40 
COMPOSITION OF SIMPLE MINERALS. 
Mica, another mineral found in granite, 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 
thus, 
Potassic mica. Magnesian mica. 
and magnesia ; 
Silica.___ 
.. . 46.10 
40.00 
Alumina,_ 
_ 31.60 
12.67 
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 substance, 
and extremely tough in the mass. It is commonly crystalline, and more or less fibrous. 
It differs 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 
Protoxide of iron._ 14.59 
Oxide of manganese, 
Fluoric acid,_ 
0.33 
1.50 
98.56 
All these substances are termed silicates; the silica uniting with each of the principal 
elements as an acid, and forming thereby silicates of alumina, potash, magnesia, and iron. 
In the northern as well as the southern highlands, pyroxene or augite enters largely into 
the constitution 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. 
Silica,_ 
Light-colored. 
.... 55.32 
Dark colored. 
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 
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 
