THE MINERALS IN GRANITE. 215 



Riesengebirge, the orthoclase is flesh-red and the oligoclase snow- 

 white, as in some Scotch granites ; and at Viborg, in Finland, the 

 orthoclase is flesh-red and the oligoclase green. Though in such 

 cases the oligoclase is formed subsequently, yet in the granite of 

 Beyrode in the Auvergne, the two minerals alternate in the same 

 crystal, or the oligoclase may have been first formed. 



Albite also has been recognised in granite, especially in the Mourne 

 Mountains, and the chemical composition appears to indicate the exis- 

 tence of labradorite and other kinds of felspar in some localities, espe- 

 cially in Ireland, and at Strontian. 



Quartz usually occurs in more or less angular grains, but not often 

 with the crystalline faces perfectly developed. It varies in colour 

 like the felspar, being blue in Monte-Rosa and sometimes blue in 

 the Mourne Mountains, and red in the Jagernthal in the Vosges, 

 though commonly colourless. At Gablonz in Bohemia the quartz 

 crystals are larger than the orthoclase. 



The mica generally occurs in thin plates which are often hexagonal. 

 Crystals are rare. It varies in colour, being silvery white, brown, or 

 black. The white potash mica is more diffused than the black mag- 

 nesian mica, which is brown or green in polarised light. Both kinds 

 often occur together. At Penig, in Saxony, the mica is olive-green. 

 Certain large-grained granites contain lithia mica. Haughton recog- 

 nises the silver-grey mica margarodite in the granite of the south-east 

 of Ireland, and other varieties of the mineral in other localities. As 

 is well known, these minerals are usually mixed together without any 

 trace of a schistose arrangement. 



Mica is the most variable, element in granite. It is more or less 

 replaced in the Alps and some parts of the Schwarzwald by talc. In 

 the granites of the Pyrenees graphite is associated with mica. Some- 

 times hornblende is associated with magnesia mica, forming a syenitic 

 granite. Apatite is generally present. Zircon is occasionally found. 

 In porphyritic granite large crystals are only formed by the orthoclase : 

 the well-known Carlsbad twins are among the finest examples of these, 

 sometimes reaching in the Pyrenees a length of six inches. Occasion- 

 ally the crystals are broken and reunited. 



Granite is usually compact, but sometimes porous, and in South 

 America allows water to pass through it so freely as to be used for 

 filters. And it may contain cavities which have the walls covered 

 with crystals of the chief constituent minerals as well as various acces- 

 sory minerals, as may be seen at Lugano, Baveno, Mourne, and in 

 many localities for European granites. 1 



Chemical Variation in Composition. The silica in granite varies 

 from 62 to 82 per cent. ; the alumina from 7 to 19 per cent. ; the iron 

 oxides from less than a quarter per cent, to 6 per cent. ; lime from 

 13 per cent, to 5-5 per cent. Magnesia may amount to 2 per cent., 

 or show little more than a trace; potash varies from 2 to 7 per cent., 



1 For a list of the accessory minerals found in the granites of different locali- 

 ties the student should consult Zirkel's " Le irbuch der Petrographie," vol. i 

 p. 481. 



