PRIMER OF PRIMERS. 187 



glaciers move, and thus aid in the formation of 

 soil. 



When water contains dissolved in it oxygen or 

 carbonic acid gas, it may gradually dissolve some 

 of the less insoluble ingredients of the hardest 

 rocks, and thus cause them to become permeable. 



Clayey soils are derived from the disintegration 

 of feldspathic rocks. 



Calcareous soils are derived from the disintegra- 

 tion of limestones. 



Some soils possess the valuable property of ab- 

 sorbing moisture directly from the vapor in the 

 air. Soils containing a large quantity of vegetable 

 humus possess this property in a more marked 

 degree than any others. Clayey soils also possess 

 it to a marked extent. 



The ability of soils to absorb the sun's heat will 

 vary with their color. Dark-colored soils absorb 

 the heat much better than light-colored soils. 



The plants that are found growing naturally in 

 any locality are those which are best fitted to grow 

 in such locality. They will continue to grow 

 naturally only so long as these favorable con- 

 ditions are maintained. 



Forests require for their continued existence a 

 certain character of soil, so that, although even all 



