crests are hydrographic entities, with no
outlet for their waters, as in the case of the _Bolson_ of Mapimi--a
vast rock-wilderness of 50,000 square miles in area, with great swamps
and lake bottoms--and the Valley of Mexico. These great depressions,
indeed, in a measure bear out the analogy or relationship with the
South American Andes, as in the case of the hydrographic entity of Lake
Titicaca in Peru and Bolivia, the great inland sea whose waters have no
outlet save by evaporation. The enormous depth of alluvial soil found
in the _bolsones_ or depressions of the Mexican plateau, formed from
rock-decay, or of volcanic material accumulated by the great lakes of
recent times which covered them in the central part of the great _mesa
central_, bear striking evidence to the filling-up process of the past.
In the neighbourhood of the River Nazas wells have been sunk to great
depths in this material without a single stone or rock of any
description being encountered. Indeed, on some of the cotton lands of
this region I have looked in vain to find even a pebble, so fine is the
alluvial soil. The stratified rocks, which are scarce upon the southern
part of the plateau, become much more prevalent in the north, and the
vast sandy, arid plains, which cover enormous areas of land in
Chihuahua and Coahuila extending thence past the valley of the Rio
Grande into the great American deserts of Texas and New Mexico, are
doubtless formed from the disintegration of the sandstone and chalk
horizons of that region.

Leaving for a moment our examination of the great plateau, let us
observe the coast. On both sides of the country--the Gulf of Mexico or
the Pacific Ocean--we observe that the littoral is composed of sandy
lowlands. On the eastern or Gulf side these coastal plains vary in
width from a few miles up to a hundred miles; for the Cordillera
approaches near to the sea at Vera Cruz, and recedes far from it in
Tamaulipas. Upon the Pacific side, however, the coastal plains are more
restricted in width, as the Cordillera runs nearer the sea-coast, but
leaving a wider strip at the north. Indeed, in the State of Guerrero
the Sierras rise almost abruptly from salt water, and the waves bathe
the roots of the trees which cover the mountain slopes. The country
rises rapidly from both oceans--more rapidly from the Pacific side--and
forms a succession of terraces upon the slopes of the Sierra Madres,
traversed by profound transversal canyons and