362 THE WORK OF RUNNING WATER 



The destructive and constructive actions of water are con- 

 stantly at work. In some places the destructive action is more 

 prominent, in other places the 'constructive action; but always 



FIG. 228. 'Veins of calcite in a volcanic rock. 



the result is to change the character of the original substances 

 and to modify the land. 



Streams wear away the land. Small streams and rivers 

 work more rapidly than ground water and are more effective 

 in producing visible changes in the land. Rivers swollen by 

 heavy rains and spring thaws sweep away bridges, railroad 

 tracks, and houses, destroy thousands of acres of crops, uproot 

 whole groves of trees, tear away banks, cut out rocky slopes, 

 and form new channels. But without streams there would be 

 no inland navigation, thousands of mills run by water would 

 be shut down, irrigation would be unknown, the air would 

 become dry for lack of moisture, and existence on the earth 

 would be impossible. 



When rain runs down a slope, it picks up soil particles and 

 carries them to the streams into which it flows, making the 

 streams muddy for miles. In some parts of the country all 

 loose soil has been torn from hillsides by heavy rains and has 



