VARIATION IN LEVEL OF GROUND WATER. 429 



GENERAL STATEMENTS. 



The variations in the level of ground water where they are consid- 

 erable are very slow, but in amount they are of great consequence. Of the 

 factors which separately, or more commonly by combination, have 

 produced marked variations in the level of ground water, uplift, subsidence, 

 denudation, valley filling, and the influence of man are the most important, 

 The lead and zinc districts of the Mississippi Valley afford excellent illus- 

 trations of variations in the level of ground water. In the lead and zinc 

 districts of the upper Mississippi Valley, near the close of the Grlacial epoch, 

 there was a subsidence and valley filling to the amount of 60 to 90 meters 

 or more. This produced a corresponding rise in the level of ground water, 

 and the transfer of a considerable belt which had been in the belt of 

 weathering to the belt of cementation. Possibly the same thing has hap- 

 pened in the lower Mississippi Valley lead and zinc region. For instance, 

 in the Joplin district of Missouri, below the level of ground water, there 

 were large caves before mining- began. These caves may have been 

 formed when the level of ground water was at a lower horizon, so that the 

 surrounding ground was in the belt of weathering. At a time when a 

 horizon now in the belt of cementation was in the belt of weathering 

 solution was the dominant process. When the level of ground water rose, 

 this horizon passed into the belt of cementation and deposition and conse- 

 quent filling of the openings began. As a result in the Joplin district the 

 caves are gigantic geodes lined with great crystals of calcite, some of them 

 from 10 to 50 or more centimeters long. By the artificial lowering of the 

 level of ground water by mining processes this belt has been again trans- 

 ferred to the belt of weathering, and reactions characteristic of this belt 

 have been set up. 



METAMOKPHISM IN THE BELT OF WEATHEEDfG. 

 VARIABLE MATERIALS AND CONDITIONS OF BELT OF WEATHERING. 



Within the belt of weathering there is the greatest variety of materials 

 and conditions. The reactions are therefore of the most extraordinarily 

 complex character. In the belt of rock Aveathering there is the greatest 

 possible variety of rocks. In fact, every known variety of rock which 

 exists anywhere upon the earth may be present. There are in this belt 



