d crystalline band there are great areas of gentle relief where an
abundant population can dwell. Westward on the edges of the plateau and
the plains beyond a still greater population can find a living, but in
the intervening space there is opportunity for only a few. The great
problem is to cross the mountains as easily as possible. Each accessible
crossing-place is associated with a city. Boston, as well as New York,
owes much to the low Mohawk-Hudson route, but is badly handicapped
because it has no easy means of crossing the eastern crystalline band.
Philadelphia, on the other hand, benefits from the fact that in its
vicinity the crystallizes are low and can readily be crossed even
without the aid of the valleys of the Delaware and Schuylkill rivers.
It is handicapped, however, by the Alleghany escarpment at Altoona, even
though this is lower there than farther south. Baltimore, in the same
way, owes much of its growth to the easy pathways of the Susquehanna
on the north and the Potomac on the south. Farther south both the
crystalline band and the Alleghany plateau become more difficult to
traverse, so that communication between the Atlantic coast and the
Mississippi Valley is reduced to small proportions. Happy is New York
in its situation where no one of the three bands of the Appalachians
opposes any obstacle. The plains of North America form the third of the
four main physical divisions of the continent. For the most part they
lie between the great western cordillera on one side and the Laurentian
and Appalachian highlands on the other. Yet they lap around the southern
end of the Appalachians and run far up the Atlantic coast to New York.
They remained beneath the sea till a late date, much later than the
other three divisions. They were not, however, covered with deep water
like that of the abysmal oceans, but only with shallow seas from which
the land at times emerged. In spite of the old belief to the contrary,
the continents appear to be so permanent that they have occupied
practically their present positions from the remotest geological times.
They have moved slowly up and down, however, so that some parts have
frequently been submerged, and the plains are the parts that remained
longest under water.

The plains of North America may be divided into four parts according
to the character of their surface: the Atlantic coastal plain, the
prairies, the northwestern peneplain, and the southwestern high plains.
T