282 ELEMENTS OF GENERAL SCIENCE 



Along rocky cliffs where water runs into crevices and 

 freezes, one may often observe its effects in breaking up the 

 rock (fig. 137). Tons of rock may be pushed aside a short 

 distance by the expansive force exerted through the freez- 

 ing of a small amount of water. When we think of all the 

 small crevices in rock into which water may run and after- 

 wards freeze, we may imagine the enormous amount of work 

 that is done in breaking up rock. 



283. Disintegration of rock by water. Rock often contains 

 substances, such as lime, salt, or compounds of sulphur, which 

 may be taken up in solution in water. If water has filtered 

 for a long time through rock which contains soluble sub- 

 stances, the rock is made porous 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, rolled over and over, and often carried 

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

 large rocks smaller pieces are broken and are then carried 

 downstream 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 show 

 the carrying power and 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 



