SOIL PHYSICS 



469 



preventing excessive evaporation from large cultivated fields. 

 What effect will such mulch have upon the cracking of the 

 soil? Explain. 



RELATION OF GROUND-WATER TO WELLS 



552. The Fallacy of Underground Streams. By many 

 people it has been supposed that to " strike a vein of water" 

 when boring a well means that the drill has tapped an UNDER- 

 GROUND STREAM. While it is true that underground streams 



FIG. 285. An underground stream in limestone. The carbonic acid has 

 dissolved the limestone and the ground-water drains away in a true under- 

 ground stream. 



exist in some places, the supposition that most wells tap 

 something that may properly be called an underground stream 

 is false. 



We have already seen that all of the free ground-water moves 

 slowly through the soil down hill, or toward a lower level. 

 In many places it is doubtless true that the nature of the soil 

 strata is such that through certain strata the water moves much 

 more rapidly than through others. If a layer of coarse gravel 



