214 ON THE GEOLOGY AND PETROGRAPHY OF BATHURST, N.S.W., 



specimen submitted for analysis was fairly typical of the general 

 character of the rock : — 



Hornblende-biotite-granite. 



Silica 66-69 



Alumina 17*03 



Ferric oxide 3- 15 



Ferrous ditto *69 



Manganous ditto trace 



Lime (Ca O) 1'82 



Magnesia (Mg O) 2-50 



Potash (K 2 O) 6-26 



Soda (Na 2 0) 1-21 



Phosphoric acid trace 



Sulphuric anhydride trace 



Titanic acid trace 



Moisture -48 



99-83 

 Comparing the above with well-known granites, it will be seen 

 that the Bathurst rock contains about 10 per cent, less silica 

 than the normal type of West of England granite, while it is 

 richer than the average granite in alumina and potash. 



Basalt. 



The basalts have been defined as dark-coloured lavas of basic 

 composition and high specific gravity, representing the extrusive 

 or volcanic type of the gabbros and dolerites. Dr. Geikie limits 

 the term basalt to the contemporaneous lavas of basic composition.* 

 They consist of a compact or finely granular ground-mass, through 

 which crystals of plagioclase, augite and olivine are scattered. 

 Again, some authors use the terms dolerite, anamesite and basaltf 

 for rocks which, chemically identical and all holo-crystalline, differ 



* British Petrography, by J. J. Harris Teall, M.A. ; London, 1888, p. 193. 

 t Professor J. G. Bonney — Anniversary Address to the Geological Society, 

 London ; Quarterly Journal Geological Society, Vol. xli. p. 70. 



