as the result of mountain growth, we may say
that here and there on the earth's crust these dislocations have
occurred in such association and of such magnitude that great areas
have been uplifted above the plain of the sea. In general, we find
these groups of elevations so arranged that they produce the
triangular form which is characteristic of the great lands. It will be
observed, for instance, that the form of North America is in general
determined by the position of the Appalachian and Cordilleran systems
on its eastern and western margins, though there are a number of
smaller chains, such as the Laurentians in Canada and the ice-covered
mountains of Greenland, which have a measure of influence in fixing
its shore lines.

[Illustration: _Waterfall near Gadsden, Alabama. The upper shelf of
rock is a hard sandstone, the lower beds are soft shale. The
conditions are those of most waterfalls, such as Niagara._]

The history of plains, as well as that of mountains, will have further
light thrown upon it when in the next chapter we come to consider the
effect of rain water on the land. We may here note the fact that the
level surfaces which are above the seashores are divisible into two
main groups--those which have been recently lifted above the sea
level, composed of materials laid down in the shallows next the shore,
and which have not yet shared in mountain-building disturbances, and
those which have been slightly tilted in the manner before indicated
in the case of the plains which border the Rocky Mountains on the
east. The great southern plain of eastern and southern United States,
extending from near New York to Mexico, is a good specimen of the
level lands common on all the continents which have recently emerged
from the sea. The table-lands on either side of the Mississippi
Valley, sloping from the Alleghanies and the Cordilleras, represent
the more ancient type of plain which has already shared in the
elevation which mountain-building brings about. In rarer cases plains
of small area are formed where mountains formerly existed by the
complete moving down of the original ridges.

There is a common opinion that the continents are liable in the course
of the geologic ages to very great changes of position; that what is
now sea may give place to new great lands, and that those already
existing may utterly disappear. This opinion was indeed generally held
by geologists not more than thirty years ago. Further stu