A 



48 GROUND-WATER 



be looked upon as a sort of chemical laboratory through which 

 waters are circulating in all directions, charged with many sorts of 

 mineral substances. Some of the substances in solution are de- 

 posited beneath the surface, and some are brought to the surface 

 where the waters issue. Much of the material brought to the sur- 

 face in solution is carried to the sea and utilized by marine organ- 

 isms in the making of shells. Without the mineral matter brought 

 to the sea by springs and river, many shell-bearing animals of 

 great importance, geologically, would perish. Biologically, there- 

 fore, as well as geologically, ground-water is of great importance. 

 It is also of prime importance in the development of ores. 



SPRINGS AND ARTESIAN WELLS " 



Springs. The term spring is applied to any water which issues 

 from beneath the surface with volume enough to form a distinct 

 current. If water issues so slowly as merely to keep the surface 

 moist, it is seepage but not a spring. 



Many springs issue from the sides of valleys (Fig. 28), the bot- 

 toms 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 the several classifi- 

 cations suggest characteristics worthy of note. They are some- 

 times classed as deep and shallow, but the idea involved in this 



Fig. 28. Diagram showing conditions favorable for springs, in the side of a 

 valley. P, porous rock, and I, impervious. Rain-water sinks to I, and, moving 

 along its surface, comes out as springs at S and S. 



grouping would be better expressed by strong and feeble. They 

 a!re also classed as cold and thermal, the latter term meaning 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. Again, some springs are continuous in their flow, while 

 others are intermittent. Most intermittent springs flow after periods 

 of rain, but dry up during droughts. Springs are also classified as 



