Federated Malay States. 



455 



A Tembeling andesite (No. 3756) was analysed by Mr. J. Shelton, 

 Chemist to Geological Department F.M.S. 



Examination under the microscope shows that the greater part of 

 the rock is made up of tiny laths of cloudy oligoclase-andesine with 

 interstitial chlorite. The zoned felspar phenocrysts are largely 

 altered to chlorite, iron-ores are abundant, and small grains of augite 

 occur in the rock. A little epidote is present. 



Si0 2 59-02 



A1 2 3 



Fe 2 3 



MgO 



CaO 



Na 2 



K 2 



Loss on ignition 



17-23 

 9-07 

 2-42 

 4-12 

 4-20 

 1-68 

 2-64 



100-38 



The large amount of iron-ores in the rock is indicated in the 

 analysis, as also is the fact that the felspar is of a soda-lime rather 

 than a lime-soda variety. 



A lava rather like this, but containing much more secondary 

 chlorite, from the Benta-Kuantan road, contained numerous amygdules, 

 the small ones being spherical and the larger being elongated along 

 the line of flow. 



Pseudomorphs of chlorite and micaceous material after olivine are 

 present in two specimens of rocks from the Tembeling district with 

 S.Gr. 2 - 68, and in large fragments of lava from a tuff-deposit near 

 Kuala Lipis, S.G. 2*75. Unfortunately nothing is known of the field 

 relations of these olivine-bearing rocks. 



The felspar phenocrysts (acid oligoclase) have rounded corners and 

 have been partially resorbed by the action of the groundmass, and 

 they have a border rich in grains of iron-ores. These phenocrysts of 

 felspar may be as much as 4 or 5 mm. in length, but there is a second 

 generation of smaller felspar phenocrysts which are sharper in outline 

 and show no reaction-border. 



The groundmass has suffered alteration, and for the most part 

 consists of an irregular mosaic of felspars and quartz, in which can 

 be distinguished occasional tiny felspars about *05 mm. in length, 

 and abundant scattered granules of dark material, some, and perhaps 

 all, of which is magnetite. 



Another type of andesite (S.G. 2*70) containing abundant large 

 phenocrysts set in the groundmass of microliths, can be seen at 

 several places for a distance of about J mile along the railway, near 

 the 117th mile at Tembeling. There are two sets of phenocrysts, 

 one is 2 or 3 mm. across, with occasional crystals as much as 1 cm. 

 across, and the other set varies between 0*1 mm. and 0-3 mm. in 

 length. Most of the phenocrysts are plagioclase showing albite and 

 microcline twinning, and having an extinction angle corresponding 

 to a composition intermediate between oligoclase and andesine. 

 The large phenocrysts are rounded and show dark reaction-borders. 

 The border is part of the groundmass, differing from the greater 

 part of the groundmass in containing a bigger proportion of a brown 



