J. A. Thomno)i — Rochs of Western Australia. 151 



The granite along its margin is distinctly gneissose, while the result 

 of contact alteration has been to reconstitute the basic rocks into 

 well-foliated schists. Goczel * and many other observers supposed 

 the granite to be a basal Archaean series on which the- younger 

 auriferous series was deposited as lavas and tuffs. Chewings ^ in 1896 

 and Blatchford^ in 1899 clearly recognized the intrusive nature of the 

 granite, and since that time granites intrusive into the auriferous 

 series have been recognized over the whole length and breadth of the 

 State. But at the same time the distinction between these granites 

 and older gneisses has not always been held in view. Each of the 

 main rock-series will now be briefly reviewed. 



Gneisses. — The gneisses have not hitherto received any petro- 

 graphical or chemical study. Their presence is noted in the 

 Northampton Lead and Copper Field by A. Gibb Maitland.^ The rocks 

 are briefly described as "granites, gneisses, mica schists and quartz 

 schists, etc., intersected by veins and masses of pegmatite ". 



"Sheeted zones of micaceous and garnetiferous schist and of 

 garnetiferous gneiss stand out in bold relief with a general north-west 

 and south-east trend." Maitland at one time believed the oldest 

 rocks of the Pilbara District to be " granites and gneisses which form 

 the platform upon which the oldest formations were laid down, and 

 which everywhere underlie the plain extending from Port Hedland 

 to Doolena Gorge on the Shaw Eiver".* In a later publication, 

 however, he concludes that the granite and gneisses are everywhere 

 intrusive into the greenstones and associated beds,^ so that it is 

 doubtful whether any basal gneisses outcrop in the Pilbara District. 



H. P. Woodward has described the occurrence of a series of 

 gneisses, mica-schists, sericite schists, and quartzites in the Menzies 

 Goldfield, but does not indicate very clearly whether he regards these 

 rocks as older than, contemporaneous with, or younger than the 

 amphibolites of the auriferous series.'' 



The Greenbushes Tinfield appears to be the chief mining area 

 wliere pre-granitic gneisses and greenstones exist side by side, but 

 again their relationship to one another is not clearly indicated. 

 Woodward^ describes the 'Crystalline Series' as consisting of 

 gneisses merging in places into dark-coloured mica-schists, and 

 intersected by dykes of albite-pegmatite, and basic rocks consisting 

 of highly weathered hornblende and mica-schists, penetrated by 

 dykes of highly foliated granite which are in turn penetrated by 

 pegmatites. Dykes of broiizite-diabase intersect both gneisses and 

 basic schists, but their relationship to the pegmatites is doubtful. 



Numerous references to gneiss and gneissic granites in the Gascoyne, 

 Ashburton, and West Pilbara Goldfields are made by Maitland,^ but 

 none of these are sufficiently definite for our present purpose. It 



^ G. Goczel, Interim Eeport, Dept. of Mines, W.A., for half-year ending 

 June 30, 1894, pp. 18-23, Perth, 1894. 



^ C. Chewings, Proc. Eoy. Col. Inst., London, vol. sxvii, pp. 263-371, 1896. 



^ Torrington Blatchford, Bull, iii, p. 20, 1899. 



* Bull, ix, p. 9, 1903. 5 Bull, xv, p. 10, 1904. 



« Bull, xxiii, pp. 79, 80, 1906. ^ Bull, xxii, pp. 14-16, 1906. 



^ Bull, xxxii, pp. 26-30, 1908. " Bull, xxxiii, 1909. 



