AGRICULTURAL TEXT-BOOK. 25 



76. The principal rocks whose decay has given rise to the formation 

 of soil may be briefly enumerated as follows : (Mulder.) 



(a,) Quartz Rocks; (ft,) Feldspar Rocks; (c,) Mica Rocks; (d.) 

 Hornblende Rocks ; (e,) Serpentine Rocks; (/,) Augite Rocks ; (#,) 

 Alumina Rocks ; (A,) Lime Rocks; (i,) Gypsum ; (j,) Iron. 



77. The value of Irrigation for the purpose of manme depends upon 

 the geological formation through which the stream passes. 



79. Soils differ much in the relative quality of the substances 

 which compose them ; but all fertile soils contain the same ma 

 terials. 



80. The great bulk of all such soils consists of three earths : 

 (a,) Silica, (or sand,) (b,) Alumina, (or clay,) (cj Lime. Gen 

 erally, these lie in juxtaposition, inert, and producing no effect 

 on each other. To these must be added (d,) Water, which is 

 always present uniting them. Deprived of water, soil becomes 

 dust ; but the water varies in its relative quantity according to 

 the proportionate mixture of these three materials. 



81. The economical character and usefulness of the soil varies 

 in proportion to the predominance of one or other of these sub 

 stances. 



82. On a general average, the earthy part of the soil consti 

 tutes about 96 per cent, of its whole weight when free from 

 water. (Johnston.) 



83. By Silica, is meant siliceous sand or siliceous gravel of various 

 degrees of fineness from an impalpable powder to sand-stones. By Clay 

 a finely divided chemical compound consisting very nearly of 60 per 

 cent, of Silica and 40 per cent, of Alumina, with a little oxide of iron, 

 and from which no sandy matter can be separated mechanically or by 

 decantation. By Lime Carbonate of Lime, in the shape of chalk, lime 

 stones, marl, shell-sand, (fee. 



84. Soils may be classified as follows : 



(a,) Pure Clay, pipe day, = 60 per cent. Silica, 40 per 

 cent. Alumina, oxide of iron chemically combined, (b,) Strong 

 est clay soil, brick clay, pure clay (a,) with 5 to 10 per cent, 

 of sand which can be separated by boiling and decantation. 

 (cj) Clay loam = 15 to 30 per cent, fine sand and pure clay. 



