FISSURE SPRINGS 



93 



as when a spring rises from a deep fissure, or bursts out upon 

 the top of a hill. But these are not real exceptions, and here 

 also the source, which may be many miles distant, is above the 

 spring, and it is this which produces the necessary pressure. 



The commonest type of spring is formed when a relatively im- 

 pervious bed of rock (usually clay in some form) overlaid by 

 porous rocks, crops out on a hillside. The rain-water descends 

 through the porous beds, saturating their lower layers, until its 



FIG. 31. — Diagram of fissure-spring. The heavy line represents the fissure along 

 which the water rises. 



descent is arrested by the impervious bed, and then the water 

 follows the upper surface of the latter. When, by some irregu- 

 larity of the ground, the impervious bed comes to the surface, the 

 water will issue as a spring, or a line of springs. (See Fig. 30.) 



A second class of springs are those which rise through a crack 

 or fissure in the rocks. Inclined porous beds, enclosed between 

 more impervious ones, allow the water to follow them downward, 

 until in its lower course such water is under great pressure, or 

 " head." On reaching a fissure opening upward, the water will rise 

 through it and, if under sufficient pressure, will come to the surface. 



