514 Geology and Magnetism, 



lamination explained in the previous chapter. In the primary 

 rocks we recognise in every crystal the action of the constant and 

 undeviating laws of the polar force and chemical affinity, giving to 

 the mass a regular grain, and to every crystal a definite form and 

 composition. Hence the above may be considered an experimental 

 and natural truth. 



The elementary substances entering into the composition of the 

 primary rocks, may on an average be considered the following : — 

 Silica, Iron, 



Aluminum, Manganese, 



Magnesia, Fluoric acid, 



Potash, Carbonic acid, 



Soda, Water. 



Lime, 

 These are united with variable proportions of the gases, hydrogen, 

 oxygen, chlorine, &c. The compound consists of the above in a 

 state of fluids, semifluids and solids, being an aggregation of the 

 separate elements in different states of crystallization. 



Silica forms Quartz. 



Silica, alumina, lime and potash, Felspar. 



Silica, alumina, potash and iron, Mica. 



Silica, magnesia and potash, Talc. 



Silica, alumina, magnesia and iron, Chlorite. 



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 proportions. 

 Numerous appellations have been from time to time suggested 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 irregularly dis- 

 seminated, and possess no distinct lines of demarcation. However, 

 in order to have some idea of their variable character, the following 



