ROCK SPECIMENS FROM CENTRAL AND WESTERN AUSTRALIA. 295 



crystal outlines, but form a polygonal mosaic of uneven 

 grain. No evidence of cataclastic structure is seen, but 

 strain shadows are not rare. The hypersthene is a little 

 schillerised, strongly pleochroic variety with rose-red to 

 green tones, and is optically negative with a fairly high 

 optic axial angle. It occurs in irregular layers which have 

 a rude parallel arrangement, but within the layer the 

 mineral is not definitely oriented. It has sometimes con- 

 siderable tendency to idiomorphism, but when surrounded 

 by felspars occurs in more rounded forms. Closely associ- 

 ated with, and often penetrating, the hypersthene is a 

 considerable amount of reddish-yellow to black biotite. 

 In Helm's specimen, but not in Gibson's, there is a little 

 common green hornblende intergrown with the hypersthene. 

 Apatite is fairly abundant in stout prisms with a general 

 elongation in the direction of the banding. Pig. 1, Plate 

 XIV, gives an adequate idea of the relative proportions of 

 the different minerals. 



While it is not impossible that the rock belongs to the 

 gneissic series, its structure and mineralogical composition 

 suggest, as more probable, that it is of directly igneous 

 origin, as Stelzner supposed, and is a norite with feeble 

 protoclastic structure and well marked fluxion banding. 

 The presence of hypersthenic dyke rocks at Norseman 1 

 makes the presence of norite dykes in the Praser Range 

 quite probable. 



The northward extension of this gneiss belt has not yet 

 been delimited by the Western Australian Geological 

 Survey. Apparently the western margin turns north 

 towards Burtville, where the gneiss is found a few miles 

 east of the town. 22 The eastern boundary is unknown in 

 the northern part. 



1 Campbell, W. D., The Geology and Mineral Resources of the Norse- 

 man District, Dundas Goldfields, Bull. 21, Geol. Surv. W.A., 1906, p. 24. 



2 Gibson, C. G., The Laverton, Burtville and Erlistown Auriferous 

 Belt, Mount Margaret Goldfield, Bull. 24, Geol. Surv.W.A., 1906, pp. 29, 30. 



