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AGRICULTURE, &c. 



TILLAGE ; the art of Husbandry as applied to 

 the cultivation of the earth. 



Origin of Soil. 



Soils appear to have been originally formed in con- 

 sequence of the decomposition of rocks, and it fre- 

 quently happens that soils, are found in an unaltered 

 state on the rocks from which they ar derived. It 

 is easy to form an idea of the manner in which rocks 

 have been converted into soils by referring to the 

 instance of soft granite or porcelain granite, a sub- 

 stance consisting of three ingredients, quartz, mica, 

 and feldspar. The quartz is almost pure silicious 

 earth, in a crystalline form. The feldspar and mica, 

 are compound substances ; both contain silica, alu- 

 mina, and iron. In the feldspar there is usually lime 

 and potash ; in the mica, lime and magnesia. 



When a granitic rock of this nature has been ex- 

 posed to the influence of air and water, the lime and 

 potash contained in its constituent parts are acted on 

 by carbonic acid or water, and the oxide of iron which 

 is almost always in its least oxidized state, tends to 

 combine with more oxygen ; the consequence is, 

 that the feldspar decomposes, and likewise the mica, 

 but the first with most rapidity. The feldspar, which 

 is as it were the cement of the stone, forms a fine 

 clay. The mica, partially decomposed, mixes with 

 it as sand, and the undecomposed quartz appears as 

 gravel or sand, of different degrees of fineness. 



After this it is necessary to suppose some of the 



