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rocks which are better designated by such terms as felsite, quartz-felsite, 

 ortho-felsite, and micro-granite. 



Granophyre, Felsophyre, Vitrophyre. These three terms were in- 

 troduced by VOGELSANG for different varieties of quartz-porphyry. The rocks 

 designated by the first two terms can only be distinguished by the use of the 

 microscope. The granophyres of VOGELSANG are rocks which possess a 

 holo-crystalline ground-mass ; the felsophyres are rocks with a crypto- 

 crystalline or micro-felsitic ground-mass. ROSENBUSCH has shown that the 

 granophyres of VOGELSANG may be divided roughly into two groups (1) those 

 with a micro-crystalline or micro-granitic structure and (2) those with a 

 micro-pegmatitic or pseudo-spherulitic structure. He proposes that the 

 rocks of the former group should be termed micro-granites and that the 

 term granophyre should be limited to those of the latter. In using the 

 term granophyre it is therefore necessary to state which author is followed. 

 The term vitrophyre is applied to the quartz-porphyries with a vitreous 

 ground-mass. 



Trachyte. This name was introduced by HAUY, but his definition is 

 not of much use in fixing the modern sense of the term. It is now 

 generally applied to rocks containing porphyritic crystals of sanidine in a 

 felsitic or vitreous ground-mass. From this point of view the rocks to 

 which it is applied differ from the quartz-free felsophyres (orthophyres) 

 merely in the fact that the large felspars are sanidine instead of orthoclase. 

 As used by RICHTOFEN and SZABO the term includes the andesites of most 

 modern authors. 



Liparite, Rhyolite, Quartz-trachyte. These terms are practically 

 synonymous. They are applied to the typical volcanic rocks of acid 

 composition. Such rocks, in their unaltered condition, contain as a rule 

 porphyritic crystals of sanidine and quartz embedded in a micro-crystalline, 

 crypto-crystalline, micro-felsitic or vitreous ground-mass. The ferro- 

 magnesian minerals occupy a very subordinate position in the composition 

 of these rocks. Some authors use liparite as the group name ; others use 

 rhyolite. The more recent liparites or rhyolites frequently contain a 

 glassy base. The corresponding rocks of the earlier geological periods 

 more frequently possess a micro- or c^pto- crystalline ground-mass which, 

 however, can be proved in many cases to owe its double-refraction to the 

 devitrification of a glassy base. 



Nevadite. This term was introduced by RICHTOFEN for certain 

 liparites or rhyolites which somewhat resemble granite. It has been 

 shown by HAGUE and IDDINGS that, in the typical nevadites, this resem- 

 blance is superficial and due to the fact that the porphyritic crystals are 

 exceptionally abundant. Just as certain basalts (e.y., that of Dunsapie 

 Loch near Edinburgh) somewhat resemble gabbros in appearance in 

 consequence of the extraordinary abundance of their porphyritic con- 

 stituents so do certain liparites resemble granites. The resemblance in both 

 cases is superficial and disappears on careful examination. 



Syenite. This term is said to have been used by PLINY for the rock 

 occurring at Syene (Assouan) in Egypt. It was subsequently applied to 



