GEOLOGY OF ERUPTIVE AND ASSOCIATED ROCKS, POKOLBIN. 397 



Apatite 



Magnetite 



Sphene 



Hornblende 



Biotite 



Plagioclase 



Orthoclase 



Quartz 



B. Hypabyssal. 



Dolerite, Matthews' Gap. 



A greenish coloured greasy-looking rock; phanerocrystal- 

 line and fine-grained. Felspar and a dark ferro-magnesian 

 mineral can be seen in hand specimen. Porphyritic cry- 

 stals of pyroxene showing hour-glass structure are also 

 visible. Under the microscope it is noncrystalline with a 

 few porphyritic crystals. The base has medium grainsize. 

 The fabric is hypidiomorphic granular. 



The minerals present are: — Plagioclase, augite, magne- 

 tite, chlorite and calcite. 



The plagioclase is in tabular idiomorphic crystals, zoned 

 and twinned after the albite, carlsbad and pericline laws. 

 It is partly decomposed to kaolin, the decomposition often 

 being zonal. A symmetrical section showing both albite 

 and carlsbad twinning gives two symmetrical extinctions 

 viz.: — 24 \° and 37i°, indicating that it is labradorite with 

 composition Ab 2 An 3 . Augite is in large subidiomorphic 

 crystals with zonally arranged inclusions. A good deal of 

 chloritic material is present as decomposition product, 

 probably from biotite. Magnetite is present in idiomorphic 

 crystals. A little secondary calcite is also present. 



0. Volcanic, (i.) acid. 

 Rhyolite. 



