's foot. The pressure involved
in the process changed and hardened the rocks so much that the coal
which they contain was converted into anthracite, the finest coal in all
the world and the only example of its kind. Even the famous Welsh coal
has not been so thoroughly hardened. During a long period of erosion the
tops of the folded layers were worn off to a depth of thousands of feet
and the whole country was converted into an almost level plain. Then
in the late geological period known as the early Tertiary the land was
lifted up again, and once more erosion went on. The soft rocks were thus
etched away until broad valleys were formed. The hard layers were left
as a bewildering succession of ridges with flat tops. A single ridge
may double back and forth so often that the region well deserves the old
Indian name of the "Endless Mountains." Southwestward the valley grows
narrower, and the ridges which break its surface become straighter.
Everywhere they are flat-topped, steep-sided, and narrow, while between
them lie parts of the main valley floor, flat and fertile. Here in the
south, even more clearly than in the north, the valley is bordered
on the east by the sharply upstanding range of the crystalline
Appalachians, while on the west with equal regularity it comes to an end
in an escarpment which rises to the Alleghany plateau.

This plateau, the third great band of the Appalachians, begins on the
south side of the Mohawk Valley. To the north its place is taken by
the Adirondacks, which are an outlier of the great Laurentian area of
Canada. The fact that the outlier and the plateau are separated by the
low strip of the Mohawk Valley makes this the one place where the highly
complex Appalachian system can easily be crossed. If the Alleghany
plateau joined the Adirondacks, Philadelphia instead of New York would
be the greatest city of America. Where the plateau first rises on the
south side of the Mohawk, it attains heights of four thousand feet in
the Catskill Mountains. We think of the Catskills as mountains, but
their steep cliffs and table-topped heights show that they are really
the remnants of a plateau, the nearly horizontal strata of which
have not yet been worn away. Westward from the Catskills the plateau
continues through central New York to western Pennsylvania. Those who
have traveled on the Pennsylvania Railroad may remember how the railroad
climbs the escarpment at Altoona. Farther east the train has pas