Federated Malay States. 511 



of the specimens in the two countries would be without value, but 

 one difference is sufficiently clear, namely that the felspar of the 

 Pahang Yolcanic Series dolerites is very much more acid in character, 

 varying, in the five or six specimens examined, from oligoclase with 

 an extinction angle of 12° to andesine. As already mentioned in 

 the detailed description of the Pahang Volcanic Series dolerites, one 

 of them contains quartz which is probably original. The alteration 

 of the augite in both series results either in the formation of chlorite 

 or of an amphibole. None of the dolerites collected from the Pahang 

 Volcanic Series contain either diallage or olivine. 



Andesites. 



Dr. Emil Gutzwiller follows the custom of the Continental 

 petrologists in using the name porphyrite for the older andesitic 

 lavas. 



Judging by the descriptions of the ten Goemaigebergte porphyrites 

 given in Mijnwezen, 1912, the most obvious difference is in colour, 

 for none of the porphyrites of South Sumatra have the red-brown 

 colour which is so typical of many of the Pahang Volcanic Series 

 andesites. A more important difference is in the composition of the 

 felspar, for only two specimens out of the ten contain oligoclase- 

 andesine, the remainder having felspars varying from andesine- 

 labradorite (three specimens), through labradorite (two specimens), 

 to bytownite (two specimens) and anorthite (one specimen), whereas 

 the felspar of the Pahang Volcanic Series andesites on the other 

 hand is never more basic than andesine. 



The mode of alteration of the felspars in both the Pahang Volcanic 

 Series and the Sumatran andesites is occasionally the same, then 

 resulting in both cases in the formation of chlorite, but usually there 

 is a considerable difference in the products of alteration of the two 

 series, which is to be expected after considering that, apart from 

 occasional specimens, the composition of the felspars in the two 

 series is so widely different. In many of the Pahang Volcanic Series 

 rocks the alteration product is a brown, opaque, extremely fine-grained 

 material, which is probably kaolin, often associated Avith tiny flakes 

 of secondary mica, not a common mode of alteration for the felspar 

 of the Sumatran rocks. On the other hand, the felspar of the 

 Sumatran rocks is often altered with the separation of epidote and 

 calcite (eaussurite). The oligoclase-andesine felspar of one of the 

 Tembeling andesites contains a good deal of epidote, but in most of 

 the Pahang Volcanic Series rocks the epidote which they contain 

 has not been formed from the felspar. These rocks usually contain 

 calcite, sometimes formed by alteration of the felspar, but often 

 simply added to the rock by infiltration from neighbouring calcareous 

 sediments. 



The porphyrites of South Sumatra are divided petrologically into 

 two groups, the labradorite-porphyrites and the augite-porphy rites, 

 the first group containing no phenocrysts of pyroxene, but in some 

 cases (Nos. 7, 8, and 9 Gloegoer) chlorite aggregates occur which are 

 undoubtedly secondary after amphibole or pyroxene. In some of 

 these, augite grains occur unaltered in the groundmass. A similar 



