line descends rapidly, some six hundred feet, into the valley of the city of Mexico; which valley is really an elevated plain, thirty-one by forty-five miles in extent, having an average altitude of about 7,500 feet above the sea level, and inclosed by high and irregular mountain-ridges, from which rise two volcanic peaks—Popocatepetl and Iztaccihuatl—to the height of 17,884 and 15,705 feet respectively, and whose summits are covered with perpetual snows. And it is at the lowest point, and near the center of this valley, or plain, and surrounded by a group of lakes, which in turn are bordered by swamps, that the city of Mexico is located.
Starting next from the city of Mexico, and going east toward the Atlantic, or west toward the Pacific, for a distance in either direction of about one hundred and sixty miles, we come to the edge or terminus of this great plateau; so well defined and so abrupt, that in places it seems as if a single vigorous jump would land the experimenter, or all that was left of him, at from two to three thousand feet lower level.
The annexed cut approximately represents the profile of the country between the two oceans, and in the latitude of the city of Mexico.
Up the side of the almost precipice, which bounds the plateau on the east—tunneling through