slands are made up of volcanic ejections of lava,
pumice, and dust, which has been thrown up from craters and fallen
around their margin or are formed of coral and other organic remains.

Next after this general statement as to the division of sea and land
we should note the peculiarities which the earth's surface exhibits
where it is bathed by the air, and where it is covered by the water.
Beginning with the best-known region, that of the dry land, we observe
that the surface is normally made up of continuous slopes of varying
declivity, which lead down from the high points to the sea. Here and
there, though rarely, these slopes centre in a basin which is occupied
by a lake or a dead sea. On the deeper ocean floors, so far as we may
judge with the defective information which the plumb line gives us,
there is no such continuity in the downward sloping of the surface,
the area being cast into numerous basins, each of great extent.

When we examine in some detail the shape of the land surface, we
readily perceive that the continuous down slopes are due to the
cutting action of rivers. In the basin of a stream the waters act to
wear away the original heights, filling them into the hollows, until
the whole area has a continuous down grade to the point where the
waters discharge into the ocean or perhaps into a lake. On the bottom
of the sea, except near the margin of the continent, where the floor
may in recent geological times have been elevated into the air, and
thus exposed to river action, there is no such agent working to
produce continuous down grades.

Looking upon a map of a continent which shows the differences in
altitude of the land, we readily perceive that the area is rather
clearly divided into two kinds of surface, mountains and plains, each
kind being sharply distinguished from the other by many important
peculiarities. Mountains are characteristically made up of distinct,
more or less parallel ridges and valleys, which are grouped in very
elongated belts, which, in the case of the American Cordilleras,
extend from the Arctic to the Antarctic Circle. Only in rare instances
do we find mountains occupying an area which is not very distinctly
elongated, and in such cases the elevations are usually of no great
height. Plains, on the other hand, commonly occupy the larger part of
the continent, and are distributed around the flanks of the mountain
systems. There is no rule as to their shape; they normally grade