398 w. r. Browne and a. b. walk<>m. 



The rhyolite varies a good deal from place to place. 

 Typically in hand specimen it is light in colour, with small 

 phenocrysts of clear glassy quartz and pink felspar in an 

 aphanitic groundmass. Plow-structure is well developed 

 in places, but is more often absent. Cavities in the rock 

 are of frequent occurrence and are almost always filled 

 with secondary material — this is generally chalcedony, 

 agate or some other form of secondary silica. Such 

 secondary material is plentiful in the Mount Bright area, 

 but is of less frequent occurrence at Drake's Hill. At 

 Drake's Hill, however, the rhyolite has been brecciated 

 and very numerous cracks, transverse to the lines of flow 

 as well as parallel to it, have been filled with a deposit of 

 black jasperoid material. 



Under the microscope it is markedly banded, and has a 

 cryptocrystalline to microcrystalline base with few pheno- 

 crysts. The phenocrysts are felspar and quartz; the felspar 

 is idiomorphic and somewhat decomposed. Drusy cavities 

 are present and are generally filled with fine quartz grains 

 and in places microscopic quartz crystals can be seen pro- 

 jecting from the sides of the cavities. Minute cracks 

 transverse to the direction of the banding are filled with 

 quartz. At Drake's Hill there is a good example of brec- 

 ciation of the rhyolite. The rock has been much crushed 

 and crumpled; the original banding of the rhyolite is 

 generally still visible, but the cracks have become filled 

 with dark coloured secondary jasperoid material. 



An analysis of rhyolite from half a mile south of Mount 

 Bright by Mr. J. C. H. Mingaye, 1 is as follows: — 



Si0 2 



77*82 



H a O + 



1-40 



Ti0 2 



0-02 



H 2 - 



0*36- 



A1 2 3 



11*46 



oo 2 



()'03 



Pe 2 3 



0*30 



p 2 o 5 



0*04 



FeO 



0*09 



so 3 



0'07 



MnO 



« o-oi) 



BaO 



0-02 



MgO 



0*23 



Li 2 



trace 



OaO 



0'22 





: — 



K 2 



7*19 





100-11 



Na 2 Q 



0-86 













Sp. gr. 



2596 



Kindly furnished by the Department of Mines. 



