566 ERUPTIVE AND METAMORPHIC ROCKS. 



Eruptive Rocks containing over 6o per cent, of Silica. 

 (Acid Rocks.) 



Chief Mineral Components or Lithological Affinities. 

 /Granite.'^ — Orthoclase (sometimes Plagioclastic Felspars, as oligoclase and 



microcline), Mica (sometimes Potash Mica, sometimes Magnesian Micas, 



and frequently both). Quartz. Schorl, Sphene, Magnetite and Pyrites, 



etc., etc., often present. 

 Pegmatite or Graphic Granite — Coarsely crystalline varieties in which the 



quartz often assumes a peculiar arrangement within the felspar, which 



rudely resembles Oriental writing. Frequently poor in, or destitute of. 



Mica. 

 Syenitic Granite. — A rock similar to ordinary Granite, except that it contains 



Hornblende, often in considerable quantity. This rock is frequently 



designated Syenite, or, more correctly, Quartz-syenite." 

 Syenite. — Orthoclase and Hornblende. 



Granitlite or Leptinite. — Orthoclase and Quartz, usually with garnets. It 

 -a I frequently has a foliated or fissile structure, and hence is regarded 



as a metamorphic rock, and is classed with those rocks which present 



a schistose character. 

 Felstone or Felsite. — Orthoclase and Quartz, occurring as a fine admixture, 



the separate minerals not being discernible by the naked eye, and 



scarcely by microscopic examination. Such a substance is termed 



felsitic. The devitrification of Obsidian results in felstone. Hallefluita 



may be included with the Felstones. 

 Felspar Porphyiy? — A felsitic matrix, containing distinct crystals of 



Orthoclase. 

 Minette or Mica Trap. — A felsitic matrix, containing crystals of Magnesian 



Mica and small Orthoclase crystals, usually with some Magnetite or 



Titaniferous Iron, etc.* 

 Quartz- Porphyry or Quartz- Felsite. — A felsitic matrix, containing distinct 

 s crystals of Orthoclase and Quartz. (El van.) 

 ' G7ieiss. — A foliated rock, having the same mineral composition as granite. 



It frequently graduates into sedimentary rocks, and hence is regarded 



as a metamorphic rock. 

 Phonolite. — Orthoclase (Sanidine), Nepheline, Hornblende, Hauyne, 



Nosean, Augite, Plagioclastic Felspar, Pyrites, etc., often present. 

 Trachyte. — Sanidine and Plagioclastic Felspars, Hornblende, Augite, and 



Magnesian Mica — frequently Tridymite. Rhyolite is sometimes in- 

 ^ i eluded in this group. ^\\&\q 2i\e. 2\i,o Quartz- Trachytes ox Dacites. 



Pitchstone. — Variable in composition, sometimes containing more and 



sometimes less than 60 per cent, of Silica. As a rule Pitchstone 



approximates to the felspars in composition. It often contains felspar 



and magnesian mica, crystals and microliths of augite, and other 



minerals. On analysis Pitchstones afford a considerable amount of 



water. 

 Obsidian. — A more vitreous condition of Pitchstone ; it usually contains 



microliths, chiefly of felspar.^ 



^ The red or grey colour usually depends upon the colour of the felspar. 



^ The name Syenite was originally given by Pliny to the hornblendic granite of 

 Syene in Egypt. 



^ Porphyries are generally known by the name of the larger disseminated 

 crystals as Quartz Porphyry ; but in other cases we speak of Porphyritic Granite. 

 The term Porphyry is now almost obsolete. 

 J^ Minette may be regarded as a mica-syenite. The term mica-trap also includes 

 Kersantite, which differs from minette in that the felspars are plagioclastic, and 

 it is therefore allied to mica-porphyrite. 



* Since Pitchstones, Obsidians and other vitreous rocks frequently exhibit perlitic 

 structure, it has been proposed to abandon the term Perlite. Q. J. xl. 345. The 

 term Perlitic is applied to delicate spheroidal cracks that often occur in vitreous and 

 in devitrified lavas. 



