119 



dote prisms all over the rock, but particularly over the colour- 

 less minerals, and work on the felspars is still more obscured 

 by some secondary mica. Quartz is present in fair amount, 

 and ilmenite, with leucoxene, also occurs. Apatite forms in 

 prisms. Xame. — Diopside-biotite granodiorite. 



A very beautiful type of syenite is to be found in a 

 small vein about 500 yards to the west of Mr. Scrymonger's 

 homestead on the hills overlooking the Torrens (Section 

 3240, Hundred of Yatala). Macroscopically it is a medium- 

 grained rock with pink-grey felspars and biotite flakes. Micro- 

 scopically (see plate iii., fig. 1) the predominant mineral is an 

 orthoclase pei'thite. The ripple-like nature of the perthitic 

 intergrowth is beautifully marked. It is a fine example also 

 of the passage of orthoclase into microcline. The straining 

 of the rock has given rise to a moire appearance on all the 

 orthoclases, and in some faintly, and in others clearly, the 

 outline of the 'gitter mikroklin" appears. The orthoclase 

 forms irregular grains, averaging about 2 mm. in diameter. 

 Plagioclase other than the albite of the perthite is quite sub- 

 ordinate ; it forms subidiomorphic grains about '5 mm. in 

 diameter and acid oligoclase in composition. It is twinned 

 on the albite and pericline laws. The ferromagnesian sili- 

 cate is a brown, slightly-chloritized biotite in irregular flakes 

 *7 mm. in length, and often clouded with secondary magne- 

 tite. A very little diopside is present in small grains. A 

 great deal of magnetite is present passing into deep-red trans- 

 lucent haematite, with a little clouding of leucoxene. Rutile 

 grains are present, and apatite is an accessory. The rock has 

 been rather shattered and altered. Between each grain and 

 fringing each fragment of iron-ore is a little fringe of colour- 

 less mica, with fine-grained quartz and rarely a little ortho- 

 clase and oligoclase. It is a little difficult to say whether the 

 quartz is entirely secondary. Some grains strongly suggest 

 a primary origin, while the mode of occurrence of the majo- 

 rity with mica in intergranular openings and filling in cracks 

 in fractured felspar crystals is surely indicative of its second- 

 ary nature. The percentages of alkalies in this rock are 

 No^O, 5-02: K, O, 478. .Yo we.— Biotite syenite. 



576. Locality. — Occurs in Sections 562, 563, 572, of the 

 Hundred of Yatala. Macroscopically it shows the gneissic 

 banding developed rather strongly. The predominant mine- 

 ral is a white felspar, but there are green crystals of uralitic 

 pyroxene about 3 mm., in length. A few grains of ilmenite 

 are recognizable. Microscopically the texture is hypidiomor- 

 phic granular, though with a markedly parallel arrangement 

 of the longer axes of the grains or aggregates of grains of 



