862 GEOLOGY OF (iLASS HOUSE MTS. AND DISTRICT, 



From faint traces of multiple twinning in the porphyritic 

 felspar phenocrysts it was suspected that they were not true 

 orthoclase. These crystals are twinned like sanidine on the 

 Carlsbad plan, and have a refractive index of 1-525. Measure- 

 ments of extinction angles and microchemical tests proved a fair 

 amount of soda and a little lime to be present. Hence some, if 

 not all, of these phenocrysts are composed of anorthoclase. 



Hand specimens of Beerburrum rock resemble specimen No. 

 9256 (Trachyte from the Canoblas) in the Sydney Mining Museum. 



Beencah Trachyte. — The specimen sectioned was obtained on 

 the N.E. tiank of the mountain, and is typical of the bulk of the 

 Mt. Beervvah rock. This trachyte separates on weathering into 

 huf^e shingle-shaped slabs. It has a very glistening, silky lustre 

 when freshly broken, apparently due to the habit of the consti- 

 tuent felspar. The rock is very soft and crumbling, and has a 

 fa-eenish-grey colour. It was taken by Mr. Stutchbury, in 1854, 

 to be metamorphic sandstone, probably on account of its tendency 

 to split into slabs and its comparative softness. With aid of a 

 pocket lens the rock can be seen to be porphyritic, containing 

 abundant tabular phenocrysts of a plagioclase felspar. A few 

 hornblende phenocrysts are also present. 



Examined under- the microscope, flow-structure is very apparent, 

 the arrangement being trachytic-pilotaxitic as in the typical 

 trachytes of the Siebengebirge (Drachenfels type). Felspar is the 

 predominant constituent, both as sanidine with characteristic cross 

 cracking, and in form of a plagioclase felspar which seems to be 

 oligoclase or andesine. The crystals are lath-shaped, with their 

 long axes all in the same direction. The base is microcrystalline 

 and displays the trachytic variety of pilotaxitic texture. No 

 glass is present. The ferromagnesian minerals are a brown 

 hornblende, often in well shaped, twinned crj^stals; a strongly 

 pleochroic horn])lende, having green, blue and slate-coloured 

 pleochroism in different sections. This latter amphibole is 

 probably arfvedsonite, and is frequently seen enveloping the 

 brown hornV)lende which is barkevicite. ^gerine is scattered 

 plentifully throughout the base in minute rods. The chief 



