72 THE MINERALS OF TASMANIA. 



inch or two in length are common in the granite of the 

 East and North-East Coasts. 



Sanidine. — This pellucid monoclinic felspar is found in 

 the alkali syenites and elaeolite syenite porphyries of Port 

 Cygnet. It frequently shows zonal structure. 



Flagioclase Felspars, albite, oligoclase, andesine, labra- 

 dorite, bytownite, anorthite, form a continuous series, in 

 which, according to Tschermak, albite and anorthite are 

 opposite extremes. The intermediate felspars have been 

 shown by Schuster to be isomorphous mixtures of albite 

 and anorthite. 



Albite occurs as replacement of the groundmass of por- 

 phyroids or keratophyres at Mt. Read ; in larger crystals 

 in the actinolitised slates in the North Dundas district. 

 Intergrown with orthoclase, it forms microperthite ; seen 

 in granite at Anderson's Creek and in alkali syenite at Port 

 Cygnet. 



Lahradorite is the felspar of our basalt and dolerite 

 (diabase). Labradorite-bytownite and bytownite-anorthite 

 felspars characterise the gabbros at the Heazlewood, Bald 

 Hill, &c. 



Oligoclase, with its narrow twin lamellae, is the plagio- 

 clastic felspar of our granites. Andesine occurs in essexite 

 at Port Cygnet. 



Microrline, though chemically identical with orthoclase, 

 is triclinic in crystallisation. Basal sections microscopically 

 show a characteristic cross-hatched twinning, due to the 

 intersection nearly at right angles of the twin lamellae of 

 two types (albite and pericline). Seen in granite porphyry 

 at St. Marys, and in granite elsewhere. 



Analysis of the flesh-coloured veins in the garnet rock 

 which occurs so abundantly at the Shepherd and Murphy 

 Mine, Bell Mount: — 



100-0 



The rock fuses before the blow-pipe, bubbling slightly, 

 and forming a somewhat blebby white mass. It appears 

 to consist mainly of felspar." (F. W. Ward, Government 

 Analyst.) 



