lts.]

The piedmont boundary also divides two areas of contrasted density of
population. Mountain regions are, as a rule, more sparsely settled than
plains. The piedmont is normally a transition region in this respect;
but where high mountains rise as climatic islands of adequate water
supply out of desert and steppes, they concentrate on their lower slopes
all the sedentary population, making their piedmonts zones of greatest
density. Low mountains in arid regions become centers of population;
here their barrier nature vanishes. In the Sudanese state of Darfur, the
Marra Mountains are the district best watered and most thickly
populated. Nowhere higher than 6000 feet (1850 meters), they afford
running water at 4000 feet elevation and water pools in the sandy beds
of their wadis at 3200 feet. Below this, water disappears from the
surface, and can be found only in wells whose depth and scarcity
increase with distance from the central mountains.[1194] The neighboring
kingdom of Wadai shows similar conditions and effects.[1195] In the heart
of Australia, where utter desert reigns, the Macdonnell Ranges form the
nucleus of the northern area occupied by the Arunta tribe of natives;
farther north the Murchison Range, usually abounding in water-holes, is
the center and stronghold of the Warramunga tribe.[1196]

Mineral wealth or waterpower in the mountains serves to collect an urban
and industrial population along their rim, as we see it about the base
of the Erz Mountains in Saxony, the Riesen range in Silesia, the
coal-bearing Pennine Mountains of northwestern England, and the
highlands of southern Wales, all which piedmont zones show a density of
over 150 to the square kilometer (385 to the square mile). Hence the
original Swiss Confederation, which included only the mountain cantons
of Schwyz, Uri and Unterwalden, was greatly strengthened by the
accession of the piedmont cantons of Lucerne, Zurich, Zug and Bern in
the early fourteenth century, as later by St. Gall, Aargau and Geneva.
These marginal cantons to-day show a density of population exceeding
385 to the square mile, and rising to 1356 in the canton of Geneva.

[Sidenote: Piedmont towns and roads.]

Piedmont belts tend strongly towards urban development, even where rural
settlement is sparse. Sparsity of population and paucity of towns within
the mountains cause main of traffic to keep outside the highlands, but
close enough to their base to tap their trade at e