ORIGIN OF SOIL 89 



which covers the surface is called mantle rock. It forms 

 a sort of mantle over the solid rock beneath. 



The weathering of rock, which leads to the formation 

 of soil, is caused by various things, and it proceeds at 

 various rates in different places. Some of the principal 

 factors in rock weathering are the following: 



I. Water. You have already noted what a large part 

 running water plays in the distribution of soil. It also 

 reduces pieces of rock to sand and clay by grinding them 

 together. This is quite evident to any one who has watched 

 the effects of water when streams are running swiftly or 

 waves are pounding on the beaches. But there is quite 

 another and an earlier part which water plays in the 

 origin of soil, a part in which movement of the water is not 

 necessary. You know that water expands when it freezes. 

 It expands about one- tenth of its volume. In this ex- 

 pansion it exerts great force. You have seen how this 

 force may cause the bursting of pipes. Similarly, it 

 may cause the bursting of rocks. Crevices in rock may be 

 filled with water. A freeze comes and the water changes 

 to ice. The expansion that results may have force enough 

 to enlarge the crevices, splitting the rock still farther. 

 After a thaw, more water runs in. Then the process may 

 be repeated. So, with alternate freezings and thawings, 

 great masses of rock may be broken up, and this process 

 will be effective wherever water can get into crevices. 

 Evidently it will be most effective where changes from water 

 to ice, and the reverse, are most frequent. So in some re- 

 gions this process produces much larger effects than in 

 others. 



II. Expansion and Contraction. You know that in 

 general heat causes substances to expand while cold causes 



