In North, as in South America, the deserts proper lie to the west of the continent, and occupy the basins commanded by the parallel or divergent walls of the Rocky Mountains. The most northerly of these American deserts occupies, to the west of Lake Utah, a part of the space called the “Great Basin,” and is comprised between the principal chain of the Rocky Mountains and the Sierra Nevada of California. The desert of Utah is an immense surface of clay, dotted over with thin tufts of artemisia; in certain places, however, it exhibits no trace of vegetation, and resembles a causeway of concrete, intersected by innumerable clefts, forming nearly regular polygons. In the midst of these solitudes no rivulet flows, and no water-spring gushes forth; only after journeying for many a long hour the traveler sometimes comes upon some field of crystallized salt, a white expanse, on which the clouds and blue sky are reflected as on the surface of a lake. On the extreme horizon some volcanic rocks may be seen, like great scoriæ, half veiled by warm atmospheric columns, quivering like the air over the flame of a hot brazier. Across these vast plains, inhabited only by a prodigious quantity of extraordinarily shaped lizards, the road employed by the emigrants used to pass, which was so soon destined to be supplanted by the Pacific Railway from New York to San Francisco. The deserts of North America, crossed here and there by fertile valleys, extend eastward toward the basins of the Red River and the Arkansas, where they blend with the savannas, and to the south into the Mexican states of Chihuahua, Sonora, and Sinaloa. But in the tropical zone, which commences beyond these points, the heavy summer rains and the much smaller extent of the Mexican territory between the two oceans, have prevented the formation of deserts. Regions destitute of trees and verdure are only again found on the coasts of Peru, to the south of the Gulf of Guayaquil. The trade-winds, after having discharged their moisture on the eastern slopes of the Andes, pass away through the air far above the seashore on the western side of the mountains, and then sweep far out to sea over the surface of the Pacific.