THE WORK OF WATERS UNDERGROUND 111 



cities grew up largely or wholly because of the value of their 

 medicinal waters. In general, the waters of mineral springs 

 either contain large quantities of mineral matter in solution, 

 or something which is striking because of its taste or odor. 

 Most medicinal springs are mineral springs, but the reverse 

 is not true. In 1908 there were 695 commercial mineral 

 springs in the United States, which sold 56,108,820 gallons 

 of mineral waters, valued at $7,287,269. 



Among the more common deposits of springs are lime 

 carbonate by calcareous springs, and iron compounds by 

 ferruginous springs. Silica, gypsum, and many other things 

 are also deposited by springs. Limestone is deposited by 

 springs at San Filippo, in Tuscany, at the rate of one foot 

 in four months, and has formed a mass 250 feet thick, a 

 mile and a quarter long, and a third of a mile wide. Each 

 year the springs at Bath, England, discharge, on the average, 

 mineral matter sufficient to cover an area of 11,340 square 

 feet with a layer one foot deep. A spring near Minden, 

 Germany, has been found to bring to the surface each year 

 salt enough to form a solid cube measuring 72 feet on a side. 



Hillside springs, flowing wells, deep-seated springs, and 

 geysers are types of springs which may be discussed briefly. 



Hillside springs. Figure 102 illustrates the occurrence 

 of the ordinary hillside spring. If the upper (P) material is 

 relatively porous 

 (for example, 

 sand), while the 

 lower (7) material 

 is relatively im- 

 pervious (like 

 clay), rain water 

 will penetrate 

 readily to the sloping surface of the impervious beds, along 

 which it will flow slowly, and issue as springs at the base of 

 the hill. The great majority of springs are of this class. 

 They are usually not of great volume. 



FIG. 102. Diagram showing conditions for ordi- 

 nary hillside springs. 



