134 VOLCANIC AREA OF EAST MOBETON, ETC., DISTRICTS, Q., 



A few seem to possess albite twinning. The segirine occurs in 

 acicular crystals. Quartz is very subordinate in amount and 

 interstitial. The magnetite is present in very minute amount in 

 idiomorphic cubes. 



(4) Nomenclature and affinities. — Owing to decomposition it is 

 hard to determine the felspars exactly. The rock seems to be a 

 Solvsbergite (using the name in Brogger's sense), closely related 

 to Grorudite (Brogger). Rosenbusch terms such rocks " quartz 

 tinguaites " on account of their close structural and mineralogical 

 resemblance to true tinguaites. 



In my previous paper reference was made to a tinguaite (hypa- 

 byssal " aegirine-syenite ") from Butler's Ck., Kilkoy,* which I 

 am now referring to the Solvsbergites. In mode of occurrence, 

 texture and composition it is extremely like Sp. No.55, the main 

 difference being that the former is quartz-free and contains more 

 segirine. Nepheline may be present in the Butler's Ck. rock, for 

 it gelatinises with HC1 and stains. However, as it is not quite 

 fresh this reaction is not very reliable, the decomposition products 

 of the felspar possibly being capable of gelatinising. 



The above-mentioned Solvsbergites are closely allied to the 

 soda-trachytes and pantellarites of the Glass House Mountains, 

 and were probably intruded at the same time. 



(e) Volcanic Rocks. 



These I intend to deal with in the following order : — 

 (a) Rhyolites; (b) Soda Trachyte, Pantellarite, Comendite Series; 

 (c) Andesites; (d) Basalts; (e) Tuffs. 



(a) Rhyolites. 

 Mt. Archer Rhyolites. 

 The chief localities for rhyolites were Mount Archer and the 

 Yandina District. 



At Mt. Archer, and likewise at Mt. Delaney close to Mt. Archer, 

 we find mixed up and apparent^ interstratified with the 



* Proc. Linn. Soc. N. S. Wales, 1903, p.869. 



