. Elementary physical geography . Fig. spring (s) at junction of permeable layer (a) and impervious layer (b). This is particularly liable to happen on hillsides where alayer of sand rests upon a stratum of clay. In the earth, certain strata are more permeable to waterthan are others; and under somecircumstances the conditions fa-voring the production of arte-sian wells (Fig. 112) may bepresent. Sandstones are themost permeable of rocks, andwhen a sandy layer crops out atthe surface, the water readilysoaks into it. If such a layer iscovered and underlaid by a moredense rock, such
. Elementary physical geography . Fig. spring (s) at junction of permeable layer (a) and impervious layer (b). This is particularly liable to happen on hillsides where alayer of sand rests upon a stratum of clay. In the earth, certain strata are more permeable to waterthan are others; and under somecircumstances the conditions fa-voring the production of arte-sian wells (Fig. 112) may bepresent. Sandstones are themost permeable of rocks, andwhen a sandy layer crops out atthe surface, the water readilysoaks into it. If such a layer iscovered and underlaid by a moredense rock, such as a clay stra-tum, the water that enters thesandy layer is in large measureimprisoned within it. If undersuch conditions the strata dipinto the earth, the water in thesandstone passes down this layerbetween the two enclosing a result of the weight of thecolumn of water in the stratum, it is under a considerable. Fig. well. 230 PHYSICAL GEOOBAPHY. pressure; and this is sufficient to force it upward towardthe surface, to a height nearly as great as that of the placewhere the water enters the ground. If this water-bearing layer is pierced by a well-boring,the water will rise in the well as high as the pressurecan force it; and if the place at which the well is bored,
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