n, in the earth, such as silica, aluminum,
calcium, iron, carbon, sodium, chlorine, potassium, etc.

The characteristics of the soil depend upon its constituents, and upon
the predominance of one or the other of its composing elements. The
nature of the soil also depends upon its physical properties. When the
disintegrated rock consists of quite large particles, the soil is
called a _gravel soil_. A _sandy soil_ is one in which the particles
are very small. _Sandstone_ is consolidated sand. _Clay_ is soil
consisting principally of aluminum silicate; in _chalk_, soft calcium
carbonate predominates.

The organic constituents of the soil are the result of vegetable and
animal growth and decomposition in the soil.

=Ground Water.=--Ground water is that continuous body or sheet of
water formed by the complete filling and saturation of the soil to a
certain level by rain water; it is that stratum of subterranean lakes
and rivers, filled up with alluvium, which we reach at a higher or
lower level when we dig wells.

The level of the ground water depends upon the underlying strata, and
also upon the movements of the subterranean water bed. The relative
position of the impermeable underlying strata varies in its distance
from the surface soil. In marshy land the ground water is at the
surface; in other places it can be reached only by deep borings. The
source of the ground water is the rainfall, part of which drains into
the porous soil until it reaches an impermeable stratum, where it
collects.

The movements of the ground water are in two directions--horizontal
and vertical. The horizontal or lateral movement is toward the seas
and adjacent water courses, and is determined by hydrostatic laws and
topographical relations. The vertical motion of the ground water is to
and from the surface, and is due to the amount of rainfall, the
pressure of tides, and water courses into which the ground water
drains. The vertical variations of the ground water determine the
distance of its surface level from the soil surface, and are divided
into a persistently low-water level, about fifteen feet from the
surface; a persistently high-water level, about five feet from the
surface, and a fluctuating level, sometimes high, sometimes low.

=Ground Air.=--Except in the hardest granite rocks and in soil
completely filled with water the interstices of the soil are filled
with a continuation of atmospheric air, the amount depending on the
degr