414 GEOLOGY AXD PETROLOGY OP THE GREAT SERPENTINE BELT OF X.S.W., 



Quartz Trachyte. 



The only potash trachyte is 1500 from the S.E. coiner of Portion lUT. It 

 is very sparingly micro-porphyritic in orthoclase, which also predominates in the 

 fine-grained gToundmass. The felspar is in ill-formed laths, much spangled with 

 alteration-products, and is accompanied by a little acid oligoclase. Quartz forms 

 an imperfect micro-poikilitic setting to the felspars and ii-on ore is fairly abundant 

 with a little apatite. A small amount of chlorite probably represents original 

 biotite. 



Quartz Latite. 



Only one specimen (1495) of this rock type has been en'countered, occurring 

 as a dyke near the centre of Portion 273, Currabubiila. It is distinguished from 

 the other felsitic rocks, from which it does not differ noticeably in appearance, by 

 the presence of a more basic plagioclase . In thin section the rock is slightly 

 porphyritic, the phenocrysts being felspar, probably andesine; a few crystals with 

 only simple twinning or with straight extinction may be orthoclase. In the 

 groundmass, zoned andesine (Abeo An4o) predominates, but there are indications 

 of the presence of orthoclase as well, though the relative proportions are impos- 

 sible to tell. The felspar laths of the groundmass average about .6 mm., and 

 there is at times a tendency to traehytic fabric. Iron ores are fairly abimdant, 

 while interstitial quartz is less important. A few remnants of a pale monoclinic 

 pyroxene ai-e seen, but much altered to carbonates, and tiny apatite needles are 

 scattered about. Chlorite is present in good amount, and there is a small pro- 

 portion of interstitial carbonates. 



Andesites. • 



These are of two distinct, though probably related types, ehai'acterised by 

 the presence of hornblende and pyroxene respectively. The former type is the 

 more acid, and this is reflected in the composition of the felspar. 



(ft) Hornblende Andesites. Rocks belonging to this type have been found 

 occurring as sills: (a) in Portion 117 of the eastern limb of the syncline and (6) 

 at the Werris Creek Gap, on the western limb. We have also been supplied by 

 IVIrs. Scott with a specimen of the same type of rock from a mass oeeun-ing near 

 "AUanbank," which lies about 13 miles slightly W. of N. from the Gap. This 

 rock-type is fairly constant in its main characteristics, and is identical with one 

 phase of the well-known Carboniferous lava in the Hunter River district quamed 

 at Martin's Creek, near Pateison . 



The rock in hand specimen is purplish-grey in colour, and is studded with 

 small phenocrysts of felspar and hornblende, the former predominating. A 

 characteristic megascopic feature of the felspars is the existence in them of kernels 

 of dark-coloured material, presumably chlorite. In thin section the Werris Creek 

 Gap specimen (1491) is seen to be hj'pocrystnlline. The plagioclase phenocrysts 

 are tabular, fairly well-formed, and slightly zoned, the general composition l)eing 

 about Abso An45 (andesine) . The felspar is much cracked and resorbed and is 

 considerably altered to kaolin, with occasional patches of carbonates. Inclusions 

 of the groundmass are not infrequent. Hornl)lende is. as usual in stich rocks, 

 much I'csorbed, and is surrounded by a dark rim composed largely of magnetite 

 granules. Magnetite is also beginning to separate out along the cleavage jilanes 

 from without inwards. The linnibleiide is green, but looks bleaclied at times and 



