140 VOLCANIC AREA OF EAST MORETON, ETC., DISTRICTS, Q., 



of biotite are i*epresented by black, opaque, rodlike pseudomorphs. 

 It may be termed Felsitic and Microspherulitic Trachy- 

 Rhyolite. 



(b) The Trachytes, Comendites, and Pantellarites 



These rocks consist essentially of glassy felspar (sanidine) with 

 one or more ferromagnesian minerals belonging to the pyroxene 

 and amphibole groups. The ratio of felspar to ferromagnesian 

 minerals is, with a few ex eptions, about 90 % of the former to 

 10% of the latter. The exceptions are certain trachytes with 

 trachytic structure from Bridge's Hill and Big Hill, in which the 

 percentage of felspar probably is as great as 95 %, and in which 

 the original ferromagnesian mineral has been wholly altered to 

 hematite. 



In the felspars three dominant shapes may be recognised — 

 (a) the orthophyric, (6) the tabular, and (c) the columnar (lath- 

 shaped). The orthophyric type prevails in the comendites from 

 Conowrin, Ewin, Coolum, Cooran and Cooroora; and associated 

 with felspars of other habits in some of the other rocks. The 

 tabular type is predominant in the phenocrysts of the Ngun-Ngun 

 and Beerwah trachytes, and in some of the Beerburrum porphy- 

 ritic trachyte. The columnar prevails in the felspars of Big Hill, 

 Bridge's Hill, Beerwah, Round Mountain and other lavas. 

 Micro-allotriomorphic granular felspars, with a tendency to 

 orthophyric habit, prevail in the fine even-grained riebeckite 

 comendite of Mt. Tibrogargan and Mt. Beerburrum. 



The orthophyric sanidines of Mt. Conowrin comendite were 

 found to be enclosed by the faces B (010), C (001), x (101), y (201), 

 with occasionally the faces n(021) and m (110) represented. 

 The square sections are enclosed by (7 and B, and sections in the 

 zone Cy are also almost square when y is strongly developed. 

 Twinning is chiefly Carlsbad parallel to B. Baveno quartets also 

 occur, and sometimes simple Baveno twinning is represented. 



In the Cooran rock the same forms were represented. 



Perfectly square and rectangular sections are practically 

 isotropic, while rhomboidal sections generally show the highest 

 colours. Hence the acute bisectrix lies in the direction a\ while 



