BY W. H. TWELVETREES AND W. F. PETTERD. 43 



of plagioclastic porphyritic felspars led us to suspect that 

 the rock belonged to the sub-group of soda felsites or 

 keratophyres (in their altered sheared form often called 

 porphyroids). To avoid all chance of error, we sent samples 

 of the rock to Professor Rosenbusch, who very kindly 

 favoured us with his opinion, as follows : — "^ Undoubtedly 

 we have here strongly dynamically altered forms of the 

 acid eruptive rocks. The typical porphyritic structure,, 

 the nature of the phenocrysts, the still recognisable fluidal 

 structure, the nearly entire absence of dark constituents, 

 the occasional spherulitic forms still recognisable in their 

 replacement products (quartz, albite), all point with cer- 

 tainty to members of the quartz porphyry family, and, 

 with great probability, not to quartz porphyry in the 

 narrower sense, but to quartz keratophyre and keratophyre.. 

 .... The rocks greatly resemble our German occurrences 

 in Westphalia, the Fichtelgebirge and Thilringen, and 

 especially the occurrences in Wales. These are the forms 

 which in Germany were originally called poi*phyroids and 

 flaserporphyries." 



Microscopical charctcten^ of tJie FeUifc. 



As the aspect of the rocks differs in the field in different 

 parts of the same mass, so their microscopical structure 

 varies to an equal extent. Sometimes they are typically 

 porphyritic, though the crystals are never very large ; or 

 the porphyritic crystals are set so closely together as to 

 resemble somewhat a plutonic rock-like granite ; or they 

 are broken and mutilated, giving a fragmentary appear- 

 ance to the rock. The mineral constitution, too, varies.- 

 There is a set in which quartz phenocrysts accompany 

 those of felspars, and another series from which they are 

 absent. Nevertheless, despite all these variations, the 

 observer recognises that he is looking at one and the same 

 group, the acid and the sub-acid eruptives. 



The mineral constituents of one or other of the members 

 of the gi*oup may be classed as under : — 



