874 THE GEOLOGY OF THE NANDEWAR MOUNTAINS, 



Minerals of The Nandewar Rocks. 



i. The Ali-trachytes, Ali-rhyolites, Bostonites, and related 

 porphyries contain many minerals in common. 



(a) Abundant or common. — Felspar occurs in phenocrysts 

 and also finely granulitic in the base. The phenocrysts are 

 usually bounded by the faces m(l]0), « (100), n(021), and 

 6(010). Sometimes they are tabular parallel to 5(010), which 

 is the typical sanidine habit, and have the usual crosscracks 

 developed. Sometimes they are prismatic, thickened also in the 

 c direction; a (crystallographic) is the usual direction of elonga- 

 tion. Carlsbad twinning is generally observed, but in some 

 rocks the Baveno type is even more frequent. Manebach 

 twinning sometimes occurs. There are, therefore, Baveno twins 

 forming nearly square prisms elongated in the direction of 

 crystallographic a, Baveno twins tabular parallel to 6, prismatic 

 Carlsbad twins, and Carlsbad twins tabular parallel to h. In 

 some Carlsbad twins the faces x and c are well developed (N.30). 

 Inclusions of albite and of groundmass are occasionally seen in 

 the phenocrysts of some rocks {e.g., N.30); in others it is not 

 unusual to find inclusions of quartz in them, in others again, 

 microperthitic intergrowths of potash and soda felspar or crypto- 

 graphic intergrowths of quartz and felspar. 



In the Dingo Creek Bostonite, JS.51, the extinction angle is 

 from S'^ to 11° on crystallographies (edge h c), indicating a 

 variety of sanidine rich in soda. The form of the crystals is 

 tabular or elongated in the a direction. The extinction is 

 shadowy, indicating ultramicroscopic twinning. Inclusions of 

 quartz and apatite occur within the phenocrysts. By picking a 

 section cut nearly parallel to the plane of the optic axis it was 

 ascertained by use of the selenite plate that a is near a normal to 

 the a axis. The square sections {i.e., sections at right angles to 

 a, the direction of elongation) showed an axial cross in the same 

 slide. Therefore 



Bx'*' = a, i.e , nearly at right angles to a., 

 These crystals are therefore negative. They must be looked 

 upon as a variety of sanidine very rich in soda. 



