194 ELEMENTS OF GENERAL SCIENCE 



freeze, we may imagine the enormous amount of work that is 

 done in breaking up rock. These broken pieces help to make 

 the soiL 



217. Other changes due to water. Rock often contains sub- 

 stances, as lime, salt, or sulphur, which may be taken up in 

 solution in water. If water has filtered for a long time through 

 rock which contains soluble substances, the rock is made por- 

 ous by the removal of these substances. Great caves, as the 

 Mammoth Cave, Kentucky, have been made in this way. If 

 the water is heated (as sometimes occurs within the earth's 

 crust) or if it contains acids, its dissolving effects are much 

 greater. 



Streams and waves of water are also important factors in 

 disorganizing rock. In streams, stones are rolled, turned, and 

 ground over one another and constantly worn thereby. In 

 swift-flowing mountain streams very large, rocks are torn 

 from their positions and rolled over and over, and often car- 

 ried down to levels where the current is not so swift. From 

 the large rocks smaller pieces are broken and then are carried 

 down the stream to places where the current is much less 

 swift than where the larger rocks are left. From the mouth 

 of a stream to its mountain source, soil and rock particles 

 furnish an index of the carrying power of the stream, and this 

 carrying power indicates the wearing power of the stream. 

 Many of the finer rock particles may be carried by the 

 stream until it reaches the large body of water into which 

 the stream empties sometimes the ocean. When streams 

 overflow their channels, there is an abundant deposit of fine 

 soil particles that were carried by the water. 



Along the shores of lakes and in the riffles of streams there 

 may constantly be seen the process of wearing rocks into smaller 

 and smaller particles. Also, this same action of the water sorts 

 the materials into the different sizes, and one may find these 

 sizes assorted in definite regions as he passes from the shore 

 into deep water. 



