The geological structure or physiognomy of Mexico is peculiar. The great Cordillera of the Andes, which traverses the whole of South America, from its southernmost limit, is exceedingly depressed at the Isthmus of Panama, where its gentle swells serve merely to form a barrier between the union of the Pacific and Atlantic. But, as soon as this massive chain enters the broader portion of North America, it divides into two gigantic arms, to the east and west along the shores of the Gulf and of the Pacific, which support between them a continuous lofty platform, or series of table lands, crossed, broken, and intersected by innumerable and abrupt sierras, some of which rise to the height of seventeen thousand feet above the level of the sea. This geological structure prevails throughout the whole of Mexico, as now bounded; for, at the Rio Grande, the southern limit of Texas, the land sinks to comparative levels, and affords channels for the numerous and important streams with which, Louisiana, Florida and Texas are abundantly irrigated. Whilst this is the case on the northern and eastern confines of Mexico, the western portion is still traversed by the main body of the gigantic Cordillera, which, penetrating California with its icy peaks of the Sierra Nevada, passes onward to the north until its rocky walls are lost, beyond Oregon, in the wilderness that bounds the Frozen Sea.[1]
The reader who pictures to himself such a country will easily understand that all temperatures are gained in Mexico on the same parallel of latitude,—or that eternal heat and eternal frost are encountered in crossing the country in a straight line from Vera Cruz to the Pacific coast. It is a country hanging on the two slopes of a mountain, one of which descends to the Gulf and the other to the Western Ocean; and the traveller, in penetrating it, even by the road usually traversed by public conveyances, must attain a height of ten thousand six hundred and sixty feet, before he begins to descend into the valley of Mexico, which is, still, seven thousand five hundred and forty-eight feet above the level of the sea! Thus
- ↑ The high table land of Mexico which we have described, is said to owe its present form to the circumstance that an ancient system of valleys in a chain of granitic mountains, has been filled up to the height of many thousand feet with various volcanic products. Five active volcanos traverse Mexico from west to east,—Tuxtla, Orizaba, Popocatepetl, Jorullo, and Colima. Jorulla which is in the centre of the great platform is no less than one hundred and twenty miles from the nearest ocean, which is an important circumstance, showing that proximity to the sea is not a necessary condition although certainly a very general characteristic of the position of active volcanos. If the line which connects these five volcanic vents in Mexico be prolonged westerly, it cuts the volcanic group in the Pacific called the group of Revilla-Gigedo.—Lyell's Geology, American edition, vol. 1, p. 294
