MEXICO, a Federal republic of North America, though be longing partly to Central America, in the geographical sense, and extending from the United States southward to Guatemala and British Honduras. Its northern boundary-line follows the Rio Grande del Norte (Rio Bravo) from its mouth northwest ward to lat. 31° 47' N., thence on that parallel westwards for Too m., thence south to lat. 31° 20' N., thence due west to the meridian, thence in a straight line, nearly west-north-west, to a point on the Colorado river 20 m. below the mouth of the Gila river, thence northward to the mouth of the Gila, and thence nearly due west along the old line between upper and lower Cali fornia, to a point on the Pacific coast one marine league south of the southernmost point of San Diego bay, this line having a total of 1,810 miles. The boundary-line with Guatemala and British Honduras is even more arbitrary, beginning at the mouth of the Suchiate river, on the Pacific coast, and following that stream to its source, thence determined by the peaks of Tacana., Buenavista and Ixbul, from there following a parallel to the Chixoy river, which is really the Upper Usumacinta, to a point on the Usumacinta itself about 16 miles S. of Tenosique (Ta basco), thence due east to the San Pedro Martir river, thence north to lat. 17° 4o' N., which it follows eastward to the border between British Honduras and Guatemala, thence north to the Hondo river which it skirts to Chetumal bay. The length of this border line is not known accurately.
Mexico consists largely of an elevated plateau, open in the direction of the United States, limited on its two maritime sides by a double chain of mountains, themselves a continuation of American ranges, which meet one another at La Junta, south-east of Puebla. To this plateau must be added a fringe of lowlands
extending on both sides between the mountains and the sea; a high region in the south-east which forms part of the Central American plateau ; and two peninsulas, one, that of Lower Cali fornia, which is evidently a continuation of the American coast range, and the other one, Yucatan, made almost entirely of a low calcareous plain.
Heilprin has shown that the great plateau of Mexico is what remains of a folded region, partly eroded, and covered with de posits of volcanic and detritic origin, and that the chains that are left are undoubtedly the continuation of the American Basin ranges deflected towards the south-east. Remains of transverse chains are to be seen in the isolated ridges and peaks, rising above the level of the table-land, and, in some cases, forming well defined basins. Two such depressions are particularly conspicu ous, the Bolson de Mapimi, in which the resemblance to the American Basin ranges is quite evident, and the Valley of Mexico, formerly provided with large bodies of water, now replaced by small lakes and marshy lagoons.
The Mexican high plateau has a general elevation of about 8,000 ft. in the States of Mexico and Puebla. Its southward slope is abrupt, while the one to the north is gradual, being about 41 ft. to the mile, as shown by the elevation at Ciudad Juarez, op posite El Paso, which is only 3,60o feet. In a general way the plateau slopes from south to north and from west to east.