widely recognized. Because of this fact its economic
applications may be summarized at somewhat greater length than those of
the other branches of geology above mentioned, which are to be more or
less taken for granted.

The central feature of physiography is the so-called erosion cycle or
topographic cycle. Erosion, acting through the agencies of wind, water,
and ice, is constantly at work on the earth's surface; the eroded
materials are in large part carried off by streams, ultimately to be
deposited in the ocean near the continental margins. The final result is
the reduction of the land surface to an approximate plain, called a
_peneplain_, somewhere near sea level. Geological history shows that
such peneplains are often elevated again with reference to sea level, by
earth forces or by subsidence of the sea, when erosion again begins its
work,--first cutting narrow, steep gulches and valleys, and leaving
broad intervening uplands, in which condition the erosion surface is
described as that of _topographic youth_; then forming wider and more
extensive valleys, leaving only points and ridges of the original
peneplains, in which stage the surface is said to represent _topographic
maturity_; then rounding off and reducing the elevations, leaving few or
none of the original points on the peneplain, widening the valleys still
further and tending to reduce the whole country to a nearly flat
surface, resulting in the condition of _topographic old age_. The final
stage is again the peneplain. This cycle of events is called the
_erosion cycle_ or _topographic cycle_. Uplift may begin again before
the surface is reduced to base level; in fact, there is a constant
oscillation and contest between erosion and relative uplift of the land
surface.

The action of the erosion cycle on rocks of differing resistance to
erosion and of diverse structure gives rise to the great variety of
surface forms. The physiographer sees these forms, not as heterogeneous
units, but as parts of a definite system and as stages in an orderly
series of events. He is able to see into the topographic conditions
beyond the range of immediate and direct observation. He is able to
determine what these forms were in the past and to predict their
condition in the future. He is able to read from the topography the
underground structure which has determined that topography. A given
structure may in different stages of topographic development give quite
diverse