64 THE SOIL. 



taining these alkalies. The reason for this must be 

 readily seen, as we have learned that no plant t an arrive 

 atmatiu'itj without the necessary supply of materials 

 required in the formation of the ash, and these mate- 

 rials can be obtained only from the soil ; consequent- 

 ly, when they do not exist there, it must be barren. 



The earthy part of soils has two distinct offices to 

 perform. The clay and sand form a mass of material 

 into which roots can penetrate, and which support 

 plants in their position. These parts also absorb 

 heat, air and moisture, to serve the purposes of growth, 

 as we shall see in a future chapter. The minute 

 portions of soil, which comprise the acids, alkalies 

 and neutrals, furnish plants with their ashes, and are 

 the most necessary to the fertility of the soil. 



GEOLOGY. 



The relation between the earthy parts of soils and 

 the rocks from which it was formed, is the foundation 

 of Agricultural Geology. Geology may be briefly 

 named the science of the rocks. It would not be ap- 

 propriate in an elementary work, to introduce much 

 of this study, and we will therefore simply state that 

 the same kind of rock is of the same composition all 

 the world over ; consequently, if we find a soil 

 in Kew England formed from any particular rock, 

 and a soil from the same rock in Asia, their natural 

 fertility will be the same in both localities. All rocks 

 consist of a mixture of difierent kinds of minerals ; 

 and some, consisting chiefly of one ingredient, are of 



