864 GEOLOGY OF GLASS HOUSE MTS. AND DISTKICT, 



This is sometimes twinned. A few grains of riebeckite were 

 present. 



The lock from Mt. Ewin is macroscopically like that of 

 Conowrin, but microscopically it was observed that the ferriferous 

 constituents had taken chiefly the form of legerine. A few pheno- 

 crysts of a brownish hornblende were also present (barkevicite). 

 Mt. Ngun Xgun Trachyte. — The main mass of Mt. Ngun Ngun 

 is built up of huge polygonal columns of porphyritic trachyte. 

 Specimens from here are macroscopically very like specimen No. 

 11227 (Trachyte from the Canoblas, Orange) in the Mining 

 Museum, Sydney. The rock is holocrystalline, consisting of 

 sanidine phenocrysts which are sometimes corroded, and a micro- 

 crystalline orthophyric base. The base contains sanidine, scattered 

 irregular granules of a colourless non-pleochi'oic pyroxene, and 

 the green pleochroic hornblende often with a nucleus of l^rownish 

 hornblende. Fragments of quartz are present as an accessory, 

 and also a few fragments of an orange-yellow mineral. The 

 (quartz is probably allogenic, derived from the sandstone in the 

 upward passage of the magma. Another variety of trachyte is 

 found on the S.E. side of Ngun Ngun; this is exactly similar in 

 structure to that of Mt. Conowrin. There is also a third variety 

 found on the E. side of the mountain; this rock is of a bluish- 

 grey colour, very hard, and emits a ringing sound when struck. 

 In section it was found to be composed of sanidine in phenocrysts, 

 lath-shaped sanidines, and deep blue hornblende and green 

 legerine in the base. 



Round Mountain Trachyte. — Hand specimens of this rock are 

 often much darker in colour than usual, so as to suggest a fine- 

 grained andesite. But the darkness is entirely due to mineral 

 solutions which have permeated the rock after its formation. 

 Sections prove the Round Mountain rock to be a holocrystalline 

 trachyte, very fine in texture, but containing a few small sanidine 

 phenocrysts scattered in a microcrystalline to cryptocrystalline 

 base. The rock consists almost entirely of sanidine felspar, 

 lugerine in minute gi'anules, and a few scattered crystals of the 

 deep blue hornblende (riebeckite) which has also been noticed in 



