direct chemical action of the moist atmosphere or the
alternation of hot and cold which crumbles rocks far above the
line where rain never falls. Once the rock is rotten and
decayed, it yields readily to the forces of degradation, which
drag it down--the beating of the rain, the rush of the
avalanche or of the landslide, the tumult of the torrent, the
quieter action of the muddy river in its lower reaches or the
mighty glacier which transfers fine and coarse material alike
toward the sea.

These actions are always going on. Are they always equally
balanced, or are there periods when the forces of elevation are
more active, the forces of degradation not so powerful, as
against other times in which the forces of degradation alone
are at work? If there is inequality in the balance and struggle
of these contending forces, the great periods or acts in the
geologic drama might thus be marked off as Chamberlin suggests.
Newbery, Schuchert and others have pointed out that there seem
to have been great cycles of sedimentation which may be
interpreted as due to the alternate success, first of the
factors of elevation, then of those of degradation.

Suppose, for instance, that there has been an epoch of
elevation, that mountain chains have been lifted far into the
sky and volcanoes have sent their floods of lava forth, and
fault-scarped cliffs run across the landscape and that then,
for a while, the forces of elevation cease their work. Little
by little, the mountains will be worn down to a surface of less
and less relief, approaching a plain as a hyperbola approaches
its asymptote--a surface which W. M. Davis has called