152 E. G. ANDREWS. 



be somewhat as follows: — The Hunter River structures, 

 although Palaeozoic, are relatively weak, while the Macleay 

 gorges were held up for a long period in the acid Oarrai 

 granites; once the Macleay had receded westward of these 

 powerful structures, it advanced rapidly through the slates, 

 until in turn it encountered the granites in the central 

 plateau. In all cases the acid granites are situated in those 

 localities where they are best protected from erosion, and 

 this not by accident, but by a beautiful selective action 

 exercised during several cycles of erosion. Thus, when the 

 granites were first exposed, the streams avoided them as 

 much as possible, because of their strength and insolubility, 

 leaving them in the main inter-stream areas as the cycles 

 progressed. Gradually they thus became localised as head- 

 waters. Each successive slight uplift merely accentuated 

 the process; until at present they are practically indestruc- 

 tible. The strike of the New England structures, moreover, 

 is characteristically meridional in disposition, and the 

 granites appear to have followed main meridional lines of 

 weakness in the earth's crust. This has had a peculiar 

 reaction on the course of the Eastern Australian streams. 

 To take a single example: — The Clarence River in its early 

 history made long subsequent courses so as to avoid attack- 

 ing the north and south acid granites lying farther west. 

 As the successive cycles of erosion resulted only in incom- 

 plete reductions to peneplains, so these subsequent stream 

 forms became accentuated. Upon the great Kosciusko 

 Uplift the streams were rejuvenated and the canons still 

 receded along the edges of the acid granites. In this way 

 has also arisen the widely varying topography of the same 

 stream along its lower course and along the great granite 

 ramps farther west. 



Similar subsequent courses may be studied along the 

 Myall, Karuah, Paterson and Williams Rivers. Here, 

 however, rhyolites take the place of granites. It is probable 



