sture intercepted by the Alps and the Himalayas has
not only created the plains of the Po and the Ganges from the rock-waste
carried from the slopes, but has also made them exceedingly fertile.

Mountain-ranges are also valuable for their contents. The broken
condition of the rock-folds and the rapid weathering to which they are
subjected have exposed the minerals and metals so useful in the arts of
commerce and civilization. Thus, the weathering of the Appalachian folds
has made accessible about the only available anthracite coal measures
yet worked; and the worn folds about Lake Superior have yielded the
ores that have made the United States the foremost copper and steel
manufacturing country of the world. Gold, silver, tin, lead, zinc,
platinum, granite, slate, and marble occur mainly in mountain-folds.

=Mountains and Valleys.=--Mountain-ranges are great obstacles to commerce
and intercommunication. The Greek peoples found it much easier to
scatter along the Mediterranean coast than to cross the Balkan
Mountains. For twenty years after the settlement of California, it was
easier and less expensive to send traffic by way of Cape Horn than to
carry it across the Rocky Mountains.

The deep canyons of mountainous regions are quite as difficult to
overcome as the high ranges. In modern methods of transportation a range
that cannot be surmounted may be tunnelled, and a tunnel five or six
miles in length is no uncommon feat of engineering. A canyon, however,
cannot be tunnelled, and if too wide for cantilever or suspension
bridges, a detour of many miles is necessary. In crossing a deep chasm
the route of transportation may aggregate ten or fifteen times the
distance spanned by a straight line.

Excepting the mining regions, the population of mountainous countries is
apt to be found mainly in the intermontane valleys. A reason for this is
not hard to find; the valleys are usually filled with rich soil brought
from the higher slopes and levelled by the water. The population,
therefore, is concentrated in the valley because of the food-producing
power of the land. For this reason the Sound, Willamette, and San
Joaquin-Sacramento Valleys contain the chief part of the Pacific coast
population. The Shenandoah and the Great Valley of Virginia are similar
instances.

What is true of the larger intermontane valleys is true also of the
narrow stream valleys of mountain and plateau regions. Thus, in the New
England plateau the