NATURE AND FORMATION OF SOILS. 33 



minerals, they are fir ir.ore floiirisliino;- in the 

 presence of deca}!:!^ organic matter. Indeed, 

 their action is l)elieved to hasten the decompo- 

 sition of organic material. So it is that the 

 plant, by its own decomposition, is through 

 these agencies made to contribute to the forma- 

 tion of hunius, which is an essential part of true 

 soil. 



(3) Vegetable Accumulations or Deposits. 

 — Not only living plants exert an influence 

 upon the soil, but when they die their remains 

 form, though very slowly to be sure, accumula- 

 tions of veofetable matter. 



(a) True soil. — Vegetable accumulation is 

 most important as well as most conspicuous as 

 a mantle of true soil, formed from the decayed 

 vegetation in the forests or grass - covered 

 prairies. 



(/?) Wherever vegetation slowly undergoes 

 decomposition under water carbonaceous accii- 

 vitilations are formed. The further decompo- 

 sition proceeds the greater the per cent, of car- 

 bon ; thus results peat, lignite, bituminous, or 

 anthracite coal, according to the stage of de- 

 composition reached. 



{f) In fresh-water lakes and ponds, as well as 

 in the sea, the siliceous cases of microscopic 

 plants known as diatoms form considerable ac- 

 cumulations. 



I. A7iimal Life. — Animals have a twofold 



