152 VOLCANIC AREA OP EAST MORETON, ETC., DISTRICTS, Q., 



(xv.) Trachyte. Loc: Dyke at Big Hill on Woodford-Caboolture 

 Road (Plate xiv. fig. 24). 



This is a hypocrystalline rock with a trachytic fabric. It is 

 composed essentially of sanidine laths with fluidal arrangement; 

 also laths of a black, almost opaque mineral^ consisting probably 

 of magnetite, pseudomorphous after hematite; also a little glass. 

 (xvi.) JEgerine-Sanidine Trachyte (Sp.No.118). Loc: Bell's Quarry 

 near Caboolture. 



This rock has a trachytic fabric. It is of a darkish grey colour 

 in handspecimen. The constituents are (a) sanidine in broken 

 and bent crystals showing Carlsbad twinning and shadowy extinc- 

 tion due to strain; (b) beautiful green nonpleochroic acmite in 

 corroded phenocrysts and smaller corroded needles of pleochroic 

 pegirine; (c) magnetite in idiomorphic crystals; (d) decomposed 

 biotite of yellowish colour from iron ores. A little zircon and 

 apatite are present as minute accessories. 



General Remark s. — The foregoing descriptions of the 

 leading types of Glass House Mountain rocks are, I think, 

 .sufficient to make their nature understood. Each specimen 

 described is typical of its locality, only minor variations from 

 the type being met with. All the rocks with orthophyric fabric, 

 and consequently with a micro- to cryptoc^stalline base, I have 

 called comendites or pantellarites, according as they resembled 

 more closely the one or the other analysed. The cryptocrystalline 

 (' felsitic ') base in these rocks must undoubtedly contain quartz 

 (see .Norms ii., iii., and iv.) whether the microscope shows it or 

 not. 



The order of crystallisation is not quite normal. Riebeckite is 

 occasionally found included in felspar, but frequently again it 

 occurs in the base, forming a graphic intergrowth with sanidine. 

 It practically crystallised simultaneously with, and as long as, 

 the felspar in many of these rocks. 



Attempts were made to measure microscopically the relative 

 proportions of sanidine (felspar), and riebeckite (amphibole) in 

 the Beerwah and Conowrin l'ocks. Calculating all the blue 

 patches as hornblende, about 14% of this mineral was estimated 

 to be present, about 86 % of the rock being felspar and base. 



