[Illustration]
When we think of the ocean with its waves, tides, and currents, of the
winds, and of the rain and snow, and the vast net-work of rivers to
which they give rise, we realize that the energy or force manifested
upon the earth’s surface resides chiefly in the _air_ and _water_—in the
earth’s fluid envelope and not in its solid crust. And it would be an
easy matter to show that, with the exception of the tidal waves and
currents, which of course are due chiefly to the attraction of the moon,
nearly all this energy is merely the transformed heat of the sun. Now
the air and water are two great geological agencies, and therefore the
geological effects which they produce are traceable back to the sun.
Organic matter is another important geological agent; but all are
familiar with the generalization that connects the energy exhibited by
every form of life with the sun; and, besides, it is scarcely necessary
to allude to the obvious fact that all animals and plants, so far at
least as any display of energy is concerned, are merely differentiated
portions of the earth’s fluid envelope. And so, if space permitted, it
might be shown that, with the exception of the tides, nearly every form
of force manifested upon the earth’s surface has its origin in the sun.
Of this trio of geological agencies operating upon the earth’s surface
and vitalized by the sun—_water_, _air_, and _organic matter_—the water
is by far the most important, and so it is common to call these
collectively the aqueous agencies. Hence we have _solar agencies_ and
_aqueous agencies_ as synonymous terms.
The aqueous agencies include, on one side, _air_ and _water_, or
_inorganic_ agencies; and, on the other, _animals_ and _plants_, or
_organic_ agencies.
Let us notice briefly the operation of these, beginning with the air and
water.