75 



(4) ORTHOCLASE - QUARTZ - HORNBLENDE - BIOTITE ROCKS. 



Hornblende-granitile. 



(5) ORTHOCLASE-QUARTZ-AUGITE ROCKS. Augite-granite. 



(6) ORTHOCLASE-QUARTZ-AUGITE -BIOTITE ROCKS. Augite- 



biotite- granite or augite-granitites. 



(7) ORTHOCLASE - QUARTZ - MUSCOVITE ROCKS. Muscovite 



granite. 



(8) ORTHOCLASE-QUARTZ-MUSCOVITE-BIOTITE ROCKS. Musco- 



vite-biotite- granite or granite proper. 



(b) Trachytic texture. 



(1) Non-porphyritic, compact. Felsite. (1) 



(2) Porphyritic, with compact ground-mass. Felspar 



orthoclase (not sanidine). 



Quartz-felsite (Field term.)< 2 > 



(i) Ground -mass, holocrystalline ; texture, micro - 

 granitic. Micro-granite (Rosenbusch) . 



(ii) Ground-mass, holocrystalline ; texture, micropeg- 

 matitic or pseudo-spherulitic. Granophyre, (Rosen- 

 busch). 



(iii) Ground-mass, cryptocrystalline or microfelsitic. 

 Felsophyre (Vogelsang). 



(3) Porphyritic, with compact ground-mass. Felspar 



sanidine. Quartz-trachyte, Liparite or SkyottU. 



(4) Porphyritic, with glassy ground -mass. Vitrophyre 



(Vogelsang) ; including porphyritic obsidians and pitch- 

 stones, and also the glassy (hyaline) rhyolites. 



The trachytic modifications of the acid magma often contain one or more 



(1) It must be remembered that many of the felsites are devitrified glassy rocks. See 

 ALLPOET, Q.J.G.S., vol. XXXIII., page 449 ; also RUTLEY, Felsitic Lavas, " Memoirs of Geological 

 Survey of England and Wales." 



(2) The term quartz-felsite as here used is not precisely equivalent to the quartz-porphyry 

 of Continental authors : the latter term is more comprehensive as it includes the porphyritic 

 pitchstones and obsidians of pre-tertiary age. 



If we compare the pre-tertiary trachytic representatives of the acid magma with the corres- 

 ponding rocks of later date we are struck with the greater frequency of the occurrence of glass, as a 

 constituent of the groundmass, in the latter rocks. This is largely, if not entirely, due to 

 devitrification having taken place in the older rocks. 



(3) The term nevadite was introduced by RICHTHOFEN for a variety of liparite in which the 

 porphyritic elements are so extremely numerous that the rock approaches in aspect to a granite. 

 The typical nevadites, however, are not granitic, but trachytic in texture (see HAGUE and 

 IDDINGS, A.J.S., vol. 27, p. 461). With regard to the use of the terms liparite and rhyolite, it 

 seems convenient to restrict the latter term to rocks in which a flow structure is well marked. 



