146 VOLCANIC AREA OF EAST MOEETON, ETC., DISTRICTS, Q., 



the segirine-pantellarite from Trachyte Range (Anal, iv.; Norm 

 iv., pp.168, 170) If all were arfvedsonite the ratio of FeO to 

 Fe 2 3 should be greater. 



The determination of cossyrite and of barkevicite is supported 

 by the proportion of Ti0 2 present, these minerals being titanium- 

 bearing. Zirconia is present (see Anals. i. and v.), and its 

 presence gives further evidence that this molecule is essential for 

 the formation of riebeckite. 



Fluorine was looked for in the Conowrin rock, and occurs only 

 to the extent of - 02 %. Murgogi considers it essential for the 

 formation of riebeckite.* In my opinion chlorine has supplied its 

 place in these rocks. The blue hornblendes are therefore chloro- 

 riebeckites and chloro-arfvedsonites. 



Chlorine is present in considerable amount (Anals. i. and v.) 

 and is only freed by digestion with nitric and hydrofluoric acids. 

 It is consequently not present in the form of sodalite, which one 

 would hardly expect in so acid a rock, but occurs in some 

 insoluble silicate like the blue hornblende or marialite (a scapo- 

 lite). The amount of the latter mineral is, however, not sufficient 

 to account for all the chlorine. 



Texture. — The trachytes and allied rocks are mostly holo- 

 crystalline or nearly so, but holo- and hypo-crystalline specimens 

 are frequently obtained from the same mountain; thus at 

 Coolum Mountain every gradation exists from holocrystalline to 

 hypohyaline. At Mt. Tinbeerwah specimens of trachyte obsidian 

 were obtained. The Conowrin, Round Mountain and other lavas 

 contain occasional small patches of glass in the base. 



The fabrics represented are : — 



(1) Pilotaxitic : e.g., Round Mountain, Coolum Mountain, tfec. 



(2) Trachytic : e.g., (a) even-grained — Bridge's Hill, Big Hill, 

 <kc; (b) porphyritic — Beerwah, Beerburrum. 



(3) Orthophyric : (a) with small phenocrysts — Mt. Cooran, Mt. 

 Conowrin, &c; (b) with large phenocrysts —Mt. Ngun-Ngun, <fec. 



(4) Granular (microgranitic) : Mt. Tibrogargan, Trachyte 

 Range, Mt. Beerburrum (riebeckite trachytes), &c. 



See Murgogi, Amer. Journ. Sci. August, 1904. 



