Geology and Magnetism. 515 



may be enumerated as very common compounds in Europe and 

 America : — 



Micaceous granite, Mica predominating. 



Chloritic „ Chlorite „ 



Talcose ,, Talc „ 



Hornblendic „ Hornblende ,, 



Quartzose ,, .. Quartz „ 



Felspathic „ Felspar „ 



Porphyritic „ Felspar in excess with crystals of 



Felspar in a base. 

 Felspar in general constitutes by far the largest part of granite : 

 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 contemporaneous- 

 ly formed, accidentally modified by an admixture of different in- 

 gredients : — 



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. 



In these masses are veins of the predominating substances of the 

 enclosing rocks. 



The granite being the fundamental base, or the crystalline shell 

 of the globe, its thickness is not known. It has a polar structure, 

 and when the quantity of mica is considerable, granite divides into 

 parallel plates, or in other words becomes laminated, and exhibits 

 the meridionally structure explained above. 



Gneiss is the laminated part of the granitic base, the same 

 identical mass ; the distinction being produced by the ingredients 

 tending to arrange themselves in parallel plates; quartz follows 

 quartz, felspar follows felspar, and mica follows mica. (See Plate 

 VI.) 



As this crystalline arrangement and lamination of the funda- 

 mental base is produced by the continual circulating action of the 

 magnetic currents through the semifluid mass, the transition of the 



