586 



right side-river of the S'' Kalemhoi near the Kampong Kota Renah. 

 Here we remark some tei)S of meters up tiie river, from the month, 

 first a larger intrusion of granite, and a few meters fartiier a 

 smaller intrusion which seems to be developed as a vein with N.W. 

 strike and a breadth of 1.8 m. 



Coarse-granular parts alternate with fine-granular ones and por- 

 phyritic structures are found too. Polysyntlietically twinned felspars 

 and untwinned ones, or felspais showing cross-hatching, can both 

 dominate so as to exclude the others. Further constituents are 

 quartz, muscovite, tourmaline and sometimes reddish-brown garnet, 

 iron ore missing almost entirely and only occurring as very fine 

 spots in the rocks. Furtiier, some light-green chlorite was found in 

 a few saui|)les when microscopically examined. The plagioclase 

 ^chiefly albite) shows ouly polysynthetical twins, according to the 

 albite-iaw. Whole crystals are sometimes characterised by cross- 

 hatciiiug. However, part of the crystals often is untwinned, entirely 

 untwinned crystals also occurring. These untwinned parts sometimes 

 show parallel extinction, often their direction of extinction in sections 

 of the symmetrical zone was observed to cut in half the angle between 

 the directions of extinction of the polysynthetical twins ; the untwinned 

 felspar often consists of irregular spots, which gradually pass into 

 one another and extinguish to different sides of the twinning plane, 

 their angle of extinction varying between the one of the distinctly 

 limited lamels and the one of the homogeneously extinguishing parts 

 mentioned above. These crystals a[)parently contain dilferent gradua- 

 tions from microcline into orthoclase (extinction in sections perpen- 

 dicular to the positive bisectrix of the obtuse angle = 5°). Similar 

 graduations were described by the author in the microcline micro- 

 perthites of Transvaal foyaites ') ; they support the truth of the 

 conception of orthoclase as a microcline in which microscopically 

 no twinning can be obserxed. 



•In the parts with porphyritic structure small crystals of musco- 

 vite, sometimes of quartz and felspar too, are to a small degree 

 enclosed by the larger felspar-crystals. The form of the larger 

 crystals of felspar, muscovite and quartz with respect to the ground- 

 mass, points to partly simultaneous crystallisation ; in the ground- 

 mass the felspar often occurs in well developed elongated sections. 

 Varieties rich in garnet and tourmaline, near the contact with 

 the hornfels, show a beautiful poikilitical structure. Large crystals 

 of felspar incliule many small crystals of muscovite, of beautifully 



1) H. A. Brouwer, Oorsprong en Samenstelling der Transvaalsche nephelien- 

 syenieten. 's Gravenhage, 1910. 



