256 H. I JENSEN. 



nitude. The hookshaped bend of the Nepean before it joins 

 the Warragamba points to origin by river capture. The 

 nature of the Glenbrook gravels and Mulgoa Heights gravels 

 at the Basin point to derivation from the granite areas at 

 the head of Oox's River. The gravels along Mulgoa Greek 

 and on the heights east of Penrith point to derivation from 

 the Hawkesbury and Permo-Oarboniferous sandstones to 

 the south in the drainage area of the present Nepean. The 

 gravels of the St. Mary's district are also typical of the 

 Nepean drainage area. 



It therefore follows that in late Tertiary times the 

 Nepean and Warragamba Rivers effected a junction some- 

 where to the north of the present town of Penrith, and the 

 capture of the Nepean by the Warragamba was in all 

 probability brought about by the igneous uplift occasioned 

 between Penrith and Mulgoa by intrusions of basalt and 

 diabase which manifest themselves on the surface in the 

 form of dykes. 



The shifting of the river to the north of Penrith from its 

 old course between Penrith and Windsor to its present 

 course is due to a distinct movement of depression in the 

 area around Richmond. In the absence of igneous intrus- 

 ions in this area, the depression of the eastern limits of the 

 Lapstone monocline was very much accentuated here, a 

 fact which is also established by the north-westerly dip of 

 the Wianamatta shales in those parts of the parishes of 

 Londonderry, Oastlereagh, Ham Common and Windsor 

 which adjoin the old river channel. Whatever may be the 

 case with the Sydney district as a whole, and the writer 

 is in agreement with Andrews in regarding it as akin to a 

 senkungsfeld, there can be no doubt that the Richmond 

 district is a subsidence area. 



