BY W. H. TWELVETREES AND W. F. PETTERD. 35 



in England for compact granitic protrusions, we propose 

 to confine the terms felsite and quartz felsite to devitrified 

 acid lavas. The term quartz-keratophyre is applied to the 

 same rocks when containing an alkali-felspar rich in soda. 

 Keratophyre, the syenitic equivalent = soda felsite : quartz 

 keratophyre = soda quartz felsite. This terminology can 

 be correlated with Rosenbusch as follows : — 



Here defined. Itosenhusch. 



Felsite. Felsophyre felsite rock. 



Quartz felsite. Felsophyric quartz porphyry. 



Keratophyre = Soda felsite. Keratophyre. 



Quartz-keratophyre = Soda- Quartz-keratophyre. 

 quartz-felsite. 



The groundmass of felsites is characteristically felsitic. 

 What felsitic matter really is has occasioned much discus- 

 sion among petrologists, and a definite result can hardly be 

 said to have been yet attained. The compact groundmass 

 irresolvable by the naked eye or the hand lens is often 

 resolved by the microscope into an intimate aggregate 

 of minute crystalline-granular quartz and felspar, giving a 

 confused speckled appearance between x nicols. 



When the component individuals of the aggregate become 

 more minute and indefinable, the groundmass is what Rosen- 

 busch calls crypto-crystalline. This is the felsitic material. 

 And, following the process of resolution still further, we 

 arrive at the ultimate isotropic vitreous base. But more 

 frequently further resolution discloses a minute aggregate 

 of isotropic granular or flaky material w^hich we assume to 

 be a devitrification product, though this is mostly not 

 susceptible of proof (miscro-felsite, Rosenbusch). From 

 Vogelsang's researches it is probable that this micro-felsitic 

 material is no longer a mechanical aggregate of quartz and 

 felspar, but an independent silicate. (See the lucid exposi- 

 tion given by Rosenbusch, Mass. Gest., p. 'o^'^.) 



Under such conditions it is not wonderful that the 

 Mount Read rocks, masked by great geological age, and 

 distorted and mineralogically reconstructed by intense 

 dynamic metamorphism, should prove puzzling to the 

 geologist. Their felsitic nature is often obscured by green 

 colouration due to the free development of chlorite, which 

 gives a very different appearance from that of the light 

 coloured halleflinta-like aspect of so many of the more typical 

 felsites. It must be premised that the rocks not only occur 

 in the Zone of the West Coast argillitic and phyllitic 

 schists, but have themselves been affected by the forces 

 which produced the foliation of the schists. Hence they 



