GEOLOGY OF AGRICULTURE. fit 



and other less extensive unstratified rocks ; and it en- 

 ters largely into the composition of the ocean. Lime 

 and magnesia are diffused almost universally among 

 the rocks, in the form of silicates and carbonates — the 

 carbonate having been estimated to form one-seventh 

 of the crust of the globe. At least three per cent, of 

 all known rocks are some binary combination of iron, 

 such as an oxide, a sulphiiret, a carburet, &c. Man- 

 ganese is widely diffused, but forms much less than 

 one per cent, of the mass of rocks." — (Hitchcock^s 

 Geol.^ ]), 45.) 



90. The foregoing is rather a geological than a 

 chemical view. Most of the substances spoken of ex- 

 ist in rocks and soils, as ternary compounds. Says 

 Dana (Muck Manual, p. 56 — an unpretending name, 

 but an excellent book), "Viewed in the light of chem- 

 istry, rocks are masses of silicates. The simple mine- 

 rals composing rocks are truly only silicates in fixed 

 proportions. The simple minerals are quartz, felspar, 

 mica, hornblende, talc, serpentine." 



91. According to this same author, chemically de- 

 fining the above minerals, quartz is nearly pure silica; 

 felspar and mica are silicates of alumina and potash ; 

 hornblende is silicate of alumina and lime, with mag- 

 nesia ; and talc and serpentine are silicates of magnesia. 

 Thus it will be seen that silex, silica, or silicic acid, as 

 unfortunately it is variously called, forms a very 

 prominent part of the principal minerals, with the' ex- 

 ception of carbonate of lime ; and consequently of ail 

 rocks, except lime-stone ; and then, as another conse- 



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