SGG (iEOLOGV OF GLASS HOUSE MTS. AND DISTRICT, 



chieriy felspar, a darker green mineral being also present in fine 

 needles. This is probably acicular microlites of segerine. Glassy 

 material seems also to be present. 



True trachyte lavas also occur on Trachyte Range, forming 

 the summits of the ridge. The}- i-esemble the rock of Mt. Ewin. 

 One specimen obtained on the southern side of the ridge is macro- 

 scopically very like specimen No. 5006 (Riebeckite Trachyte, 

 Warrumbungle Mts.), in the Mining Museum, Sydney. Micro- 

 scopically examined, it is seen to consist of a holocrystalline, 

 even-textured sanidine ground-mass, containing peculiar dark 

 blue to black, arborescent aggregates of ultra-microscopic crystals, 

 probably a hornblende, arfvedsonite or riebeckite. 



The trachytes of Mt. Miketeebumulgrai are partly fine in 

 textuie like that of Mt. Conowrin, and partly coarse and porphy- 

 ritic like that of Mt. Beerburrum. 



Mt. Tibrogargan is composed of trachytes uf a fine texture, 

 resembling those of Mt. Conowrin and Mt. Ewin. They seem 

 to be essentially segerine trachytes. 



To sum up and generalise, it might be said that most of the 

 Glass House Mountains are composed of columnar trach3^te. The 

 core of the mountain usually consists of vertical columns, and the 

 sides often of horizontal and slanting columns {e.g., Ngun Ngun 

 and Tibrogargan). The trachyte rocks are usually of a grey 

 colour and dull lustre. The more porphyritic trachytes (<^.^., 

 Beerwah, Beerburrum, Ngun Ngun) contain more brown and 

 greenish blue hornblende (barkevicite and arfvedsonite) than the 

 more even-textured rocks. The hornblendes seem to have been 

 the first mineral to crystallise, being usually very corroded, and 

 often occurring as inclusions in sanidine phenocr^^sts. The sani- 

 dine phenocrysts are often corroded and partially resorbed; they 

 possess the cljaracteristic cross cracks parallel to the (100) j^lane. 

 In the instance of the Beerburrum rock, the phenocrysts proved 

 to be anorthoclase, containing a considerable amount of soda and 

 some lime. In the coarsely porphyritic rocks a?gerine is less 

 plentiful than hornblende. The amphibole sometimes occurs in 

 twinned phenocrysts. 



