ed break across the entire width of
the long Appalachian system. This fact, together with its ready
accessibility from the Hudson on the east and Lake Ontario on the west,
lent it importance in the early history of the colonies, as well as in
the later history of New York. It was an easy line of communication with
the Great Lakes, and gave the colonists access to the fur trade of the
Northwest, then in the hands of the French. So when French and English
fought for supremacy in the New World, the Mohawk and Hudson valleys
were their chief battleground; elsewhere the broad Appalachian barrier
held them apart. Again in the Revolution, control of the Mohawk-Hudson
route was the objective of the British armies mobilized on the Canadian
frontier, because it alone would enable them to co-operate with the
British fleet blockading the coast cities of the colonies. In the War of
1812, it was along this natural transmontane highway that supplies were
forwarded to the remote frontier, to support Perry's fight for control
of the Great Lakes. The war demonstrated the strategic necessity of a
protected, wholly American line of water communication between the
Hudson and our western frontier, while the commercial and political
advantage was obvious. Hence a decade after the conclusion of the war,
this depression was traced by the Erie Canal, through which passed long
lines of boats to build up the commercial greatness of New York City.

[Sidenote: Height in mountain barriers.]

Other structural features being the same, mountains are barriers also in
proportion to their height; for, with few exceptions, the various
anthropo-geographic effects of upheaved areas are intensified with
increase of elevation. Old, worn-down mountains, like the Appalachians
and the Ural, broad as they are, have been less effective obstacles than
the towering crests of the Alps and Caucasus. The form of the elevation
also counts. Easy slopes and flat or rounded summits make readier
transit regions than high, thin ridges with escarpment-like flanks.
Mountains of plateau form, though reaching a great altitude, may be
relatively hospitable to the historical movement and even have a regular
nomadic population in summer. The central and western Tian Shan system
is in reality a broad, high plateau, divided into a series of smoothly
floored basins and gently rolling ridges lying at an elevation of 10,000
to 12,000 feet above the sea. Its pamirs or plains of thick grass