456 E. 3. Willboum — The Pahang Volcanic Series — 



dust pigment (probably iron-ores) which gives nearly all the Tem- 

 beling andesites their dai'k-purple colour. Some big phenocrysts have 

 a border of a different type. The crystal is filled with an opaque 

 dust, possibly kaolinite, and a binary twinning can be distinguished, 

 indicating that this central part is orthoclase felspar, but surrounding 

 the crystal is a narrow band of clearer felspar which is not in 

 crystalline continuity with the remainder of thephenocryst. Needles 

 of ilmenite are scattered about in the crystal, in the clear band more 

 abundantly than elsewhere. 



The groundmass is made up of twinned microliths of felspar, 

 which have a low extinction angle, with a certain amount of 

 interstitial quartz, which is probably the result of devitrification 

 of pre-existing glass. There are abundant iron-ores scattered 

 throughout the rock, much of it associated with chlorite, which was 

 formed by the alteration of augite, for pseudomorphs having the 

 form of augite are the rule. Apatite is a common accessory, and an 

 irregular-shaped grain of a mineral like sphene was seen in the rock. 



Andesites are fcmnd interstratified with Raub and Gondwana rocks 

 both as lavas and tuffs, and it appears that the andesite phase was 

 predominant during the whole period of the Pahang Volcanic Series. 



Q,UARTZ-PORPHYRY AND GRANOPHYRE. 



Between the 101st and 102nd mile on the Pahang Railway there is 

 a considerable development of intrusive rocks consisting of quartz- 

 porphyry and granophyre. They probably belong to the same 

 period of eruption as the rhyolites of Lubok Plang, a few miles to 

 the east, and contributed to the formation of certain fragmental 

 deposits in the neighbourhood by adding more acid material to what 

 would otherwise have been andesite tuffs and breccia. The rocks 

 are associated with intrusions of a more basic composition, both 

 dolerite and porphyrite. 



About 10 miles further north there are extensive exposures of 

 granophyre both in the river and the railway cuttings, and there is 

 a strong development of quartz-porphyry near the railway near 

 Sibah and Lanna. Other exposures occur along the Benta-Kuantan 

 road and in the valley of the Kechau River. 



Quartz-porphyry. 



The rock consists of felspar phenocrysts, quartz phenocrysts, 

 chlorite, and iron-ores probably formed by the alteration of augite, 

 apatite, a fine-grained holocrystalliue quartz-felspar groundmass, 

 and secondary minerals. Its specific gravity varies between 257 

 and 2-75. 



The felspar phenocrysts are most commonly of a plagioclase with 

 composition between oligoclase and andesine, but orthoclase is usually 

 present in less quantity. Occasionally there are two sizes of felspar 

 phenocrysts, all being altered, either to kaolin or to a micaceous 

 mineral. The quartz crystals are practically always corroded by the 

 groundmass. Augite is rarely seen in the quartz-porphyries, but 

 they usually contain pseudomorphs of chlorite associated with 

 magnetite, whose form resembles basal sections of augite, and one 



