iron, through which water
passes very slowly, or not at all. These strata are sometimes regular,
extending at an equal depth over large tracts, and having a uniform dip,
or inclination. Oftener, however, in hilly regions especially, they are
quite irregular--the impervious stratum frequently having depressions of
greater or less extent, and holding water, like a bowl. Not
unfrequently, as we cut a ditch upon a declivity, we find that the dip
of the strata below has no correspondence with the visible surface of
the field, but that the different strata lie nearly level, or are much
broken, while the surface has a regular inclination.

Underlying all soils, at greater or less depth, is found some bed of
rock, or clay, impervious to water, usually at but few feet below the
surface--the descending water meeting with obstacles to its regular
descent. The tendency of the rain-water which falls upon the earth, is
to sink directly downward by gravitation. Turned aside, however, by the
many obstacles referred to, it often passes obliquely, or almost
horizontally, through the soil. The drop which falls upon the hill-top
sinks, perhaps, a few inches, meets with a bed of clay, glides along
upon it for many days, and is at last borne out to be drunk up by the
sun on some far-off slope; another, falling upon the sand-plain, sinks
at once to the "water-line," or line of level water, which rests on clay
beneath, and, slowly creeping along, helps to form a swamp or bog in the
valley.

Sometimes, the rain which falls upon the high land is collected together
by fissures in the rocks, or by seams or ruptures in the impervious
strata below the surface, and finds vent in a gushing spring on the
hill-side.

We feel confident that no better illustration of the theory of springs,
as connected with our subject, can be found, than that of Mr. Girdwood,
in the Cyclopedia of Agriculture--a work from which we quote the more
liberally, because it is very expensive and rare in America:

"When rain falls on a tract of country, part of it flows over the
surface, and makes its escape by the numerous natural and
artificial courses which may exist, while another portion is
absorbed by the soil and the porous strata which lie under it.

"Let the following diagram represent such a tract of country, and
let the dark portions represent clay or other impervious strata,
while the lighter portions represent layers of gravel