66 PHYSICAL GEOLOGY 



algae may cause lime to deposit. In this way beds of so-called 

 " petrified moss," more than ten feet thick, have been formed. 

 In the Yellowstone National Park the deposits about the geysers 

 were built up both by the evaporation of the water and by algae 

 (p. 65). A reduction in temperature and pressure may also cause 

 minerals in solution to be deposited, as may also the mingling 

 of waters carrying in solution substances of different composition 



(p. 372). 



Mineral Springs. — Mineral springs contain various salts or 

 gases. Such springs are often called " medicinal " because of their 

 supposed curative properties. The total value of mineral waters 

 is large, amounting to $5,631,391, in 1913, in the United States 

 alone. 



' Temperature of Springs. — The temperature of springs is usually 

 much lower than that of the air in summer, being about 47 F. in 

 Connecticut; and the water is often described as being " icy cold." 

 The temperature of such springs in middle latitudes is fairly constant 

 if they come from a depth greater than 50 or 60 feet, since at this 

 depth the water is not affected by daily or seasonal changes and has, 

 consequently, about the average temperature of the region. 



Thermal Springs. — The temperature of many so-called hot 

 springs varies from lukewarm to boiling. (1) The heat is sometimes 

 due to the presence of deep fissures through which the surface water 

 has percolated until it has reached great depths, where its temperature 

 has been raised by the interior heat of the earth. After being thus 

 heated, the water is forced by hydrostatic pressure to a point on the 

 surface which is lower than the point of ingress. The depth from 

 which come springs like those of Bath, England, which have a tem- 

 perature of 120 F., may be approximately told from well borings, such 

 as that of a well at Berlin, Germany, the water of which has a tem- 

 perature of no.5 F. at a depth of 3390 feet. Springs located along 

 fissures in the earth's crust occur in Virginia, Arkansas, Colorado, 

 Nevada, and South Dakota, and are often the seat of popular health 

 resorts. (2) The water of some springs is heated by chemical action. 

 (3) Water in volcanic regions may be heated at comparatively shallow 

 depths by the presence of uncooled lava. Of this class there are more 

 than 3000 in the Yellowstone National Park, some of which deposit 

 limestone (travertine) and others silica (geyserite). Hot springs may 

 bring about a considerable change in the character of the rocks in the 

 regions in which they occur, by causing the disintegration of some and 



