120 G. D. OSBORNE. 



deposits of river gravel occur. Here it might be noted 

 that the deposits on the southern side of the Basin extend 

 some distance to the south. (On the map these gravels 

 are only indicated, with no suggestion of extent.) Further 

 other river gravels occur near the Wallacia Bridge, quite 

 close to the Nepean, but comprising rocks which are not 

 found in the present drainage area of this river. 



The curious bends taken by the Nepean River from the 

 Wallacia Bridge to its confluence with the Warragamba 

 are suggestive of river capture and the present junction of 

 the two rivers is within the Basin neck. These facts make 

 it clear that the Basin neck, composed of comparatively 

 soft material, has, on account of its location, formed an 

 important unit in the latest stages of the evolution of the 

 Nepean- Warragamba river systems during the Oainozoic 

 Era. 



Part II. Petrology. 



1. The Breccia. — The agglomerate has a fairly uniform 

 grainsize of about *3 mm. Dark in colour, it weathers to 

 a greenish-grey. Under the microscope it is seen to consist 

 of quartz and acid plagioclase grains, and fragments of 

 sandstone and chert. These constituents are cemented 

 together by a base in which chloritic and kaolinic material 

 preponderate. Oalcite as small veins and irregular 

 secondary masses also occurs throughout the rock. Only 

 one small fragment of basaltic nature was noted in the 

 slides of the breccia. This was similar to the "trachytic" 

 basalt of Hornsby. 



2. Inclusions ivithin the Breccia, (a) Calcareous Clastic 

 Rocks. — The occurrence of these was mentioned in Part I. 

 In some parts of the neck they are numerous. Microscopic 

 investigation reveals a suite of rocks with variable grain- 

 size, the chief constituents being fragments of lava, chert, 

 grains of quartz, plagioclase, chlorite and an indeterminate 



