33 



grown with quartz : prevailing forms columnar Carlsbad 

 twins. 



Plagioclase in some quantity, but less than orthoclase. 

 Generally in short stout idiomorphic prisms. Extinction 

 angles, those of oligoclase-andesine. 



Biotite represented by pseudomorphs of a green pleochroic 

 mineral, polarising in the steel grey tints of chlorite. This 

 chlorite is also plentifully distributed throughout the rock in 

 aggregations, and between the boundaries of the felspars. 



There is not quite so much quartz as in normal granites : 

 a little sphene occurs in grains, and magnetite in grains and 

 crystals. 



8. Hornblende Granite. 



Min. Const. : Orthoclase, plagioclase, hornblende, quartz, sphene. 



Sp. Gr. 27. 



Nearly two miles north-west of the main road, and near 

 the south bank of the Heazlewood Eiver, is a small boss of 

 this holocrystalline rock. It projects but a few feet above 

 the level of the surrounding country, which is mainly serpen- 

 tine, of the usual character common to the district. So far 

 as known it is exceptionally local in its occurrence. 



Macroscopical Character.— In general features this granite 

 bears a strong resemblance to its congeners, being dense and 

 compact. The exposed surface is but slightly decomposed to 

 a thin incrustation of kaolinic matter of a pale rusty tint. 

 The colour of the ground mass is light grey, approaching 

 white, with indistinct cloudiugs of a somewhat darker hue. 

 Throughout the substance of the rock are scattered irregular 

 elongated flakes of hornblende, which appear black to the 

 unaided vision. These give it an obscure graphic appearance. 

 Small patches of copper and iron pyrites may be frequently 

 detected, which decompose to rusty stains. 



Microscopical Structure. — The quartz is intergrown very 

 generally with the felspars, especially with the triclinic ones, 

 in a micrographic way. The hornblende is usually without 

 terminal outlines, and pale green in colour, preserving its 

 pleochroism, but not intensely. Crystals of sphene are 

 present in the usual forms. There is some tendency to a 

 micro-porphyritic architecture, small prisms of orthoclase and 

 plagioclase, especially the latter, abounding as a kind of 

 ground mass. Small grains of ilmenite may be seen scattered 

 throughout the section, and this is often converted into the 

 white variety of sphene named leucoxene, as may be seen 

 by changing the transmitted light to reflected, when the 

 black iron mineral becomes white. 



