57. Having considered the composition, structure, and arrangement of the
rock-masses which form the solid crust of our globe, we have next to
inquire into the nature of those physical agencies by the action of
which the rocks, as we now see them, have been produced. The work
performed by the various forces employed in modifying the earth's crust
is at one and the same time destructive and reconstructive. Rocks are
being continually demolished, and out of their ruins new rocks are being
built. In other words, matter is constantly entering into new
relations--now existing as solid rock, or in solution in water, or
carried as the lightest dust on the wings of the wind; now being swept
down by rivers into the sea, or brought under the influence of
subterranean heat--but always changing, sooner or later, slowly or
rapidly, from one form to another. The great geological agents of change
are these: 1. THE ATMOSPHERE; 2. WATER; 3. PLANTS AND ANIMALS; 4.
SUBTERRANEAN FORCES. We shall consider these in succession.