rn Russian
military road through the Pass of Dariel across the Caucasus, and the
yet more recent Indian railroad to Darjeeling, with the highway
extension beyond to the Tibetan frontier through Himalayan Sikkim.

Such mountain regions attain independent historical importance when
their population increases enough to form the nucleus of a state, and to
acquire additional territory about the highland base either by conquest
or voluntary union, while they utilize their naturally protected
location and their power to grant safe transit to their allies, as means
to secure their political autonomy. Therefore to mountain regions so
often falls the role of buffer states. Such were medieval Burgundy and
modern Savoy, which occupied part of the same territory, Navarre which
in the late Middle Ages controlled the important passway around the
western end of the Pyrenees, and Switzerland which commands the passes
of the central Alps. The position of such mountain states is, however,
always fraught with danger, owing to the weakness inherent in their
small area and yet smaller allowance of productive soil, to their
diverse ethnic elements, and the forces working against political
consolidation in their deeply dissected surface. Political solidarity
has a hard, slow birth in the mountains.

[Sidenote: Transition forms of relief between highlands and lowlands.]

In view of the barrier character of mountains, a fact of immense
importance to the distribution of man and his activities is the rarity
of abrupt, ungraded forms of relief on the earth's surface. The
physiographic cause lies in the elasticity of the earth's crust and the
leveling effect of weathering and denudation. Everywhere mountains are
worn down and rounded off, while valleys broaden and fill up to shallow
trough outlines. Transition forms of relief abound. Human intercourse
meets therefore few absolute barriers on the land; but these few reveal
the obstacles to historical movement in perpendicular reliefs. The
mile-high walls of the Grand Canon of the Colorado are an insuperable
obstacle to intercourse for a stretch of three hundred miles. The
glacier-crowned ridge of the Bernese Alps is crossed by no wagon road
between the Grimsel Pass and the upper Rhone highway around their
western end, a distance of 100 kilometers (62 miles). The Pennine Alps
have no pass between the Great St. Bernard and the Simplon, a distance
of 90 kilometers (54 miles).

[Sidenote: Importance