GEOLOGY. 311 



identified, are those of its mineral composition, tex- 

 ture, the relative disposition of the component mi- 

 nerals, (if a compound rock,) its internal structure, 

 and natural divisions. 



The colour, lustre, fracture, hardness, fusibility, 

 and specific gravity of rocks, are obviously deter- 

 mined by their qualities of mineral composition and 

 texture, and must vary with them ; they are conse- 

 quently secondary qualities, not characteristics. Of 

 the primary characters, mineral composition is the 

 most important. 



Granite. 



A primary unstratified* rock, consisting of fel- 

 spar, quartz, and mica. It is found almost invaria- 

 bly under all the others, yet it occupies the highest 

 points of the earth's surface. It never contains any 

 organic remains. The felspar of granite is usually 

 white, and most commonly greyish yellowish white ; 

 also reddish, or milk white, or flesh red.f It is sel- 

 dom grey, yellowish, or green. 



The quartz is usually grey, seldom milk white, 

 and usually translucent. 



The mica is commonly grey, and sometimes dark 

 brown, or nearly black. 



The felspar of granite has usually a vitreous lus- 

 tre, and perfectly foliated fracture; yet in some va- 

 rieties it passes into earthy, with loss of its lustre and 

 hardness, to the state of porcelain earth. 



The mica of granite is also sometimes decomposed 

 by exposure to the atmosphere, but the quartz is 

 never altered. 



* The unstratified rocks, under a great variety of form, may 

 be all reduced to the two grand divisions of granite and trap. 

 Granite may be called the backbone of the earth, and from it all 

 the hottest springs issue. 



t This conveys no clear idea of the substance, and shows the 

 difficulty of rendering the subject intelligible by mere description. 



