GEOLOGY OF ERUPTIVE AND ASSOCIATED ROCKS, POKOLBIN. 399 



Rhyolite Tuffs. 



These occur abundantly throughout the whole district. 

 They are generally light in colour and are very solid. Their 

 texture varies from very fine to very coarse. In the 

 coarser ones angular fragments varying from half an inch 

 to several inches in diameter are cemented together by 

 extremely fine grained siliceous material; the very fine 

 ones on the other hand are composed completely of crypto- 

 crystalline to microcrystalline siliceous material, with a 

 small amount of felspathic material and glass. The base 

 in places shows obscure traces of lamination and contains 

 fragments of felspar crystals and quartz. Occasionally 

 large quartz grains are corroded and much granulated. 



Dacite, Matthews' Gap Road. 



These rocks vary in colour from a light brown to dark 

 reddish-brown. The lighter coloured varieties appear to be 

 slightly more acid than the darker ones. They are porphy- 

 ritic in texture with aphanitic ground mass. The pheno- 

 crysts in the lighter coloured ones are chiefly quartz and 

 felspar ; biotite is only sparingly present. In the darker 

 ones quartz and felspar are abundant, as is also biotite, the 

 latter being well developed in hexagonal plates. In places 

 they are somewhat tuffaceous. 



Under the microscope they are all hypocrystalline, with 

 porphyritic texture. The phenocrysts vary in size from 

 medium to large. The base is generally made up of brown 

 glass, but occasionally it is partly cryptocrystalline ; the 

 fabric of the ground mass is fluidal. Plagioclase and 

 quartz form the bulk of the phenocrysts; plagioclase is the 

 more abundant of the two. The quartz is in corroded 

 subidiomorphic to idiomorphic grains. It contains inclus- 

 ions of the ground mass and shows very little evidence of 

 strain. Plagioclase is abundant, but rather decomposed. 



