In North Africa that portion of the great Sahara which has heretofore been considered a low plain, lying between Tripoli and Lake Tchad, has been ascertained by the German explorers, Overweg and Vogel, to be a table-land of the second class, ranging in elevation from 1000 to 2000 feet. It begins at the Chorean plateau (2000 feet) in the south of Tripoli, and sinks to an elevation of 800 feet in the neighborhood of Lake Tchad. The average altitude is about 1500 feet. This moderate elevation of Sahara corresponds with the equally high plateau of Cyrenaica, 2000 feet. The Atlas plateau, in the northwest of Africa, rises to a greater height—2000 to 3000 feet; the upper course of the Draa, near the Sahara, being 3000 feet; the high, broad table-land on which Timbuctoo lies, according to Renon’s measurement, is 1500 to 1800 feet above the sea. In south Africa the low, or rather the moderate plateau, which borders the district of the Bechuanas on the north, rises, as it advances toward the lower rim of Africa, at Cape Colony, to an altitude of 3000 feet. America has many plateaus of the second range of elevation, but her highlands of the first class are so imposing in extent, as well as in elevation, that they have been more carefully observed than the table-lands of the second class. Along the eastern slope of the Andes, on the same parallel with the great plains of the Orinoco, the Amazon, and the La Plata, these plateaus extend, touching the base of the mountains, and appearing rather as terraces, or vast plains of transition, from the highlands to the lowlands, than as independent forms. Where Alexander von Humboldt measured them, west of the low plains of the Amazon, he found their height, measured from the sea, to range from 1050 to 1200 feet; he describes them as having the appearance of vast plains, and as differing from the lowlands of the Amazon only in their greater elevation; their slope toward the narrowing of the Pongo de Mauseriche being too slight to be appreciable. Between the threefold forks of the Northern Andes, Humboldt ascertained the heights of ten plateaus, extending as far as the plains of Orinoco, and called by the various names, according to their elevation—Tierras templadas, or temperate districts, Tierras calientas, or warm districts, and Tierras frias, or cold districts—varying in height from 1800 to 6600 feet, the highest belonging clearly to the first class of plateaus. The mountains of Brazil are interspersed among plateaus of the second class. The Brazilian mountains are not true ranges, but lie in groups, their height varying from 2700 to 5700 feet, and between them are the vast elevated plains, called Campas, which are true plateaus of the second class.