BY E. C. ANDREWS. (S05 



of basalt poured thence out over the plains,"^'* whereas the great 

 lava plains in New England are suggestive of dyke-aetion. The 

 Hawkesbury all this time had been developing itself at the 

 expense of its neighbours. Originally it was an insignificant 

 stream with a course almost due east to the sea. Thence, how- 

 ever, during the cutting down of the hills it discovered the 

 slightly upturned edges of softer layers of the sandstone cap — 

 places, for instance, in which whole areas of shale existed with 

 sandstone, like that belt of country lying between Penrith and 

 Pic ton. 



Straightway on the discovery it developed a lengthwise course 

 therein, and, pushing its way south, it encountered other conse- 

 uent or east and west streams. These beintc unable to cut their 



way into the surface as quickly as the Haw^kesbury, owing to 

 lack of w^ater and load, had their head waters captured by its 

 lengthw^ise course. 



If observations be confined to the eastern portions of the 

 elevated area, localities such as Berowra, Waterfall, Helensburgh, 

 Loddon or Moss Vale, the Lithgow Plain appears to be as com- 

 pletely developed as the older peneplains. Studies conducted in 

 the central areas, howcAer, show that the age of the Lithgow 

 cycle is insignificant compared with the older cycles. Final 

 reduction of the central plateau is the criterion of excessive 

 old age of a cycle of erosion. The Jenolan Plain satisfies this 

 test of senilit}'', and the discrepancy of age between its stage of 

 development and the recent plateau is very pronounced. 



The Canon Cycle (Tertiary). 



This period was initiated by pronounced and long continued 

 uplift. This was the great elevation in late geological time 

 (PL xxxix., fig.y )• The Lithgow Plain was forced upward until it 

 reached a maximum height of 3, 100 feet above the sea in its central 

 portions. The movement was not rapid, but so gradual as to 

 probably occupy very many thousands of years in its completion 

 and not to materially alter the Tertiary (?) river system. 



* J. E. Carne, in I'M. 



