90 GEOLOGY OF MT, FLINDERS AKD FASSIFERN DISTRICTS, Q., 



Microscopic structure. — Texture. a.Crystallinity : holocrys- 

 talline. 6. Grain-size : rather even-grained, microcrystalline, with 

 a few srtiall phenocrysts. c. Fabric : trachytic, with well-marked 

 flow-arrangement. 



Mineral Composition. — The rock is composed essentially of 

 felspar and segirine-augite, with magnetite, apatite, and a little 

 fluorite present as accessories, and some reddish iron-ore and 

 brownish serpentine as decomposition-products. The felspar 

 consists entirely of lath-shaped microlites (of tabular and pris- 

 matic habits), some of which show Carlsbad twinning. The com- 

 position-plane is in the direction of the length of the laths. The 

 extinction is almost straight in sections parallel to a(lOO), and in 

 sections parallel to 6(010) the extinction angle varies from nearly 

 straight in the direction of the c axis to about 12° on the edge 

 be. Extinction is, however, usually shadowy, a feature due either 

 to strain induced by close packing of the microlites, or more pro- 

 bably to ultramicroscopic twinning. The glassy clearness of 

 sanidine, and the cross-cracking parallel to <x(100) are character- 

 istic properties. The cleavage parallel to c(OOl) is good. With 

 some difficulty the following optical properties were determined 

 by the aid of the selenite plate; a and c are closely approximated 

 so that the felspar is nearly uniaxial. It is optically negative 

 (Bxa = a). The birefringence-colours of the felspar and the 

 selenite plate add when the a of the plate is inserted in the long 

 direction of the laths, and subtract when inserted transversely. 

 The prisms and tables are elongated on the face c along the a 

 axis, and the tables are tabular on b. These properties fix the 

 felspar as being a soda-orthoclase or anorthoclase. 



The ferromagnesian mineral is in the form of minute greenish 

 rods and grains, abundantly distributed in the dark patches of 

 the rock, there forming ophitic or pcecilitic aggregates; and it 

 also occurs as the nucleus of small phenocrj^sts, whose outer zone 

 is a yellowish, feebly pleochroic acmite. The greenish pyroxene has 

 the pleochroism: a dark sea-green, b light leek-green, c yellowish- 

 green, and absorption-scheme a>>b^C- When a selenite plate is 

 introduced in the direction of the length of the crystals, the 



