GROUND WATERS AND THEIR OCCURRENCE 31 



conditions most favorable for direct absorption are the sand- 

 stones and certain of the porous limestones. In the case of the 

 granites, slates and other massive rocks the direct absorption is 

 very slight. 



Besides the character of the material the amount of absorption 

 depends very largely upon the inclination of the porous beds, the 

 amount being much greater in the gently inclined beds than in 

 those having steep dips. Thus in Figure 14 the two beds represented 

 as outcropping on a level surface present widely different absorptive 



FIG. 14. Relation of areas of outcrops to dip. 



conditions owing to the difference in area of their absorptive sur- 

 faces, the exposed surface of the gently sloping bed (a! b'} being 

 several times greater than that of the highly inclined bed (a b). 



The quantity of water absorbed by rocks that are directly ex- 

 posed to the falling rain is slight in comparison to that taken up 

 by those covered with coatings of soil, loam or sand and gravel. 

 Such materials take up the rain as would a sponge and keep the 

 water in constant contact with the underlying rock surface, by 

 which it is slowly absorbed, instead of running off as does the 

 main portion of the precipitation falling on the bare ledges. 



Waters from Lakes and Streams. One of the most common 

 popular conceptions is that ground waters are derived either 

 from neighboring or from more remote lakes and streams. It is 

 too well known to require more than the simple statement, how- 

 ever, that the movement of such water is normally toward and not 

 away from the water bodies, the surfaces of which are below and 

 not above the water table. It is only when there is some sudden 

 rise of water in the lake or stream due to causes independent of 

 local rainfall that the level becomes higher than the adjacent 

 water table and a landward movement takes place. These con- 

 ditions are illustrated by Figure 15. Such movements occur 



