416 GEOLOGY AXD PETROLOGY OF THE GREAT SERPEXTIXE BELT OF X.S.W., 



tield-examination of the relations of the glassy and stony types lias not been made, 

 but tbey are known to be at times associated in the same sill. A similar close 

 association has been obser\ed in the Hunter ^'alley area, and it is ijossible that 

 the question of the origin of the stony gToundmass will eventually be solved in 

 the field rather than in the laboratory. It is to be noted, however, that in none 

 of the slides examined does the stony type contain anj' traces of glassy base, or 

 vice versa, although in the hornblende andesite the association of glassy and crys- 

 talline patches appeal's to be the rule. 



Lithoidal Type. The rocks belonging to this type exhibit a hard stony base 

 of a purplish-grey colour, with very numerous and conspicuous tabular felspar 

 l^henoerysts up to 4 mm. in length and smaller and subordinate pyroxene pheno- 

 crysts, without any definite arrangement. The rock weathers to a very charac- 

 teristic, soft reddish crust which is pitted by the alteration and removal of the 

 felspars . 



Microscopically, in specimen 1444, from near the summit of Duri Peak, the 

 felspar phenoerysts prove to be basic labradorite, zoned, but not conspicuously so, 

 and showing well-marked albite and occasionally carlsbad and i^erieline twinning. 

 The crystals are somewhat cracked and altered, sometimes zonallj', and are often 

 crowded with tiny pyroxene and magnetite inclusions, with a few apatite needles. 

 The pyroxene phenoerysts comprise both augite and hypei-sthene, the former being 

 tlie more abundant ; both occur in well-formed stout prisms, the augite being occa- 

 sionally twinned. Hypersthene is notably pleochroic, and shows a disposition 

 to alteration along the periphery. Ilmenite is fairly plentiful. The gi-oundmass 

 at first sight appears cryptocrystalline, but really consists of an ill-defined mosaic 

 of spongy-looking, mitwinned felspar, of indeterminate composition, but with a 

 refractive index apparently higher than that of Canada Balsam. This groundmass 

 is plentifully dotted with iron ore and less almndantly with pyroxene granules, 

 and is microporph;iTitic in plagioclase. 



Specimen 1503, from the Summit of Duri Peak, is exactly similar to the 

 foregoing except that the rock has been more weathered, while in 1454, from tlie 

 lower portion of ISTorth Cobla, the only differences are that the proportions of the 

 pyroxenes are approximately e(|ual and Miat the groundmass is liut sparsely micro- 

 porphyritic. 



Vitropht/ric Type. When fresh the rock is a brownish-black colour, very 

 compact, with a resinous or pitchy lustre, and porpliyi-itic in felspar. Some- 

 times the phenocrysfs show marked flow structure, but mostly this is absent. In 

 oonnnon with ilie lithoidal type, tlie pitchstones have tY))ically a i-atlier smooth 

 fracture. 



For the microscopic features we may take as a type specimen 1531. from the 

 west face of Duri Peak (PI. xxv.. fig. 6) . The porphyritic felspar is basio labra- 

 dorite (Ab35 An.'.o) or even ftcid bytownite; it is tabular prismatic in habit and 

 may be up to 2.5 mm. in length. The crystals are much fresher than in any 

 of the lithoidal andesites examined, but are much corroded and cracked. Twin- 

 ning on albite and carlsbad laws is present, and zoning is very marked in some 

 crj-stals. There are very abundant inclusions of pyroxene and to a less extent 

 of magnetite gri-anules, as well as of the groundmass, the latter sometimes having 

 a schiller-like arrangement, while the mineral grains may be disposed in a rude 

 zonal fashion. Hypersthene and augite are fairly abundant, tlie former perhaps 

 Ijeing in excess, in much cracked prisms up to about 1.5 mm. long. Occasional 



