Federated Malay States. 453 



associated with Iiaub rocks, which, however, differ from those of 

 other known localities in including a band of chert. 



Trachytes. 



No very typical members of this group of lavas are known, but 

 some rocks occur between Jeransong and Jerantut which are related 

 to the trachyte family on the one hand and to the andesitic rocks of 

 Tembeling on the other. 



One rock is red-brown in colour with numerous tiny iron-stained 

 patches, and looks very much like a compact quartzite in the hand- 

 specimen. It has S.Gr. 2 - fi4, and contains very scanty small pheno- 

 crysts of felspar up to 1-^ mm. in length. 



Under the microscope the rock is seen to be holocrystalline and to 

 contain these minerals : orthoclase, oligoclase, calcite, and limonite. 

 The orthoclase forms irregular-shaped prisms about - 03 X *01 mm., 

 and the oligoclase occurs as prisms with a better shape and about the 

 same size. There is a well-marked flow-structure, and a number of 

 the prisms are slightly bent, owing, probably, to movement while 

 they were still hot and plastic. Large crystals of calcite are 

 common, and there are abundant small areas of calcite scattered 

 throughout the rock, interstitial to the felspar prisms, all of it 

 probably having been derived from neighbouring limestones. 

 Limonite as scattered grains is an important constituent of this rock, 

 whereas another rock from 101^ mile is grey in colour owing to the 

 great amount of pyrites which it contains. 



Fairly numerous phenocrysts of felspar are set in a groundmass of 

 felspar prisms, and the whole of the rock presents a cloudy appear- 

 ance under the microscope, due to the presence of abundant secondary 

 white-mica flakes. The phenocrysts are very much altered, being 

 almost entirely obscured by the mica flakes, but certain clear patches 

 in the mica aggregate consist of albite felspar. Some of the felspar 

 prisms in one specimen were orthoclase, and that is the reason for 

 placing the rock amongst the trachytes (it may be a porphyry) ; the 

 majority of the felspars, however, are albite and oligoclase. It is 

 possible that the albite is a secondary mineral formed by the altera- 

 tion of a more basic felspar, but this is not probable, for no remnants 

 of the basic felspar are left. 



There are cracks in the rock which are lined with small irregular 

 plates of albite, the centre being occupied by a zeolite, which has 

 a spherical form with fibrous structure, the fibres being arranged to 

 radiate from the centre, and in nearly all cases the centres of the 

 spheres lie on the edge of the felspar which lines the cavity walls. 

 The zeolite fibres under crossed nicols extinguish parallel to their 

 length, and have low double refraction and low refractive index, so 

 it is probably natrolite. 



The presence of abundant secondary mica with clear albite and 

 natrolite is fairly strong evidence that the original felspar of this 

 rock was more basic, but, as already mentioned, this is not confirmed 

 by any remains of a more basic felspar. 



The pyrites occurs as small grains and never has a good crystal 

 form. 



