I 



BY H. I. JENSEN. 865 



some of the fine-grained Conowrin rock. The phenocrysts of 

 sanidine are frequently strongly arched, having evidently been 

 subjected to very great pressure in the upward passage of the 

 magma. The sanidines are twinned on the Baveno, Carlsbad, 

 and Manebach laws. 



Some of the hand specimens of Round Mountain trachyte are 

 not unlike specimen No. 10559 (from | mile N. of Tondeburine 

 Ck., Warrumbungle Mts.), Mining Museum, Sydney. 



Mt. Cooee Trachyte. — The rock composing Mt. Cooee varies 

 widely in macroscopic appearance. Some is dark-coloured, 

 coarsely porphyritic, and resembles the andesitic rock of Grigor's 

 Estate, into which it seems to merge. The specimen sectioned 

 was of a bluish-grey colour; this rock forms irregular columns, 

 and weathers into rounded boulders. It is comparatively rich 

 in quartz, which occurs in large cr^^stals easily seen with the 

 naked eye. In colour and texture this rock resembles specimen 

 No. 11215 from Orange, in the Sydney Mining Museum. 



The constituents of the quartz trachyte are sanidine — the most 

 abundant component — a considerable amount of quartz, and a 

 small proportion of dark blue hornblende. The central part of 

 Mt. Cooee consists of square columns similar in colour, size and 

 shape to those of Mt. Conowrin and Mt. Ewin. Whether 

 the quartz- trachyte, quartz-andesite, and true trachyte of this 

 mountain are contemporaneous or not, and whether the}'^ are 

 derived from the same magma, I have not yet been able 

 to determine; but the order of superposition in places where 

 superposition could be ascertained, is — (1) trachyte, (2) quartz- 

 trachyte, and (3) quartz-andesite. 



Trachyte Range Rock. — This rock is a true pyroclastic rock or 

 tuff. It is of a dark green colour, very hard, and emits a ringing 

 sound when struck; it also contains angular opaque fragments of a 

 dark colour. Under the microscope it is seen to consist of crypto- 

 crystalline and amorphous material, forming a base containing 

 scattered sanidine crystals and angular fragments. The substance 

 of the base is in the form of minute needles and granules, and is 



