ORIGIN OF SOILS 



55 



action. Chemical processes often disintegrate the rock into very 

 fine particles. 



Changes of temperature, moving water, and ice, act mechanic- 

 ally. The chemical agencies are chiefly water and air. Plant 

 and animal life act both mechanically and chemically. 



Changes of Temperature. Changes of temperature act in sev- 

 eral ways. 



(i) Molten rock masses contract on cooling and become per- 

 meated by fissures, cracks, and joints. 



Fig. 10. Rock split by heat and cold. 



(2) Water enters into the cracks between rock masses, and, to 

 some extent, into the pores of the rock. When this water freezes, 

 it expands one-fifteenth of its bulk and exerts a tremendous force. 

 It thus splits up rock masses, and disintegrates rocks which are 

 porous. 



(3) Rocks are usually composed of two or more minerals, 

 which expand differently under the influence of heat. Heat 

 causes the different minerals to expand and to press on one an- 

 other, and cold makes them contract and move apart. In time 

 these movements so impair the coherence of the particles as to 

 cause gradual disintegration of the rock. 



(4) Large pieces have been observed to split off from bare 

 rocks exposed to the sun. This is due to expansion under 

 the influence of heat, and can take place to any extent 

 only on mountain sides where the fragments fall away from the 

 rock surface. 



