It would, perhaps, be 

 desirable to define the sense in which the term peneplain 

 is here used, as the usage seems to vary with different 

 writers. When a tableland is uplifted above sea level, a 

 cycle of erosion is initiated, which, if continued sufficiently 

 long without interruption, may result in the entire removal 

 of that tableland, with the production of what is essentially 

 a plain at or near sea-level. As such a surface is hardly 

 likely to ever be perfectly level it has been called a pene- 

 plain. 



Complete planation, however, seldom occurs during a 

 single cycle of erosion, residuals of the tableland still 

 remaining when the cycle is interrupted by some new earth 

 movement. To what extent may residuals remain and the 

 feature be still called a peneplain ? If the cycle of erosion 

 has been long continued, very extensive areas may have 

 suffered complete pene-planation, yet in other parts, particu- 

 larly in the neighbourhood of the main divides, numerous 

 residuals, the tops of many of which are portions of the 

 original tableland surface, and even considerable flat- topped 

 areas of the original tableland may remain ; so that, what 



