part of the country's area is taken up by this great plateau of
Anahuac, as it is sometimes termed. The tableland is bounded both on
the east and the west by ranges of mountains, known as the Eastern
Sierra Madre and the Western Sierra Madre respectively. These mountains
close in towards the south, enclosing the tableland in a tapering form,
and the Valley of Mexico which forms its extremity. On the north the
_mesa central_ is intersected by the Rio Grande, which forms the
boundary of Mexico with the United States; and the plateau continues
thence into the territory of that country. The length of this plateau,
from north-west to south-east, or, roughly, upon its longitudinal axis,
is approximately 800 miles, and its greatest width between the summits
of the enclosing mountains about 500 miles. The tableland is, however,
intersected by various lesser ranges of hills, and is not by any means
a flat, unbroken expanse. Nevertheless, its formation is such that a
vehicle might be driven from the City of Mexico for vast distances
without having resort to roads. It may be looked upon physically as a
great plane, inclined or tipped from south to north, or from the City
of Mexico to the United States border. The general elevation above
sea-level of the inclined plane at its southern end is 8,000 feet, and
that at its northern 4,000 feet--a slope of 4,000 feet in a direction
away from the Equator; and a fact which greatly influences its climate.
The Mexican plateau is the result of after-formation from the mountain
system of the country. The Sierra Madres are the Mexican Andes, part of
the chain-formation of those vast Cordilleras which are most developed
in South America, on the one hand, and are encountered in the Rocky
Mountains of North America on the other. In South America the Andes
consist of huge parallel chains with river and lake-basins of profound
depth between them. In Mexico the same formation must have existed, but
the basins have been filled up by material discharged from volcanoes
and from the erosion of the mountains themselves, doubtless caused by
the severe and sudden rain-storms and rapid changes of temperature
characteristic of these regions. Thus the great plateau may be likened
to a number of filled-up troughs, through whose general surface the
tops of mountain ranges still protrude in "islands" or groups, whose
crests form the intersecting hills of the plateau. Some of the plains
of the plateau between these