SUB-SURFACE WATER OR GROUND WATER 197 



porous rock layer below which is a rock through which 

 it cannot penetrate. The water will then accumulate 



HOT SPRINGS IN THE YELLOWSTONE NATIONAL PARK, U. S. A. 



throughout the porous rock. If this rock layer in another 

 part of its extent is overlaid by an impermeable layer, its 

 water is held in by the impermeable rocks above and below, 

 and so is under hydraulic pressure. When a hole is made 



FIGURE 81 



in the upper rock layer (Figure 81), the water will flow to 

 the surface, and if the pressure is sufficient it may gush out 

 of the hole. 



