levelled and graded route, highly
suitable for the tracks of the railway. As a result, outside of the
level lands of the Great Central Plain, not far from eighty per cent. of
the railway mileage of the United States is constructed along
river-valleys.

=Plateaus.=--Plateaus are usually characterized by broken and more or less
rugged surface features. As a rule they are deficient in the amount of
rainfall necessary to produce an abundance of the grains and similar
food-stuffs, although this is by no means the case with all.

Most plateaus produce an abundance of grass, and cattle-growing is
therefore an important industry in such regions. Thus, the plateaus of
the Rocky Mountains are famous for cattle, and the same is true of the
Mexican and the South American plateaus. The Iberian plateau, including
Spain and Portugal, is noted for the merino sheep, which furnish the
finest wool known. The plateau of Iran is also noted for its wool, and
the rugs from this region cannot be imitated elsewhere in the world.

=Plains.=--Plains are of the highest importance to life and its
activities. Not only do they present fewer obstacles to
intercommunication than any other topographic features, but almost
always they are deeply covered with the fine rock-waste that forms the
chief components of soil. Plains, therefore, contain the elements of
nutrition, and are capable of supporting life to a greater extent than
either mountains or plateaus. About ninety per cent. of the world's
population dwell in the lowland plains.

The Great Central Plain of North America produces more than one-quarter
of the world's wheat, and about four-fifths of the corn. The southern
part of the great Arctic plain, and its extension, the plains of the
Baltic also yield immense quantities of grain and cattle products. The
coast-plains of the Atlantic Ocean, on both the American and the
European side, are highly productive.

River flood-plains are almost always densely peopled because of their
productivity. The bottom-lands of the Mississippi and the Yangtze Rivers
are among the chief food-producing regions of the world. Lacustrine
plains, the beds of former lakes, are also highly productive regions.
The valley of the Red River of the North is an example, and its wheat is
of a very high quality.

Fertile coast-plains and lowlands that are adjacent to good harbors, as
a rule are the most thickly peopled regions of the world. In many such
regions the density