THE WORK OF WATERS UNDERGROUND 123 



Importance of chemical work of ground water. From 



the preceding paragraphs it is apparent that ground water 

 may modify the character of rocks in several ways : (1) by 

 removing soluble constituents ; (2) by depositing new ma- 

 terial in rock cavities; (3) by replacing old material with 

 new; and (4) by forming new chemical combinations. The 

 result is often to alter profoundly the character of the rocks 

 affected. Thus water is one of the leading agents of meta- 

 morphism (p. 78). Furthermore, changes of the sort sug- 

 gested above must have occurred on a vast scale, for ground 

 waters have been at work throughout the zone of fracture 

 for untold millions of years. 



QUESTIONS 



1. In order to contain water permanently, must wells be sunk 

 deeper below the surface in valleys or on uplands ? 



2. Make a diagram showing (1) the position of the water table 

 during the rainy and during the dry months, and (2) two wells, one 

 of which goes dry at times, while the other always contains water. 



FIG. 115. An exposure of rocks in a railroad cut, Columbia, S. C. 

 (Trowbridge.) 



3. Describe the characteristics of a climate that should (1) favor, 

 and (2) hinder, the work of solution by ground water. 



4. Would caves be more likely to develop in limestone regions 

 whose surfaces were well above or near to sea level ? 



5. Why cannot extensive caverns in a given region be formed 



