The rainfall may be divided into three parts: the first DRIES UP,
being discharged into the air by evaporation either directly from
the soil or through vegetation; the second RUNS OFF over the
surface to flood the streams; the third SOAKS IN the ground and is
henceforth known as GROUND or UNDERGROUND WATER.

THE DESCENT OF GROUND WATER. Seeping through the mantle of waste,
ground water soaks into the pores and crevices of the underlying
rock. All rocks of the upper crust of the earth are more or less
porous, and all drink in water. IMPERVIOUS ROCKS, such as granite,
clay, and shale, have pores so minute that the water which they
take in is held fast within them by capillary attraction, and none
drains through. PERVIOUS ROCKS, on the other hand, such as many
sandstones, have pore spaces so large that water filters through
them more or less freely. Besides its seepage through the pores of
pervious rocks, water passes to lower levels through the joints
and cracks by which all rocks, near the surface are broken.

Even the closest-grained granite has a pore space of 1 in 400,
while sandstone may have a pore space of 1 in 4. Sand is so porous
that it may absorb a third of its volume of water, and a loose
loam even as much as one half.

THE GROUND-WATER SURFACE is the name given the upper surface of
ground water, the level below which all rocks are saturated. In
dry seasons the ground-water surface sinks. For ground water is
constantly seeping downward under gravity, it is evaporated in the
waste and its moisture is carried upward by capillarity and the
roots of plants to the surface to be evaporated in the air. In wet
seasons these constant losses are more than made good by fresh