CHAPTER VIL 



' COMPOSITION OF VARIOUS HOCKS. 



Granite. Schists. Gneiss. Serpentine. Baealt. Pitchstone. 

 Obsidian. Pumicestone. Sandstones. Limestones. Dolomite. 

 Clays. Nature of certain minerals in igneous and metamorphio 

 rocks ; quartz; felspar; mica; talc; chlorite; hornblende; augite ; 

 olivine. Matrices of veins ; quartz ; fluor-spar ; calc-spar. 



Granite. 



Composed of quartz white, black, grey, &c. in rather 

 irregular grains ; mica, silvery white ormetallic black (some- 

 times replaced by hornblende) ; potash felspar of a white, 

 pink red, or yellowish colour, and crystallized. Contains 

 70 per cent, silica, with alumina, lime, magnesia, alkalies, 

 oxide of iron, &c. ; or 40 per cent, felspar, 30 to 40 per 

 cent, quartz, 10 to 20 per cent. mica. 



In foliated granite the grains are arranged in layers. In 

 graphic granite the felspar is arranged in the quartz, or the 

 quartz in the felspar, something like the letters in Oriental 

 writing. Micaceous, quartzose, f elspathic granite are varieties 

 in which mica, quartz, felspar respectively predominate. 

 Syenite is a variety of granite free from quartz, and chiefly 

 composed of hornblende and potash felspar. 



Porphyry is a compact f elspathic rock of the nature of 

 granite, having felspar crystals, mica, quartz, chlorite, &c., 

 imbedded in it, which give it a speckled appearance. 



Schists. * 



Mica schist consists of fine layers of quartz and mica ; talc 

 schist consists of fine layers of quartz and talc ; chlorite 



* It is of the utmost importance for the prospector to become 

 thoroughly acquainted with the appearance of these, as a schist 

 country is always worth prospecting. 



