212 GEOLOGY 



and where the valley slopes cut joints, porous beds, or other struc- 

 tures 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 

 grouping 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. Between deep springs and shallow ones, 

 and between cold springs and warm ones, respectively, there is no 

 sharp line of demarkation. Again, some springs are continuous 



Fig. 172. Diagram showing conditions favorable for a spring in the side of a 

 valley. P and P represent porous beds of rock, and I, an impervious bed. 

 Rainwater sinks to I, and moving along its surface, may come out as a 

 spring at S. 



in their flow, while others are intermittent. Most intermittent 

 springs flow after periods of precipitation, but dry up during 

 droughts. Springs are also classified as mineral and common. 

 Mineral springs, in the popular sense of the term, are of two types: 

 (1) Those which contain an unusual amount of mineral matter, 

 and (2) those which contain some unusual mineral. All springs 

 which are not mineral are common. This classification is not very 

 significant, for all springs contain more or less mineral matter, 

 and many springs which are common contain more mineral matter 

 than some springs that are mineral. Mineral springs are them- 

 selves classified according to the kind and amount of mineral matter 

 they contain. Thus saline springs contain salt; sulphur springs 

 contain compounds (especially gaseous) of sulphur; c/w//A'"/i 

 springs contain iron compounds, especially the sulphate; calcareous 

 springs contain abundant lime carbonate, etc. These various 

 mineral substances have been extracted from the rock. The salt 



