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 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.