40 COMPOSITION OF SIMPLE MINERALS. 



Mica, anotlier mineral found in grnnito, gneiss and mica slate, has a composition much 

 like that of the felspars, or at least is analogous to them, as containing two alkalies, potash 

 and magnesia ; thus, 



POTASSIC MICA. MagNESIAN MICA. 



Silica, 46.10 40.00 



Alumina, 31.60 12.07 



Protoxide of 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 suhstance, 

 and extremely tough in the mass. It is commonly crystalline, and more or less fibrous. 

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

 It consists of 



Hornblende. 



Silica, 42.24 



Alumina, 13.92 



Lime, 12.24 



Magnesia, 13.74 



Proto.xide of iron, 14.59 



Oxide of manganese, 0.33 



Fluoric acid, 1.50 



98.50 



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

 elements as an acid, and forming thereby silicates of altimina, potash, magnesia, and iron. 

 In the northern as well as the southern highlands, pyroxene or augite enters largely into 

 the co!istitution of the primary rocks. Its composition does not differ materially, so far as 

 its effects upon a soil is concerned, from that of hornblende ; thus, 



Pyroxene. 

 Light-colored. Dark coloied. 



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, I.59 0.6I ^ 



103.15 99.67 Rose. 



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

 highest grounds of Essex, namely, hypersthene rock. This substance contains less lime 

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

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

 it is quite barren. It is composed of 



