44 ON FELSITES AND ASSOCIATED ROCKS. 



The changes are all rung on these minerals, the secon- 

 'dary ones replacing or obscuring the essential constituents 

 in varying degrees. 



In addition to orthoclase, a felspar of the albite or 

 ^oligoclase-albite series appears as a porphyritic constituent, 

 and is sometimes very abundant, becoming the prevailing 

 felspar. This shows us that these are not the ordinary 

 ^orthoclastic f elsites. 



The material upon which we have founded these pre- 

 liminary microscopical studies comprises an extensive 

 series of rocks collected from Mount Read, on the sections 

 owned by the South Hercules, North Hercules, East Her- 

 <3ules. Crown Hercules, and Ring River Companies, from 

 Tipperarj' Creek on the west side of Mount Read, from the 

 White Spur between Dun das and Mount Hamilton, from the 

 Tyndal Track and Creek south of Mount Read, from the Red 

 Hills east of Mount Read, from Mount Black and the Tas- 

 manian Copper Company's property north of Rosebery, &c. 

 The following micro, details will be of interest to 

 students : — 



We note that in the porphyritic types on the North 

 Hercules section the felspars have a habit of collecting in 

 nests, and there is a good deal of water-clear secondai*y 

 felspar surrounding the phenocrj-sts. The phenocrysts 

 float in a matrix of this secondary felspar, with which 

 calcite is sometimes associated. Carlsbad twins may be 

 •often seen bent by dynamical stress, and stronglj' seriti- 

 cised. In the more granitic or crystalline forms, on this 

 property, there is abundant quartz, which often has a 

 "fragmentary aspect, being in all sorts of irregular sha23es. 

 Hexahedral forms are i-are. Some of the grains are 

 embayed ; others are stretched, cracked, or broken. Strain 

 shadows are frequent : minute fluid cavities present occa- 

 sionally. Between the felspars there is a good deal of 

 bright green chlorite. Sericitic streaks curve between the 

 phenocrysts : this may represent original flow structure. 

 'The rock on the White Spur is a counterpart of that of the 

 North Hercules. The crystalline form is the dominant 

 type. Fragmentary deformed quartz frequent. Schistose 

 ^structure marked ; the lines of schistosity bend round the 

 unyielding quartz grains, causing a streaky appearance, 

 and probably marking former flow lines. The ordinary 

 porphyritic form of rock also occurs on the White Spur. 



In the keratophyre of Tipperary Creek the felspars are 

 strongly sericitic, and the rock is veined with albite. This 

 felspar veining occurs also in the same rock at Red Hills, 



