SOILS. T3 



such as alter the real nature of bodies, as in the 

 rusting of iron and the burning of wood. The rust 

 has different properties from the metallic iron, and 

 the gases and ash formed in burning are very differ- 

 ent from the original wood. In such cases, chemical 

 action has brought about the change. Both these 

 agencies are all the time at work upon the rocks, and, 

 though they are generally slow in their action, vast 

 results are finally produced. 



14. The geologist includes under the term rock 

 all soils and loose material, clays, and gravel, as well 

 as the large solid masses which compose the earth. 

 The disintegrating, or crumbling action by which 

 soils are formed is always going on, plants and ani- 

 mals doing a great deal toward bringing about the 

 changes that are produced. The most powerful 

 agencies in the crumbling of rocks are air and wa- 

 ter. Water not only wears away rocks and reduces 

 them to powder by mechanical action, but it dis- 

 solves them all in a greater or less degree. This 

 solvent action is greatly increased by the air which 

 it contains. Water also soaks into soft rocks and 

 runs into holes and crevices, where freezing it ex- 

 pands, and forces masses and particles of rock asun- 

 der. One great advantage of deep, thorough culti- 

 vation is that air and moisture may penetrate the 

 soil, and continue their disintegrating or crumbling 

 action upon the particles of rock of which the soil 

 is composed, thus setting free the plant-food and pre- 

 paring it as proper nourishment for the hungry plant 



