SUBTERRANEAN WATERS, SPRINGS, ETC. 



75 



Fig. 70.— Fissure-Spring. 



with c, and then by hydrostatic pressure moves laterally until it emerges 

 at a. Sometimes this is a geological agent of considerable importance, 

 modifying even the forms of mountains, and producing land-slips, etc. 

 Thus the Lookout and Eaccoon Mountains, in Tennessee, are table- 

 mountains of nearly horizontal strata, separated by erosion-valleys. 

 These mountains are all of them capped by a sandstone stratum about 

 100 feet thick, underlaid by shale. The water which falls upon the 

 mountain emerges in numerous springs all around where the sandstone 

 cap comes in contact with the underlying shale. The sandstone is 

 gradually undermined, and falls 

 from time to time, and thus the 

 cliff remains always perpendic- 

 ular (Fig. 69). 



Large springs generally is- 

 sue from fissures. Water pass- 

 ing along the porous stratum b, 

 perhaps from great distance, and prevented from rising by the over- 

 lying impervious stratum c, coming in contact with a fissure, immedi- 

 ately rises through it to the surface at a (Fig. 70). 



Artesian Wells. — If subterranean streams have their origin in an 

 elevated region, a d, composed of regular strata dipping under a lower 

 flat country, c, then the subterranean waters passing along any porous 

 stratum as a (Fig. 71), and confined by two impermeable strata, b and 



d, will be under powerful 

 hydrostatic pressure, and 

 will, therefore, rise to the 

 surface, perhaps with con- 

 siderable force, if the stream 

 be tapped by boring at c. 

 Borings by which water is 

 obtained in this manner are 

 called Artesian wells, from the French province Artois, where they were 

 first successfully attempted. The source of the water may be 100 miles 

 or more distant from the well. Some of these wells are very deep. The 

 Grenelle Artesian in Paris is 2,000 feet deep. At the moment of tapping 

 the stream, a powerful jet was thrown 112 feet high. One in West- 

 phalia, Germany, is 2,385 feet deep ; one at St. Louis, 3,843 feet ; one 

 at Louisville, Kentucky, 2,852 ; one near Berlin, 4,172 feet ; one near 

 Pittsburg, Pa., 4,625 ; and one near Leipsic, 5,735 feet.* In parts of 

 Alabama and California the principal supply of water for agricultural 

 purposes is drawn from these wells. 



Thus there is on all coasts a constant flowing of water, both super- 



Fig. 71.— Artesian Well. 



* Science, xiv, p. 250, 1889. 



