The nature of the soil and climate of great plateaus is in general such
as to render them the least useful portions of the continents. Sahara,
with an average altitude of 1,000 feet, and the higher plateaus of
Mongolia, Iran and parts of the American Basin, may serve as types.
Their surface consists of hardened sand and rock; of hillocks and plains
of loose sand constantly shifting by the wind; and of immense tracts, as
in Mongolia, covered with pebbles varying from the size of a walnut, or
even less, to a foot in diameter: all indicating the original
transporting, grinding and depositing of these materials by water.
Salt lakes without outlet occur in each, and salt efflorescence often
covers the ground. A lack of rain to wash from the soil substances
injurious to vegetation, and supply the water necessary for the growth
of plants, leaves these plateaus generally sterile, and some of the most
extensive are in part, if not wholly, deserts.