98

FORMATION OF SOILS.

dependent on the subsoil and stony matters which areburied beneath the surface, many of which are slowlydecomposing or crumbling away, and adding to thesoil the substances of which they consisted.

300. This gradual decomposition of stony particlesin the soil is caused by the action of the air. Many ofthe common rocks are compounds of several differentearthy and alkaline substances, in which silica, unitedto lime, alumine, potash, and soda, forms silicates ofthose or similar bases.

301. Silicates of this kind, or natural compoundscontaining silica in combination with several earthy andalkaline bases, are quite insoluble in water, and arescarcely acted on even by the strongest acids ; neverthe-less they gradually decompose when exposed to the air.Under the joint action of the moisture and carbonic acidin the air, these compounds are disintegrated, carbonateof potash or soda is formed, and in part washed awayby the rain, whilst the insoluble earthy bases are leftin the form of a very fine powder.

302. The most abundant constituent of soil is com-monly silica, which frequently forms nearly nine-tenthsof the whole of its weight ; but this is by no means al-ways the case, for in calcareous or limestone countries wefrequently find soils containing a very large quantity oflime ; whilst others again contain a large proportion ofalumine. These differences in the proportion of theearthy components of the soil give rise to the varietiesof light or free, and stiff or clayey soils, which are also