452 E. S. Willbourn — Tlie Pahang Volcanic Series — 



The rocks mentioned in the table show a decreasing percentage of 

 silica moving from left to right, but as unfortunately it has not been 

 possible to make more than one analysis, the silica percentages in the 

 above table can only be regarded as approximate. It will be seen 

 that no basic volcanic rocks are represented, for it is not known 

 whether the serpentine outcrops which occur in several localities in 

 the Peninsula should be classed as lavas or intrusions, but there is 

 little doubt that the serpentine is a basic member of the Pahang 

 Volcanic Series. If further field-work shows that the serpentine 

 outcrops all occur at the boundary of Paub and Gondwana rocks, as 

 seems to be the case in Negri Sembilan*, it will be clear that the 

 serpentine is an altered lava. 



Rhtolites. 



Rhyolite lavas have been found in situ at Pulau Chengai and 

 Lubok Plang on the Pahang Piver, and as derived water-worn 

 boulders embedded in tuff on the railway at Kuala Tekal (PI. XXIX, 

 Fig. 2) and at the 1 05th mile, and in the Pahang Piver at Pulau 

 Guai, Lubok Plang, and Pulau Chengai. In addition there are many 

 rocks which may be altered rhyolites exposed in the valley of Sungei 

 Kechau. The lava in situ at Lubok Plang is in layers, one layer 

 being entirely spherulitic and another showing pheuocrysts of felspar 

 and quartz set in a fine-grained crystalline gronndmass. There 

 is a good deal of secondary quartz in the rock and a flow-structure is 

 very pronounced in the groundmass, which contains unaltered bands 

 of spherulites, and curved lines marked by opaque secondary material 

 in the remaining finely crystalline part of the groundmass probably 

 indicate that the whole of it was once spherulitic. The rhyolite 

 boulders in the ashy boulder-beds here are very similar to the rock 

 in situ. 



Certain boulders in the ashy boulder-beds of Pulau Guai are of 

 rhyolitic lava containing numerous inclusions of shale. Por the 

 most part the non-porphyritic portion of the rock is finely crystalline, 

 but here and there the remains of spherulites can be distinguished, 

 and although it has undergone considerable alteration, yet a well- 

 marked flow-structure can still be seen. Probably the groundmass 

 originally was a glass. The quartz phenocrysts are corroded and 

 contain inlets and inclusions of the groundmass. The orthoclase 

 and oligoclase crystals are altered to an opaque brown material and 

 probably also to calcite, for they contain a considerable amount of 

 that mineral, but the whole of the rock is rich in calcite, and perhaps 

 all of it is derived from neighbouring sediments by infiltration. In 

 addition the rock contains small altered crystals of biotite. 



Some of the inclusions of the sedimentary rocks are of dark shale, 

 and one slide showed chiastolite crystals in the shale-inclusions, which 

 differ from the rest of the rock in containing no calcite. Other 

 inclusions were of fine-grained volcanic tuff. 



This striking rock is found in situ at Pulau Chengai, but none of 

 the boulders of rhyolite embedded in tuff which are found within 

 100 yards of it were seen to contain shale-inclusions. 



The age of the rhyolite of Lubok Plang is Paub period, for it is 



