234 GEOLOGY. 



importance, geologically, would perish. Biologically, therefore, as 

 v/ell as geologically, ground-water is of great importance 



SPRINGS AND FLOWING WELLS. 



The term spring is appHed to any water which issues from beneath 

 the surface with sufficient volume to cause a distinct current. If the 

 water issues so slowly as to merely keep the surface moist, it is not 

 called a spring, but seepage. The spring from which water issues with 

 a strong current, especially if it be upward, is comparable to a flowing 

 well, while the spring from which water issues with little force, and 

 without upward movement, is comparable to the flow of water into 

 a common well. 



Springs often issue from the sides of valleys (Fig. 212), the bottoms 



, Fig. 212. — Diagram illustrating positions, a and h, favorable for springs. 



of which are below ground-water level. They are especially likely 

 to issue at the surface of relatively impervious layers, and where the 

 valley slopes cut joints, porous beds, or other structures which allow 

 free flow of ground-water. 



Springs are classified in various ways, and these several classifica- 

 tions suggest characteristics worthy of note. They are sometimes 

 said to be deep and shallow. The ''deep" spring, as the term is ordin- 

 arily used, is one which issues with great force, and with something 

 of upward movement, and the ' ' shallow " spring, one which issues 

 with little force, and without upward movement; but the spring which 

 issues with force is not necessarily deep, nor is the one which issues 

 with little force necessarily shallow. The idea involved in this group- 

 ing would be better expressed by strong and feeble. Springs are also 

 classified as cold and thermal, the latter term meaning simply that the 

 temperature is such as to make the springs seem warm or hot. The 

 temperature of thermal springs ranges up to the boiling-point of water. 



