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



Andesites. 



They are dark-purple or green-black rocks containing small pheno- 

 crysts of felspar which are visible in the hand-specimen. The S.Gr. 

 usually varies between 2*68 and 2 - 76, and the following minerals 

 are usually present : porphyritic felspars, augite, groundmass 

 felspars, a black iron-ore (magnetite or ilmenite), and chlorite. 



The felspar phenocrysts as a rule are not more than 1 mm. across, 

 and are strongly zoned and often rounded and corroded by the 

 groundmass. Albite twinning is the rule, and the extinction angle 

 never exceeds 18°, usually being about 10°. The crystals are often 

 very much altered, usually containing abundant mica flakes, but the 

 felspars of the Tembeling andesites often consist of merely a thin 

 shell of felspar, enclosing a pale-green central portion made up of 

 a network of chlorite and felspar. In one suchphenocryst the .albite 

 twinning could still be distinguished. It can be seen in several 

 specimens without chlorite that the outer shell of the zoned felspars 

 is less altered to mica than is the central zone, and it would appear 

 that the striking chlorite-felspar network, which makes up the 

 greater part of the phenocrysts of the Tembeling andesites, was 

 formed by the mica flakes in the felspar being altered to chlorite. 

 The interference colours of the chlorite are usually blue, and the 

 double refraction is considerably less than that of quartz. The 

 smaller felspar phenocrysts have a sharp outline, in contrast to the 

 larger rounded phenocrysts. 



The augite is of a very pale-green colour, occasionally occurring 

 as fairly large well-shaped crystals (PI. XXIX, Fig. 1), but more 

 often the crystals are small (compared with the felspar pheno- 

 crysts) and ill-shaped. Sometimes the augite occurs only as tiny 

 grains interstitial to the felspars of the groundmass, and sometimes 

 none at all can be distinguished in the rock, but this does not mean 

 that augite was not at one time present, for such a rock always 

 contains small areas of chlorite, which probably represent augite 

 grains, distributed throughout the groundmass. The alteration of 

 a phenocryst of augite to chlorite can often be seen in an intermediate 

 stage. Xo rhombic pyroxene could be seen in any of the specimens, 

 and all the chlorite pseudomorphs after pyroxene seemed to have 

 been formed from monoclinic pyroxene. 



The felspars of the groundmass often occur as untwinned lath- 

 shaped crystals measuring about "06 mm. X '01 mm. with a low 

 extinction angle, corresponding to albite-oligoclase, and a well- 

 marked flow-structure is often present. They are packed so densely 

 together as to leave no room for a glassy residuum. Iron-ores, 

 probably both magnetite and ilmenite, are usually scattered through- 

 out the groundmass, and often occur as inclusions in the border of 

 the large rounded felspar phenocrysts. Chlorite is abundant, and it 

 is clear that it is usually the result of alteration of augite and felspar. 

 Epidote is common both as a granular alteration-product of augite 

 and felspar and also in thin veins, and calcite is common as in all the 

 Pahang Volcanic Series rocks. It is probably in many cases the 

 result of infiltration from neighbouring sedimentary rocks. 



