88 GEOLOGY OF MT. FLINDERS AND FASSIFERN DISTRICTS, Q., 



Texture holocr3^stalline, hiatal-por[)hyritic,with phaneric pheno- 

 crysts in an aphanitic cryptocrystalline base. The rock is 

 perpatic, the phenocrysts forming less than 10 per cent. With 

 the high power, the base is resolved into microlites and crystallites 

 having a trachytic fabric. 



Component minerals : the chief constituent is felspar. The 

 phenocrysts consist of felspar, with glassy inclusions. The extinc- 

 tion angle makes 9° to 10° with the composition-plane of the 

 Carlsbad twins, and the extinction is somewhat shadowy. Small 

 lamellae of albite occur in some. Cross-cracking, as in sanidine, 

 is universal. These characters point to the felspar being soda- 

 sanidine or anorthoclase. The crystals are irregular in outline, 

 some or other of the faces being always mutilated. Sometimes 

 they occur in a group, and often they are corroded. These 

 features suggest that they are torn out of a previously consolidated 

 rock, as in F1.14. Grthoclase and anorthoclase form the bulk of 

 the base. They occur in the form of stunted laths and squares 

 (sections of square prisms) ; they show shadosvy and almost 

 straight extinction and Carlsbad twinning. Other minerals occur 

 only in minute amount, and consist of (1) minute needles of 

 segirine-augite; (2) irregular grains of magnetite; (3) granules of 

 htematite and reddish iron-ores. There is also a little interstitial 

 isotropic material, and some interstitial birefringent colourless 

 matter is also present. This may be chalcedonic silica. 



Analysis : the chemical composition of the 

 determined. 



SiO., 



rock 



Fe,0, 

 FeO .. 

 xMgO., 

 CaO.. 



K,0. 

 TiO„ 



* Some impurity accidentally got into the alkali, hence the Na^O was 

 estimated by difference. 



