The general structure of North America is still more simple than that of the southern part of the continent. The table-lands of Mexico and the Rocky Mountains, which are the continuation of the high land of the Andes, run along the western side, but at so great a distance from the Pacific as to admit of another system of mountains along the coast. The immense plains to the east are divided longitudinally by the Alleghany Mountains, which stretch from the Carolinas to the Gulf of St. Lawrence, parallel to the Atlantic, and at no great distance from it. Although the general direction of the mountains is from south to north, yet, as they maintain a degree of parallelism to the two coasts, they diverge towards the north—one in clinging towards the north-west, and the other towards the north-east. The long narrow plain between the Atlantic and the Alleghanies is divided, throughout its length, by a line of cliffs not more than 200 or 300 feet above the Atlantic plain, the out-cropping edge of the Second Terrace, or Atlantic Slope, whose rolling surface goes west to the foot of the mountains. An enormous table-land occupies the greater part of Mexico or Anahuac. It begins at the Isthmus of Tehuantepec, and extends north-west to the 42d parallel of north latitude, a distance of about 1600 miles, which is nearly equal to the distance from the north extremity of Scotland to Gibraltar. It is narrow towards the south, and expands towards the north-west till about the latitude of the city of Mexico, where it attains its greatest breadth of 360 miles, and there also it is highest. The most easterly part in that parallel is 7500 feet above the sea, from whence it extends towards the west, the height being 7430 feet at the city of Mexico, and then gradually diminishes to 4000 feet towards the Pacific. Its height in California is not known, but it still bears the character of a table-land, and maintains an elevation of 6000 feet along the east side of the Sierra Madre, even to the 32d degree of north latitude, where it sinks to a lower level before joining the Rocky Mountains. The descent from this plateau to the low lands is very steep on all sides; on the east, especially, it is so precipitous that, seen from a distance, it is like a range of high mountains. There are only two carriage-roads to it from the Mexican Gulf, by passes 500 miles asunder— one at Xalapa, near Vera Cruz, the other at Saltillo, west of Monterey. The descent to the shores of the Pacific is almost equally rapid, and that to the south no less so, where, for 300 miles between the plains of Tehuantepec and the Rio Yopez, it presses on the shores of the Pacific, and terminates in high mountains, leaving only a narrow margin of hilly maritime coast. Where the surface of the table-land is not traversed by mountains, it is as level as the ocean. There is a carriage-road over it for 1500 miles, without hills, from the city of Mexico to Santa Fé.