by e. c. andrews. 791 



Thesis. 



Several peneplains* were developed at various times at sea-level, 

 and elevated successively to varying heights. The elevations were 

 of the nature of broad vertical uplifts for the centre of the dis- 

 turbed area, with gradual slopes east and west of the high central 

 plains, the axis of uplift being meridionally disposed. The 

 imperfect reduction of the old plateaus in their central portions 

 permits of the deciphering of the relative amounts of erosion 

 performed during the successive cj^cles of land-degradation, and 

 thus affords physipgraphical criteria of the relative duration in 

 time of such geological divisions as the Cretaceous and Tertiary 

 periods. 



A study of the Hawkesbury River shows that progressive 

 stream to be the outcome of several periods of stream-revival," 

 during which its original and unimportant consequent direction 

 was succeeded by its present pronounced lengthwise course, as it 

 adjusted itself wonderfully to the surrounding rock structures. 

 The present canons of the Lower Hawkesbury point to a rejuven- 

 escence of the Pliocene stream, the river incising its way through 

 the differential uplifts along its old course. 



The youth of the latest uplift of importance is evidenced by 

 the appearance of the present canons. The influence of differential 

 erosion on the rock structures is also well seen. An oscillation 

 of subsidence in Pleistocene time is indicated by the conversion 

 of the Pliocene (?) canons along their lower courses into magnificent 

 harbours, while subsequent stream-action and a very recent 

 vibration of elevation accentuate such features as coastal plain- 

 formations. 



The commercial significance of the recent drowning and the 

 piracy of the Hawkesbury by beheading of streams, with conse- 

 quent diminishing of river competence to silt up the harbours, is 

 well seen by a study of the coast. 



* One of these may hereafter be referable to "benching" in horizontally 

 bedded strata. 



