190 



EARTH FEATURES AND THEIR MEANING 



concentrated locally at the intersections of the fracture network, 

 where it issues in lines of fissure springs (Fig. 200) ; but at the time 

 of earthquakes the water may rise above the surface in lines of 

 fountains (p. 83), or occasionally as sheets of water which may 

 mount some tens of feet into the air. 



In contrast to the flow of surface springs, which varies with the 

 season through wide ranges both in its volume and in temperature 



of the water, the volume of 

 fissure springs is but slightly 

 affected by the seasonal pre- 

 cipitation, and the water tem- 

 perature is maintained rela- 

 tively constant. Rock is but 

 a poor heat conductor, and the 

 seasonal temperature changes 

 descend a few feet only into the 

 ground. Thus water which 

 rises from depths of a few hun- 

 dred feet only is apt to be icy 

 cold, while from greater depths 

 the effect of the earth's internal 

 heat is apparent in a uniform 

 but relatively higher temperature of the water. Such "warm" 

 or thermal springs are apt to contain considerable mineral matter 

 in solution, both because the water is far traveled and because its 

 higher temperature has considerably increased its solvent properties. 

 It has long been recognized that lines of junction of different 

 rock formations at the base of mountain ranges are localities fa- 

 vorable for the occurrence of thermal springs. These junction 

 lines are usually within zones where by movement upon fractures 

 the widest openings in the rock have formed, and the catchment 

 area of the neighboring mountain highland has supplied head for 

 the ground water. A map of the hot springs within the Great 

 Basin of the western United States would present in the main a 

 map of its principal faults. 



Artesian wells. From the natural fissure spring an artesian 

 well differs in the artificial character of the perforation of the im- 

 pervious cover to the water layer. The water of artesian wells 

 may flow out at the surface under pressure, or it may require 



FIG. 200. Fissure springs arranged upon 

 lines of rock fracture at intersections, 

 Pomperaug valley, Connecticut. 



