ORIGIN AND FOBMATION OF SOILS. 21 



of different sizes. The force of the water in rains and 

 streams grinds the fragments together continually, wear- 

 ing them smaller, and the smallest are first carried away 

 by the water to lower levels. 



The overflow of rivers always leaves a deposit of soil 

 particles, more or less finely divided, carried in suspen- 

 sion in the water; this sediment gradually accumulating 

 forms soil. What are termed by geologists " sedimentary, 

 or aqueous, rocks " have also been formed by the action 

 of water, followed by heat or pressure, or both. 



Water also has a decided effect chemically. The 

 carbonic acid in the water absorbed from the atmosphere 

 dissolves certain of the chemical ingredients of the soil, 

 particularly lime, though its solvent effect is not confined 

 to this substance; soda, potash, silica, and iron are also 

 attacked to a greater or less degree. 



The Growth of Plants. — The growth of plants is 

 also active in the rotting of rocks. In their growth 

 the roots penetrate the crevices and force the particles 

 of rock to separate ; they also attack and absorb certain 

 portions of the constituents that are made soluble. In 

 the decay of plants, the soil is kept moist, gases are 

 generated and absorbed by the water, which, again pene- 

 trating the rocks, has a solvent effect upon them. 



It is a matter of common observation to see rocks 

 covered with the lower orders of plants, like lichens 

 and mosses ; the removal of these frequently shows the 

 rocks furrowed and eaten into by the roots, due to the 

 action described. This growth and this decay of vege- 

 table life, continued through a long series of years, gradu- 

 ally deepen the soil, and prepare it to produce the higher 



