530 THE GEOLOGY OF NEWBRIDGE. NEAR BATHURST, N.S.W., 



Microscopical. — Fabric inicroporphyritic. Ground-mass: quartz 

 and orthoclase in a fine granular base, with no sign of grano- 

 phyric intergrowth. Phenocrysts: predominant oligoclase, almost 

 idiomorphic and slightly decomposed ; untwinned orthoclase 

 crystals; quartz with shadowy extinction, and strongly pleochroic 

 biotite occur, the last changing into chlorite, JMuscovite is 

 present, but appears to be due merely to the decomposition of 

 the felspars. 



(b) Granophyric Veins. — There are several rocks present in this 

 district w^hich may be classed as granophyres. One of them 

 immediately adjoins the granite-porphyry last described. It also 

 has been traced as far as possible; it crosses the railway about 

 100 yards east of the 16-if-mile peg, and is about two yards 

 thick. It bends round to the N.W., and runs about a quarter 

 of a mile before it is lost to sight. 



Macroscopically it is a fine, even-grained, (0-5 to 1 mm.) hard, 

 bluish rock in which only quartz and felspar are recognisable. 



Microscopically it is seen to be composed of plagioclase, ortho- 

 clase, and subordinate quartz. Biotite is present in small 

 greenish flakes, and muscovite also, the latter being derived 

 from the decomposition o\ the felspar. The plagioclase is pre- 

 dominant in fairly idiomorphic crystals, probably oligoclase, 

 though the twinning is not very distinct. The orthoclase is 

 idiomorphic, and generally twinned. The quartz is usually very 

 irregular in shape, but in one or two instances shows an hexagonal 

 section, with included biotite fibres placed parallel to the prism 

 faces. The ground-mass is almost entirely quartz and orthoclase 

 in a very fine microgranophyric intergrowth generally arranged 

 radiating from some crystal, not a mica, as centre; that is, a 

 pseudospherulitic structure. Apatite is an accessory mineral, 

 and there is a little magnetite. 



In another specimen from the same vein, however, the grano- 

 phyric structure is almost entirely absent, its place being taken 

 by a fine-grained mixture of quartz and orthoclase, like the 

 " panidiomorphic " structure of aplites. Also in this specimen 

 the twinning of the plagioclase phenocrysts is more distinct, 



