36 ON FELSITES AND ASSOCIATED ROCKS. 



have a more or less banded or schistose appearance, though, . 

 owing to their, greater hardness compared with the slates, 

 they are not foliated to anything like the same extent. 

 The effect is often only shown by obscm'e banding or 

 streaking, while elsewhere there is a more decided approach 

 to schistosit5\ Sometimes a few reddish porphyritic fel- 

 spars are discernible, but as a rule the rock has a streaked 

 reddish and greenish flinty aspect, a,s if the original 

 porphyritic crystals had been rolled out and their material 

 diffused. The colour of different varieties, however, varies 

 a good deal, ranging from yellowish white to reddish. 



Sideclfic Gravity. 



Teall states the sp. gr. of f elsites and liparites as ranging 

 from 2-53 to 2-7. Our determinations of the Mount Read 

 felsites, comprising numerous selected specimens^ are : — 



2-6, 2-62, 2-63, 2-65, 2*68, 2-7, 2-74. 

 The specific gravities of the Lenne-porphyries of West- 

 phalia (Keratophj^res and Quartz Keratophyres) given in 

 0. Miigge's important paper may be usefully compared. 

 Quartz-keratophyre, 2-648, 2-654, 2-647. 

 Felso-keratophyre, 2-62 (non-schistose), 2-638, 

 2-74 (high sp. gr. due to chlorite), 2*6.5, 2-75. 

 Also those furnished by Rosenbusch : — 

 Keratophyre, 2-611, 2-677. 



Quartz-keratophyre, 2-709, 2-647, 2-620, 2*64 

 2-634, 2-614, "2-632. 



The specific gravity of the Tasmanian f elsite is consequently ' 

 quite in accord with what has been observed elsewhere. 



Intrusive or Effusive. 



The relations of the rocks to the argillitic metalliferous 

 schists are far from being definitely established, and require 

 closfe working out over an extended area. Owing to the 

 densely timbered country and paucity of serious mining 

 operations, this is a task of extreme difficulty. All that 

 has been done hitherto has been to notice the fact of their 

 occurrence here and there, especially where the country 

 has been cleared a little for mining work. On the north 

 side of Mount Read on the North Hercules, Barlen, and 

 Consols sections, this rock is prevalent, sometimes as a 

 greenish flinty schistose or fissile felsite, and towards the 

 bottom of the track going down to the Ring River valley 

 in al)rupt massive cliffs of a green and reddish streaked 

 compact siliceous felsite. It has here the appearance of an 



