40 COMPOSITION OF SIMFLE MINERALS. 



Mica, another mineral found in granite, gneiss and mica slate, has a composition much 



like thai "i the felspars, 01 al least is analogous to them, as containing two alkalies, potash 



ami magnesia ; thus, 



Potassic nil \. Magnesias mica. 



Silica 4G.10 40.00 



Alumina 31.60 12.07 



1' il I iron, 8.65 19.03 



Potash 8.39 



Magnesia, 15.70 



Together with a variable proportion of oxide of manganese and fluoric acid. 



Hornblende, which often replaces mica in the granites, is usually a dark green substance, 

 and extremely lough in the mass. It !■- commonly crystalline, and more or less fibrous. 

 It differs essentially from the micas and felspar, in containing larger proportions of lime. 

 It consist^ of 



Horn in. i \m . 



Silica 42.24 



Alumina, 13.92 



Lime, 12.24 



Ma piesia 13.74 



Protoxide of iron, 14.59 



le of manganese, 0.33 



Fluoric acid, 1.50 



98.56 



All these substances are termed silicates; the silica uniting with each of the principal 

 element- a- aii acid, and forming thereby silicates of alumina, potash, magnesia, and iron. 

 In the northern a- well as the southern highlands, pyroxene or augite enters Largely into 

 the constitution of the primarj rocks. lis composition does not differ materially, so far as 



:t- effects upon a soil is concerned, from that of hornblende ; thus, 



Pyro.vi \ I 

 Light-ct.l'ued. Dark eoloiad. 



Silica, 55.32 54.08 



Lime, 27.01 23.47 



Magnesia 16.99 11.49 



Protoxide of iron,... 2.16 10.02 



Alumina, 0.28 0.14 



Manganese, 1.59 0.61 



103.15 99.67 Rose. 



To the same famil) belongs the hypersthene, which gives name to the rock forming the 

 highest grounds I • ••. . namely, hypersthene rock. This substance contains less lime 

 than hornblende or augite, and hence i- less favorable as an element of soil; in fact, it is 

 remarked, that when- it exists in sufficient abundance to influence the nature of the soil, 

 n is quite barren. It is composed of 



