34 ON FELSITES AND ASSOCIATED ROCKS. 



Teall* — Acid rock8='* Felsite, Eurite petrosilex. These terms are 

 practically synonymous. 'Vhi'\ have l»«^en applied to compact 

 stony rocks, the mineralogical composition of which cannot be 

 ascertained by examination with the naked eye or with a lens.'' 



Jas. Geikie.i' — Quartz-felsite=Quartz-porphyry. " In this rock 

 we have a compact <i round mass of lidsitic matter, through 

 which are scattered nuicroscopie or microscopic crystals or 



crystalline «;ranules of (piartz and orthoclase 



It seems probable that micro-felsitic matter is simply the result 

 of devitritication ot a jirlassy base." 



Sir A. Geihie.X — Felsite (felstone.) " Originally vitreous lavas 

 like the rhyclites, but which have undergone complete devitrifi- 

 cation, though frequently the i)er]itic, spherulitic, and How 

 structures." 



Teall.% — Acid intrusives=felsophyre, granophyre, micro-granite. 

 Acid volcanic=devitrified rhyolites, obsidians, and pitchstones. 



These samples show that, what with acid intrusives, 

 :acid volcanics, elvans, deTitrified obsidians and rhyolites, 

 granophyres, micro-granites, quartz-f el spathic lavas and 

 the rest, the term has come to mean nothing more precise 

 than a compact ancient acid rock. Hence some petro- 

 graphers abandon its use altogether, and parcel out the 

 Tocks covered by it, some among the ancient rlwolites, the 

 rest among micro-granites. 



A different nomenclature is adopted in Germany. This 

 •family is included by Rosenbusch in Liparites and quartz 

 porphyries. In his recent work J he says : — " The differ- 

 ence between liparite and quartz porphyry is one of age : 

 .quartz porphyries are pretertiary palseo-volcanics, liparites 

 are neo-volcanic tertiary rocks, consequently young quartz 

 porphyries." He defines them as effusive rocks of normal 

 granitic magma, and divides them into two sections, viz,, 

 i(a) microgranitic and gi^anophyric quartz porphyries, 

 (6) felsophyric and vitrophyric quartz porphyries. Under 

 •different names we see there are still two main groups, the 

 microgranitic and the rhyolitic : though, as they are both 

 •classed as effusive rocks, we cannot carrj' out any exact 

 comparison with the English usage. 



It is accordingly necessary to explain the sense in which 

 vs^e attach names to the Tasmanian rocks. While the 

 particular name has a certain importance, it is equally 

 important for us to understand the rocks to which it is 

 applied. Seeing that quartz porphyry is so widely used 



* British Petrography, p. 291. 



t Outlines of Geology, 1888, p.p. lo-2-15:3. 



t Text Book ot Geology, 189:3, p. 161. 



§ British Petrography, p. 290. 



jl Eleniente der Gesteinslehre, 1898, p. 239. 



I 



