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PETROLOGICAL NOTES ON VARIOUS NEW SOUTH 

 WALES ROCKS. 



By W. N. Benson, B.A., B.Sc, F.G.S., Linnean Macleay 

 Fellow of the Society in Geology. 



i.Rocks of Nullum Mountain, near Murwillumhah. 

 Some years ago, the writer spent a day at Murwillumbah, and 

 examined Nullum Mountain, which lies five miles to the south- 

 west of the town. Nothing yet has appeared concerning- the geology 

 of this area, so that a few notes may be given here to call attention 

 to the spot. The mountain forms a short ridge, standing promi- 

 nently in front of the range. Its relief is due to the presence of an 

 inclined sheet of granophyre, which dips towards the north. The 

 main mass of the mountain is composed of gnarled slaty rocks of 

 the Brisbane Schists, of Lower Palaeozoic or even earlier age. The 

 inclined sheet outcrops on the southern side of the ridge, and is 

 exposed on the northern slopes of the mountain for some distance 

 down its face. At the base, various dykes have been noted. The 

 sill consists chiefly of granophyre composed of small crystals of 

 orthoclase and acid plagioclase, partly allotriomorphic, partly idio- 

 morphic, surrounded by a granophyric intergrowth of quartz and 

 orthoclase. A little biotite occurs and magnetite, but the bulk of 

 the ferromagnesian constituents are altered into regular patches 

 and spherulites of chlorite, and grains of epidote. A few apatite 

 crystals are also present. 



The rock of the upper surface of the sheet exposed on the north 

 slope shows frequently no granophyric structure, but has a traehy- 

 tic habit. It consists of a pilotaxitie felt of felspar-laths, both 

 orthoclase and acid plagioclase, and frequently an untwinned fels- 

 par of the same refractive index as Canada balsam, possibly anor- 

 thoclase, together with a fair amount of interstitial quartz. The 



