s not continuously and fully saturated.

The water table or ground-water level may be near or at the surface in
low and humid areas, and it may be two thousand feet or more below the
surface in arid regions of high topographic relief. Because of the
influence of capillarity, the water table is not a horizontal surface.
It shows irregularities more or less following the surface contours,
though not nearly so sharply accentuated.

The lower limit of the ground-water is more irregular than the upper
surface and is less definitely known. In general, openings in rocks tend
to diminish with depth, due to cementation and to closing of cavities by
pressures which are too great for the rock to withstand. But rocks
differ so widely in their original character, and in their response to
physical and chemical environment, that it is not unusual to find dense
and impervious rocks above, and open and porous rocks below. The lower
limit of the zone of abundant underground water varies accordingly. A
well may encounter nearly dry rock at a comparatively shallow depth, or
it may reach a porous water-bearing stratum at considerable depth. At
the greater depths pockets of water are sometimes found which have a
composition different from that of the surface water, and which
evidently are isolated from the surface water by zones of non-pervious
rock.

Attempts have been made to calculate the total volume of underground
water by measuring the openings of rocks and making assumptions as to
the depth to which such openings may extend. In this manner it has been
estimated that, if all the ground-water were assembled in a single body,
it would make a shell between eighty and two hundred feet thick
(depending on the assumptions) over all the continental areas.

MOVEMENT OF UNDERGROUND WATER

Availability of water supplies is determined by the movement or flow of
water as well as by its distribution and amount. The natural flow of
water underground is caused by gravity in the larger openings, but in
the smaller openings adhesion and capillarity are also important forces.

Of all the water falling on the surface, some may not go below the
surface at all but may immediately evaporate or join the runoff--that
is, the surface streams. Another part may penetrate a little distance
into the zone of weathering and then join the runoff. Of the water which
reaches the zone of saturation, a part may soon come to the surface in
low areas and join the