458 E. S. Willbourn — The Pahang Volcanic Series — 



much altered to chlorite with the separation of iron-ores, and very 

 often they have been bent by shearing movements which have 

 imparted a flow-structure to the groundrnass, these same shearing 

 movements in all probability being responsible for the angularity of 

 the quartz and felspar crystals. The groundrnass, in those specimens 

 which have not been sheared, is that typical of a quartz-porphyry or 

 rhyolite, and occasionally it is made up of microspherulitic structures. 

 Some specimens much resemble tuffs, but no fragments of lava have 

 been observed in them, and their tuff-like appearance is probably due 

 to brecciation by shearing movements. 



From the uniformity in the appearance of quartz-porphyry both in 

 the hand-specimen and under the microscope, when examined as 

 boulders in the different localities above described and in situ, it 

 seems to be clear that the rock is an intrusion and not a lava, 

 and this view is supported by the total absence of any appearance of 

 stratification or variation when traced through the 100 feet or so 

 of thickness of rock which is exposed at Jeram. Gading*. It was not 

 possible to go more than £ mile upstream from the point in the lliver 

 Seli* where the quartz-porphyry was first encountered in situ, and 

 the extent of the outcrop is unknown, but from the rocks exposed 

 downstream it appears that the quartz-porphyry was intruded into 

 cherts, grits, and quartzites, and is therefore of a later age than the 

 other intrusions of the Pahang Volcanic Series. 



The granite-porphyry also referred to in the account of the 

 distribution of the Pahang Yolcanic Series is not definitely proved to 

 belong to the series, but it is usually associated with the purple 

 quartz-porphyry, and its occurrence at Ginting Sempak* and 

 Gin ting Bidei* suggests that it was lifted to form the caps of those 

 hills by the intrusion of the granite, and that it is therefore older 

 than the granite. 



The hand-specimen is always green in colour, and is apparently 

 composed of a mosaic of quartz and felspar of uniform grain with also 

 abundant flakes of biotite, but under the microscope the rock is seen 

 to be markedly porphyritic. The phenocrysts are felspar, varying 

 from oligoclase to labradorite, quartz, biotite, both rhombic and 

 monoclinic pyroxene, with smaller quantities of black iron-ores, 

 apatite, zircon, and blue tourmaline, and the groundrnass is a perfectly 

 clear mosaic of quartz and felspar (oligoclase to labradorite), which 

 has sometimes the appearance of micropegmatite. 



The phenocrysts of felspar and quartz usually have an irregular 

 form and are encroached upon by the groundrnass. The form of the 

 zoning of some of the felspar phenocrysts shows that, after they were 

 formed, they were reduced to a mere fraction of their original 

 dimensions, and a phenocryst was noted with one end altered to an 

 aggregate of mica flakes, the aggregate extending on one side much 

 beyond the present boundary of the felspar crystal and yet containing 

 no small blebs of quartz corresponding with those of the groundrnass. 

 It is clear that the phenocrysts have been resorbed by the ground- 

 mass, but after consolidation, the action being probably the result of 

 shearing movements. The biotite is deeply pleochroic and is of 

 a deep brown colour. Some of the crystals are bent, like the biotite 



