SPRINGS' 261 



ous Benton clay of the Cretacic. The water enters along the outcrop 

 of this formation more than 3,000 feet above sea-level. Over parts 

 of the Great Plains the Dakota has descended near to or even below 

 sea-level, while perhaps 2,000 feet of other strata are piled above 

 it. When tapped by artesian wells the water will rise to within 2,400 

 to 1,500 feet above sea-level, according to the distance from the 

 mountains. As the surface over most of the country is lower than 

 this, the result is the formation of actively flowing artesian wells 

 (see Fig. 37). 



Springs. 



Springs are the escape of ground water at the surface of the 

 lithosphere. r)y surface of the lithosphere is here meant any point 

 where the lithosphere is in contact with a mass of water or air; 

 thus we have sublacustrine springs and springs formed in subterra- 

 nean channels, in wells, etc. 



Springs commonly issue where the contact between a pervious 

 and an impervious lower layer is exposed in section. In the Helder- 

 berg Mountains the spring line is at the base of Siluric or Devonic 

 limestones, where they rest upon impervious clays (Brayman), or 

 unconformably upon the Hudson River beds. Along the line of 

 contact betw^een the Devonic Onondaga limestone, a porous rock, 

 and the Siluric Monroe limestone, a compact, impervious one in 

 western New York. Canada, and Michigan, occurs a line of copious 

 springs. The contact line here is a disconformity, and the water 

 sometimes gushes out in great volume. The contact between the 

 Lockport dolomite and Rochester shales along Niagara gorge and 

 the whole front of the Niagara escarpment constitutes another such 

 spring line, and so does the upper surface of the Black Shale 

 throughout the southern Appalachians. Where the section is made 

 only into the pervious layer, as in a well, a general seeping or 

 "welling" of the ground water takes place ; where the surface is de- 

 pressed to the ground-water level, or below it, a similar "welling" 

 of the ground water takes place. Swamps are formed in this way, 

 which may be regarded as natural wells on a large scale. 



Water in the Solid Form, 



Snow and ice may. on the one hand, be considered as water in 

 the solid state, and so referred to the hydrosphere ; or, on the other 

 hand, as a rock, solid only at low temperatures. In the latter case 

 it belongs either to the igneous or pyrogenic rocks when due to 



