M ON FELSITES AND ASSOCIATED ROCKS. 



Teall.*— Acid rocks=" Felsite, Eurite petrosilex. These terms are 

 practically synonymous. They have been applied to compact 

 stony rocks, the raineralogical composition of which cannot be 

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

 J^as. Gelkle.f — Qudrtz-telsite=Quartz-porphyry. " In this rock 

 we have a compact groundmass of felsitic matter, throug'h 

 which are scattered macroscopic or microscopic crystals or 



crystalline granules of quartz and orthoclase 



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

 of devitrification ot a glassy base." 



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

 like the rhyolites, but which have undergone complete devitrifi- 

 cation, though frequently the perlitic, spherulitic, and flow 

 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, devitrihed obsidians and rhyolites, 

 :granophyres, micro-granites, quartz-felspathic lavas and 

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

 -than a compact ancient acid rock. Hence some petro- 

 gi-aphers abandon its use altogether, and parcel out the 

 irocks covered by it, some among the ancient rhyolites, the 

 rest among micro-granites. 



A different nomenclature is adopted in Germany. This 

 ifamily is included by Rosenbusch in Liparites and quartz 

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

 ■enee 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 granophyric quartz porphyries, 

 t(&) 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 p]nglish usage. 



It is accordingly necessary to explain the sense in which 

 we attach names to the Tasmanian rocks. While the 

 particular name has a certain importance, it is equally 

 im.portant for us to understand the rocks to which it is 

 applied. Seeing that quartz porjDhyry is so widely used 



* British Petrography, p. 291. 



t Outlines of Geology, 1888, p.p. 152-1 53. 



t Text Book ot Geology, 189a, p. 161. 



§ British Petrography, p, 296. 



jl Elemente der Gesteinslehre, 1898, p. 239. 



