ORIGIN OF SOIL 91 



is more conspicuous at high altitudes than at low ones. 

 Why? 



III. Chemical Action. The changes in rock just de- 

 scribed are physical changes. There are also certain chem- 

 ical changes constantly occurring which also contribute 

 to the reduction of rock to soil. Chief of these is the proc- 



FIG. 39. Talus slope. 



ess of oxidation, which is the chemical union of oxygen 

 with any other substances. Similarly, hydration is the 

 chemical union of water with any other substance. The 

 rusting of iron includes both oxidation and hydration. 

 Now many rocks contain iron. When this iron is exposed 

 to the atmosphere, especially to a damp atmosphere, it 

 becomes changed to iron-rust, and this tends to make the 

 rock crumble. 

 You have learned (seepage 52) how water may " act on" 



