FIELD GEOLOGY. 221 



runs to the rivers, the rest being absorbed by the water- 

 bearing strata, and again thrown out at a lower level, in 

 the form of springs, on the hill-sides ; or, the conditions 

 being favourable, it is retained by those strata at various 

 depths, which thus are constituted huge underground 

 reservoirs. (Section 3.) 



The supply from the natural founts will vary as the 

 seasons ; in periods of drought the streams run dry, and 

 the springs fall off, while shallow wells, which derive 

 their water either by soakage from a stream or from a 

 so-called land-spring, become exhausted in consequence. 

 Neither streams nor springs should be depended on for 

 a supply of water, not only on account of their inter- 

 mittent nature, but because of their liability to pollu- 

 tion. 



The strata that now throw out springs would, if 

 occurring at a different level or if inclined at a suitable 

 angle, become the means of draining water from the 

 surface, and the springs of one locality are, in fact, but 

 the natural drainage of another. 



3. Wells. This section involves the " theory of 

 springs," and relates to artesian or deep wells and 

 borings, and it may be illustrated by performing in 

 imagination a simple experiment. Take two shallow 

 dishes and place one on the other with a thin layer of 

 sand between them ; pour in water at one edge of this 

 layer, and it will be found that in a very short time the 

 sand is saturated alike throughout. A portion of the 

 water has, in fact, first descended by gravitation under 

 the centre of the upper dish, then risen to the other 

 edge of the sand, through the force of hydrostatic pres- 

 sure, to the same level as that at which it entered. A 



