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