C04 THE GEOLOGY OF THE WAliRUMBUNGLE MOUNTAINS, 



mentary flakes and is evidently foreign to the magma, having 

 been snatched up in the upward passage of the lava from the 

 strata penetrated. A little arfvedsonite, some grains of segirine, 

 and a few grains of magnetite are also present. 



4. Name : Porphyritic Albite-Biotite Soda-Trachyte. 



W.22. Loc: Nandi Mountain, Coonabarabran. (Plate xxx., 

 fig.6). 



1. Handspecimen dark in colour, porphyritic and rather more 

 coarse-grained than those previously described. 



2. Texture : holocrystalline, fairly even-grained but for the 

 phenocrysts : grain-size averaging less than 1 mm., hence fine ; 

 fabric panidiomorphic-granular. 



3. Composition : the essential constituent is felspar of prismatic 

 habit, giving square, rectangular, and lath-shaped sections. 

 Twinning is on the Carlsbad law. The crystals are zoned, the 

 interior portion being full of dark inclusions of magnetite and 

 chlorite, the outer portions being usually clear. The felspar has 

 usually slight extinction, but occasionally it extinguishes at low 

 angles up to 10°. The refractive index is lower than that of 

 Canada balsam. It appears to be essentially orthoclase and anor- 

 thoclase. The mineral next in order of decreasing abundance is 

 olivine, which occurs in partially resorbed phenocrysts showing 

 incipient decomposition to serpentine. ^girine-augite occurs 

 both included in felspar and interstitially. It has the acicular 

 habit. Magnetite occurs, primary in idiomorphic cubes as inclu- 

 sions in felspar, and also interstitially in the ground-mass. 

 Secondary magnetite in mossy aggregates ia also present. 

 Serpentine and chlorite occur sparingly as decomposition- 

 products. 



4. Order of consolidation : the felspar is of two generations, 

 the portion abounding in inclusions having probably formed in a 

 deep-seated magmatic reservoir. Crystallisation commenced 

 with the olivine which is devoid of inclusions. 



