[Illustration: FIG. 38.--How springs are formed. _A_, porous layer;
_B_, non-porous layer; _C_, spring.]
Water which soaks into the ground moves slowly downward and after a
longer or shorter journey, meets with a non-porous layer of rock
through which it cannot pass, and which effectually hinders its
downward passage. In such regions, there is an accumulation of water,
and a well dug there would have an abundant supply of water. The
non-porous layer is rarely level, and hence the water whose vertical
path is obstructed does not "back up" on the soil, but flows down hill
parallel with the obstructing non-porous layer, and in some distant
region makes an outlet for itself, forming a spring (Fig. 38). The
streams originating in the springs flow through the land and
eventually join larger streams or rivers; from the surface of streams
and rivers evaporation occurs, the water once more becomes vapor and
passes into the atmosphere, where it is condensed and again falls to
the earth.
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