-512 E. S. Willbourn — The Paltany Volcanic Series — 



•division of the Pah an g Volcanic Series andesites into those with 

 and those without augite could be made, but, as already mentioned, 

 it is quite possible that all the andesites originally contained augite, 

 which in some cases has been weathered to chlorite. The shape of 

 the chlorite aggregates in some of the lavas confirms this idea, but 

 in others chlorite occurs only as irregular areas in the groundmass, 

 and there is no confirmatory evidence as to its origin. 



Titanite is an alteration product common in the Sumatran rocks, 

 whereas it was seen in only one of andesites of Pahang ; some of the 

 Sumatran augite-porphyrites are uralitized, and titanite occurs as 

 a secondary product formed during this change. Another point of 

 difference is that some of the Pahang Volcanic Series andesites 

 contain olivine, whereas none occurs in the Sumatran porphyrites. 



Apart from the differences mentioned above, the andesites of the 

 two series bear a certain resemblance one to the other. Both are 

 practically always holocrystalline with plagioclase felspar making up 

 the greater part of the rocks as phenocrysts and groundmass, the 

 latter containing little or no glassy base. Apatite is widespread in 

 small quantities in all the Pahang Volcanic Series and Sumatran 

 andesites, and a colourless augite with its alteration products are also 

 widely distributed. Rhombic pyroxene is always absent. 



Serpentine. 



Serpentine and peridotites are found in many of the islands of the 

 Dutch East Indies, including Java, Borneo, the Moluccas, and the 

 west coast of Sumatra. Dr. Verbeek writes a description of all these 

 occurrences in Mijnivezen, Wetenschappelijh gedeelte, 1905, and from 

 this it appears that the serpentine in all these places was derived 

 from olivine. Most of the serpentine outcrops in the Peninsula show 

 no traces of the original rock, but occasionally, as in Negri Sembilan, 

 remnants of amphibole suggest that the serpentine owes its origin to 

 that mineral. An outcrop in the Perak River, however, resembles 

 serpentine of the islands of the Archipelago in containing remnants 

 of olivine. 



Tuffs and Breccias. 



They differ in much the same way as do the Sumatran lavas from 

 the Pahang Volcanic Series lavas. The andesitic constituents are 

 similar, except in the composition of the felspars, but no simultaneous 

 effusion of rhyolite material took place in Sumatra, and so the 

 Sumatran fragmental rocks differ from those of Malaya in that the 

 tuffs are of an unmixed andesitic composition. The same difference 

 applies to the breccias of the two areas. There is nothing similar to 

 the remarkable deposits of boulders in tuff in the Sumatran area. 



The age of the Sumatran volcanic rocks is given by Verbeek 1 

 as youngest Palaeozoic, while Volz s gives them a pre-Triassic 



1 E. D. M. Verbeek, "Top. En. geol. beschrijving van een gedeelte van 

 Sumatra's West Kust " : Batavia Landsdrukkerij , 1883, p. 270. 



2 W. Volz, " Zur Geologie von Sumatra " : Geol. und palaontol. Abhand- 

 lungen herausgegeben von E. Koken, Neue Folge, Bd. vi, Hft. ii, pp. 87 ft'., 

 Jena, 1904. 



