GROUND WATERS AND THEIR OCCURRENCE 33 



ground streams are not entirely mythical, although few are of a 

 size that would lift them to the dignity of rivers. Instances are 

 numerous in limestone regions where streams several feet, or per- 

 haps a rod or more in width, flow through underground channels, 

 are joined by tributaries, plunge over ledges as waterfalls, and, in 

 fact, behave much like surface streams. Such a stream is shown 

 in Figure 5. 



Outside the limestone regions there is, in general, no moving 

 ground water of a size to warrant the term river. The entire 

 ground water body, as explained elsewhere, is in motion, but the 

 movement is slow, often but a few inches a day, and the move- 

 ment is that of a sheet of water rather than an underground 

 " river. " 



In places, nevertheless, where porous materials lie between 

 masses of unporous materials, as in the case of gravels lying be- 

 tween the granite walls of valleys, etc., the motion is more rapid 

 - perhaps several feet a day and a sort of sluggish stream may 

 slowly push its way through the soil. Nothing of the nature of 

 the free flowing stream of the limestone regions exists, however. 



Underground Lakes. - - There is little to warrant the common 

 belief in the existence of " underground lakes." A few small 

 pools are found in limestone caverns, but in the great majority of 

 soils and rocks the water occurs only in the pores or occupies 

 minute fissures or other parting plains. 



In the more porous materials, however, especially in sand and 

 gravel, the volume of water is often very large and is given up 

 freely to wells. This, and the fact that the water is commonly 

 struck everywhere at about the same depth gives rise to the 

 belief of an underground lake with a definite upper surface, a con- 

 ception that is not far from the truth, except that the water ex- 

 ists not as a free body but as a body filling the pores of the sand 

 or gravel as it would a sponge. 



Temperature of Underground Waters. In all wells there is a 

 certain depth, which differs in different localities, at which there 

 is practically no difference in the temperature of the water from 



