60 PHYSICAL GEOLOGY 



Hills from the rain that falls on it and the streams that flow over it. 

 It is covered by clays and shales as it extends eastward, and when 

 borings are made at elevations lower than its source in the Black 

 Hills, the water rises and supplies wells even 350 miles from this source. 





&*W 



1 I 



2500 FT. 

 1000 FT. 

 Sea.leyel. 



Fig. 35. — Diagram showing the conditions favorable for artesian water, from the 

 Rocky Mountains to eastern Nebraska. The Dakota sandstone under the imper- 

 vious Pierre clay carries water from the Rocky Mountains and supplies artesian wells 

 on the plains. (U. S. Geol. Surv. ) 



Artesian wells vary in depth, some being 4000 feet deep, while others 

 may be less than 100 feet. Artesian water, both for drinking pur- 

 poses and for irrigation, is of great importance. It varies greatly in 

 composition, some wells affording excellent water, while others may 

 be so charged with salts as to be useless for drinking or irrigation. 



Springs corresponding to artesian wells are formed if the impervious 

 bed overlying the porous bed is broken by a fissure or fault (p. 25). 

 These springs may be of great volume. 



Chemical Work of Ground Water. — (1) Solution. Pure water has 

 little power to dissolve the minerals of which rocks are composed, but 

 rain water is seldom pure since it receives carbon dioxide from the air, 

 and, in passing through the soil, takes up this and other acids formed 

 by the decay of organic matter. It may be heated in its downward 

 course and is subjected to great pressure. Thus equipped, its solvent 

 power is greatly increased, and in its descent through the rocks it 

 carries away the more soluble minerals and the cement of many of the 

 rocks, rendering them more porous and causing their decay. At or 

 near the surface, water is an active agent in causing the disintegration 

 of the rocks, both by the mechanical work of the frost and by its 

 chemical action. 



(2) Replacement and (3) Deposition. — When ground water contains 

 much mineral matter, a slight change in temperature or pressure, 

 or a mingling with other waters of a slightly different composition, may 

 cause the dissolved material to be deposited. This results in replace- 

 ment, and deposition in cavities. (2) Replacement results when in its 

 descent ground water dissolves and carries away one mineral, deposit- 



