596 THE GEOLOGY OF THE WARRUMBUNGLE MOUNTAINS, 



W.109. Loc: Gibb's Gap, near Naman. (Plate xxx., fig.4). 



1. Handspecimen shining, silky, dark greenish-grey and vesicular.. 



2. Microscopic texture : holocrystalline, microcrystalline, and 

 even-grained base, with trachytic fabric and flow-s'tructure, and 

 with occasional sanidine phenocrysts. 



3. Composition : the essential constituent is felspar (soda- 

 sanidine) of prismatic habit, sometimes tabular, ^girine-augite 

 and blue soda-hornblende (arfvedsonite) are present in about 

 equal proportions. The former occurs in grains interstitially; the 

 latter in ragged grains exhibiting the characteristic pleochroism 

 in blue, green and yellowish-green colours. A few dark opaque 

 grains of what is probably a titanium mineral {e.g., pseudo- 

 brookite 1) are also present. 



4. Name : Vesicular Soda-Trachyte. 



The rocks so far described, W.215, W.45, W.50, W.39, W.16,, 

 W.17, VV.114, and W.109, exhibit close petrological and miner- 

 alogical affinity, though coming from different parts of the War- 

 rumbungle area. They all come from plugs outstanding like 

 monoliths, with precipitous walls; and such structures are almost 

 universally composed of this rock-type. 



W.222. Loc: Bingy Grumble Mountain, summit. (Plat& 

 xxxi., figs. 7-8). 



1. Handspecimen dark grey, with white specks, and with a 

 greasy lustre. 



2. Texture holocrystalline, fine and uneven-grained, with 

 trachytic fabric. 



3. Composition : the main constituents are albite, pseudoleucite, 

 nosean, and analcite; with segirine, arfvedsonite, and, in smaller 

 amount, a highly pleochroic, brownish-black amphibole in notable 

 quantity and also in minute amount, nepheline, and augite- 

 acmite, the last of which is colourless and has a double refraction 

 of 0040. The bulk of the felspar exhibits lath-shaped sections. 

 Only Carlsbad twinning is common. Albite twinning is occa- 

 sionally seen. The felspar is of two generations, there being 

 lath-shaped phenocrysts of albite, and minute needles of a felspar 



