9 2 ELEMENTARY SCIENCE 



limestone and sandstone. This action is a combination 

 of solution and hydration. You have noted its effects. 

 Similar effects are produced by the oxidation and hydra- 

 tion of iron in rock; the rock becomes weakened and 

 presently crumbles. Thus we see how a chemical change 

 may also produce physical change as one of its results. 



Other chemical changes produce similar results. Some 

 of the substances which result from chemical changes in 



FIG. 40. Talus slope. 



the rocks are thereby rendered soluble in water, while 

 other minerals are soluble without chemical change. So 

 we see that the combination of chemical change and solu- 

 tion is one of the most important of the causes of the 

 breaking down or disintegration of rock. As rock breaks 

 down, soil builds up; in other words, rock disintegration is 

 soil formation. 



IV. Organisms. You have learned that plants and ani- 

 mals contribute directly to soil formation, their dead parts 

 forming humus or mould. But they also contribute in- 

 directly by being an important factor in this great process 



