CLASSIFICATION OF SPRINGS 531 



at the surface and move through openings, which are generally 

 supercapillary in size. Their movement is due to gravitative 

 pressure transmitted through a continuous body of water, lying 

 in the pore spaces and fractures of the rocks, i.e., by hydrostatic 

 head. Since both kinds of openings decrease rapidly in number 

 and size below 1,500 feet, these waters are limited in amount below 

 that depth. Deep-seated waters have a complex origin. They 

 doubtless include water derived by absorption from the surface, 

 water entrapped in sedimentary rocks at the time of their deposi- 

 tion, and water expelled during the crystallization of igneous rocks. 

 It is believed that these waters do not move because of hydrostatic 

 head, that is, that they are not connected with any overlying and 

 connecting body of water, but that flow is the result of other agencies 

 operative deep within the earth. 



Evidence that a spring water has a deep origin may be positive 

 or negative. Thus the water of a spring that has a strong uniform 

 flow not subject to seasonal changes and a high temperature 

 probably has a deep-seated origin. The minimum depth from 

 which the water may come can be roughly estimated from the 

 temperature of the water, on the assumption that there is 1° F. 

 increase in temperature for every 60 to 100 feet of increase in depth. 

 In volcanic regions, however, the increase may be more rapid. 



The presence of important breaks in the earth's crust or of other 

 structures along which water could rise furnishes additional positive 

 evidence. Negative evidence is usually easier to obtain and con- 

 sists of the absence of any structure which could lead the water 

 from the surface to the necessary depth and then return the water 

 again to the surface. 



CLASSIFICATION 



I. SPRINGS DUE TO DEEP-SEATED WATER 



Springs due to deep-seated water may be divided into two 

 classes, according to their geographic distribution, with respect 

 to localities of volcanic or tectonic disturbance. Their relations 

 to the structure of the upper part of the earth's crust and the prob- 

 able character of the fissures in the zones of fracture which permit 

 the water to rise are shown diagrammatically in Figure i . In this 



