DEEP WELLS 109 



increasing amounts at least down to the drainage level. Again, 

 the surface beds may be of non-porous nature and may therefore 

 be destitute of water, while the underlying beds, if porous and 

 below drainage level, are likely to be saturated. 



Of course there is a constant tendency for the surface waters 

 to penetrate downward and fill the porous rocks below. That 

 these are at present destitute of water may be due, at least in 

 certain rugged regions, to the draining of the deeper and in places 

 relatively porous beds by deep valleys. Elsewhere, and this is 

 doubtless the most common cause, the water is kept from per- 

 colating downward by impervious beds near the surface. The 

 deeper rocks are largely of the granitic type and hold but little 

 water. Except where they constitute the surface rock and are 

 somewhat broken by joints it is of little use to penetrate them in 

 search of water. 



To speak broadly, it may be said that there is no general in- 

 crease of water with depth and that the finding of deep supplies 

 is entirely dependent on local geologic conditions. Unless there is 

 some proof that deep water-bearing beds exist, the sinking of a 

 well more than a few hundred feet in depth should be regarded 

 wholly in the light of an experiment, although in sedimentary 

 rocks it has the decided advantage that it may penetrate a num- 

 ber of water strata, which may afford in the aggregate a fair 

 supply where a single stratum might not suffice. 



Relation of Depth and Head. -- The relation of head to depth 

 has been discussed in connection with artesian flows (p. 58). It 

 may be said that, while there is no fixed relation between the two, 

 it is perhaps more common than otherwise for the deeper strata 

 of the structural troughs giving rise to artesian flows to outcrop 

 at higher levels than the overlying beds, although the reverse is 

 often true. In many structural basins, the water-bearing l>eds 

 rise toward the rims and outcrop in plateaus or other drvutimis 

 high above the low plains over the center of the basins. 



Relation of Depth and Quality. Another prevailing idea is 

 that the deeper waters are purer. Within limitations this is gen- 



