38 



Silica, alumina, magnesia, lime and iron . Hornblende. 



Silica, alumina, magnesia, potash and iron Schorl. 



Lime and carbonic acid Carbonate of lime. 



Besides the above compound ingredients, there are also dis- 

 seminated in the primary mass all the known mineral substances ; 

 these may likewise be compounds, of which hydrogen forms a 

 part. 



Granite may be considered as the fundamental crystalline 

 base, a compound of the above ingredients in variable propor- 

 tions. Numerous appellations have been from time to time sug- 

 gested for the different kinds of granites, but it is very evident 

 that such distinctions cannot be established, inasmuch as the 

 variety of crystals constituting the granitic masses are very ir- 

 regularly disseminated, and possess no distinct lines of demar- 

 cation. However, in order to have some idea of their variable 

 character, the following may be enumerated as very common 

 compounds in Europe and America : — 



Micaceous granite .... Mica predominating. 



. Chlorite „ 



. Talc „ 



. Hornblende „ 



. Quartz „ 



. Felspar „ 



. Felspar in excess with crystals of 

 felspar in a base. 



Felspar in general constitutes by far the largest part of gra- 

 nite : it is often in a soft and fluid state, and in small and large 

 grains. 



The following varieties of granitic rocks are often associated 

 in the same mountain mass, and may be regarded as contempo- 

 raneously formed, accidentally modified by an admixture of dif- 

 ferent ingredients : — 



Common granite ; the felspar white or red, composed of 

 quartz, felspar and mica. 



Chloritic granite ; quartz, felspar and chlorite. 



Felspathic granite, in which felspar is the principal ingredient, 

 and the quartz, and particularly the mica, very rare, with large 

 crystals of felspar. 



Chloritic „ 

 Talcose „ 



Hornblendic „ 

 Quartzose „ 

 Felspathic „ 

 Porphyritic „ 



