( 409 ) 



III. Biotiteamphiboleandesite (Riot i team phibolepliyrovitritonaJose). 

 The rock of which Boekit Loeboek is composed, and of which 

 the handspecimen II 749, struck over against Nangah Oeroei on the 

 Soengei Tèbaoeng, represents a sample, belongs to the freshest old 

 andesites of the Western Müller-mountains. After the macroscopic 

 habitus it takes a place quite by itself; a darkgreen ground mass, 

 strongly fading in the more weathered rock, forms the cement 

 between irregularly scattered plagioclases with microtine-habitus, some- 

 times densely crowded together and reaching a diameter of 6 mm., 

 slender amphibole-crystals (to 8x2 mm.) and hexagonal little tables 

 of bronze-coloured biotite (3 — 4 mm. diam.). The twinned, often very 

 distinctly zonal plagioclase is not really different from the one described 

 above. The very first beginning of decomposition shows itself in a 

 slight development of albite, secondary amphibole and troubling 

 substances along cracks. The plagioclase, for the rest limpid, contains 

 as inclusions primary amphibole, often twinned parallel to (100), 

 magnetite, apatite, and occasional sharp prisms of zircone, whilst 

 peripherically sometimes augite-grains are inclosed, which are charac- 

 teristic of the resorption-borders of the amphibole-phenocrysts, and 

 from this appears that the crystallization of the plagioclase did not 

 come to an end until during or after the eruption. 



The amphibole forms prismatic individuals, which are strongly 

 corroded by the magma and surrounded by a broad, loose border of 

 chloritizing amphibole-scales, pyroxene-, magnetite- and titanite-grains, 

 mixed with the usual groundmass-components. Slender prisms are 

 sometimes, to within a narow, notched lath, entirely changed info 

 these products. The extinction with respect to the c-axis reaches a 

 value of 20° ; the pleoehroism varies between dead brownish green 

 and light brownish green ; the usual twins parallel to (100) occur. 

 As for inclusions the amphibole contains basic plagioclase, magnetite 

 and cloudlike accumulations of extremely line, parallel microlites, 

 which probably, quite like a part of the magnetite-globulites, owe 

 their individualisation to a chemical dissociation of the amphibole 

 substance. The examination of the biotite yields no particular points 

 of view. The acid groundmass consists of probably primary grains 

 of quartz with wavy extinction containing colourless limpid glass- 

 inclusions, sanidine and acid, short prismatic and zonal, but also 

 slender lath-shaped plagioclase with polysynthetic twinning after the 

 albite-law ; these minerals are cemented by a small amount of colourless 

 glass, strongly troubled by globulites. Here and there some grains of 

 almost colourless augite, sometimes twinned and strongly laden with 

 glass, are found, which as appears from their occurrence near the 



27* 



