262 PETROLOGICAL NOTES, ii., 



alluvials, about a mile north of Roelands. The matrix is a fine- 

 grained, reddish-brown grit, and contains well rounded pebbles 

 and boulders of pegmatite, aplite, light quartzite, dark sandstone, 

 and decomposed, basic, igneous rocks, from |" to 2' in diameter. 



The granodiorite, gneiss, basic dykes, and pegmatites form all 

 the high land east of the railway, except where capped by laterite. 

 They are intimately associated in the field. 



The basic dykes intrude the acid rocks, forming a plexus, with 

 only occasional approaches to parallelism. They show no distinct 

 trend, and vary in width from mere veins to 50 feet and more. 

 They are coarsely holocrystalline, variable in grainsize and 

 basicity, and appear in most cases to be amphibolitic. They are 

 often gneissic, sometimes passing from a granitoid texture in 

 the middle, through a gneissic or schistose phase, to biotite- 

 schist on the edges. They are generally associated with gneiss, 

 rather than granodiorite. 



The Granodiorite-Porphyry forms a number of sporadic out- 

 crops, some of which are of large dimensions, and are fairly free 

 from basic dykes. It is well developed just north of the Collie 

 River, half-way between Shenton Elbow and the railway, also 

 around Olive Hill Siding, and between Brunswick Junction and 

 the Roelands quarry. It is a coarse, granitic rock, with very 

 numerous felspar-phenocrysts, up to 2" long, distributed entirely 

 without arrangement in the most characteristic outcrops, where 

 it weathers into small, rough tors. In the neighboui'hood of the 

 gneiss, it is rudely fluidal. 



The Gneiss is most varied in character, and outcrops as irregu- 

 larly as the granodiorite, but shows no definite boundarv-rela- 

 tions to it. It is very well exposed along the Collie Tramway. 

 There is not the slightest evidence that the granodiorite intruded 

 it. All varieties, from fine, even-grained gneiss to coarse augen- 

 gneiss occur, some of the latter bearing a suspicious resemblance 

 to the fluidal modification of the granodiorite. The foliation 

 follows no constant direction; on the other hand, it is best 

 developed in the neighbourhood of basic dykes, and runs parallel 

 to them. The more intense the foliation in the basic dykes, the 

 more perfect are the neighbouring gneisses. In places, it is much 



