77 

 APPENDIX TO GROUP D. 



Here belong the Liebenerite-porphyry (altered phonolite) and possibly 

 also certain Teschenites. 



GROUP E. 



ROCKS NOT INCLUDED IN ANY OF THE PRECEDING GROUPS. 



Notwithstanding the fact that the preceding classification has been 

 made as elastic as possible, so that many mineral-aggregates, not actually 

 mentioned, readily find a natural place within it, yet it does not include all 

 known igneous rocks. It becomes necessary therefore to recognise another 

 group for rocks which cannot be placed in any of the preceding. As 

 illustrating this group we may mention the following : 



Limbnryite (Roscnbmch) = augite, olivine, magnetite, and a glassy 

 base. 



Pi/roxenitc (Docltcr) , (l) Magma-basalt (BMeky) = augite, mag- 

 netite, and glassy base. 



GROUP F. 



VITREOUS ROCKS. 



Rocks almost wholly composed of glass have no place in a purely 

 mineralogical classification. They may be classified according to chemical 

 composition and physical characters. As the glassy rocks are often only 

 local variations of the crystalline and semi- crystalline products of volcanic 

 action it seems most convenient to designate them by means of the terms 

 applied to the latter, in the manner proposed by Messrs. JUDD AJSID COLE. (S) 

 Thus we have liparite- glass, trachyte-glass, andesite-glass, phonolite-glass, 

 basalt-glass, &c. 



The liparite- glasses are the most abundant in nature. They are divided 

 into two main groups according to lustre and fracture. 



(1) Vitreous lustre and conchoidal fracture ; Obsidian 



(2) Resinous lustre and splintery fracture ; Pitchstone. 



Subordinate varieties depending on the presence or absence of perlitic, 

 spherulitic and other structures are also recognised. 



Glasses of basic composition are often termed tachylytes. They usually 



(1) Die Vulcane der Capverden. Graz, 1882. 



(2) Petrographische Studien an den Basaltgesteinen Bohmens. Prag., 1874. 



(3) On the basalt-glass of the Western Isles of Scotland. Q.J.G.S.. Vol. XXXIX., p. 444. 



