428 A TREATISE ON METAMORPHISM. 



districts, where within comparatively small areas vast quantities of water 

 are raised to the surface. The ground-water level in many mining areas 

 has been locally lowered scores or even hundreds of meters. From the 

 Comstock lode enormous quantities of water were pumped." From a single 

 iron mine in the Lake Superior region 19,000 liters per minute have been 

 raised for considerable periods of time. In the lead and zinc district of 

 Missouri many small pumps located close together have lowered the level 

 of ground water at various places from 30 to 50 meters. Drafts upon 

 underground water by pumping for irrigation and for city supplies have 

 lowered the level of ground water to a very considerable extent in many of 

 the semiarid regions. An excellent illustration is furnished by southern 

 California, where during the last ten years the level of ground water has 

 been lowered at various places over extensive areas from 10 to 30 meters. 

 Depression of the level of ground water due to pumping rapidly 

 transfers a considerable horizon from the conditions of the belt of cementa- 

 tion to those of the belt of weathering, and thus accomplishes the same 

 effects which are slowly produced by the downward migration of the level 

 of ground water due to other causes. 



BAROMETRIC PRESSURE. 



King has shown that barometric pressure may slightly raise and lower 

 the level of ground water. The fluctuations due to this cause are, however, 

 usually but a fraction of a centimeter.'' 



TEMPERATURE. 



It has already been seen that increase in temperature decreases the 

 viscosity of ground water. At times of increase of temperature, therefore, 

 the escape of the underground waters through springs and seepag-e is pro- 

 moted, and the level of ground water is lowered. At times of decrease 

 of temperature the escape is stayed. Also rise of temperature produces 

 increased pressure, due to the expansion of the g*ases confined in the soil 

 above the ground water, and King holds this to be the chief cause of 

 increase of flowage with increase of temperature." However, the effect 

 of the temperature factor upon the level of ground water is slight, its most 

 important effect being slight daily oscillations. 



"Lord, Eliot, Comstock mining and miners: Mon. U. S. Geol. Survey, vol. 4, 1883, pp. 231-232. 

 &King, F. H., cit., p. 76. 

 <King, F. H., cit., p. 75. 



