BOCKS CHANGED INTO SOIL. WEATHERING. 81 



penetrates their surface and moistens and softens them. 

 Frost turns this moisture into innumerable little wedges 

 of ice, which split the thin outer coat of the rocks into 

 minute fragments. The hardest rocks are thus gradually 

 crumbled into dust. 



Besides these agencies, oxygen is constantly acting. So 

 are other gases ; and so are carbonic acid and other acids, 

 and lime, and the salts of potash, and other salts. These 

 are dissolving, disintegrating and crumbling the rocks; 

 and water, in streams and torrents, is constantly rubbing 

 off and dashing together the fragments. 



All these causes are still and constantly acting, not only 

 upon the surface of the great rocks, but upon the surface 

 of the particles of the soil in the cultivated or unculti 

 vated fields. The ceaseless action of all these and of 

 other forces is called weathering. 



280. The important question with the farmer is, Which 

 is the best soil ? Neither of the three kinds of earth 

 spoken of forms by itself a good soil. Indeed, each, by 

 itself, forms a soil absolutely barren. The best natural 

 soil is one formed by the due mixture of all the three, 

 the bad qualities of each being corrected by the good 

 qualities of the others. 



The chemical analysis of a vast number of soils shows 

 that the most fertile arc those into which these three 

 important classes of elements enter abundantly, but not 

 in equal quantities ; and that the fertility diminishes just 

 in proportion as any one of the three comes near to be 

 exhausted. 



281. All the innumerable soils have essentially the 

 same elements. Clay, lime and sand are the basis of all. 

 But soils vary as one or another of these prevails, or as 

 one or another is wanting. 



