The coast of Natal is rich in trees and herbage. The Zambesi, and other rivers which descend from the central plateau, refresh the plains of Mozambique and Zanzibar. But from the 4th parallel of north latitude to Cape Guardafui extends an almost continuous desert. The southern extremity of the Lupata chain also presents a vast naked country, where the presence of gold has encouraged the Portuguese to found some establishments. The neighbouring zone of Kaffraria consists of great far-spreading, gently-undulating plains, characterized by extreme aridity. The western districts are much less broken than the central, and exhibit no undulations except in the vicinity of the ocean. There an immense level territory exists under the name of the Kalihari Desert, whose southern boundary is marked by the Gariep or Orange River, which drains rather than waters it. To the north this awful wilderness stretches as far as the Lake Ngami, thus covering the area comprised between the 29th and 30th parallels of south latitude. The pastoral country of Namaqua and Damaras bounds it on the west. Eastward it extends to the 24th meridian of west longitude. Moisture is not wholly wanting in this vast region. The Kalihari has been called a desert, says Livingstone,[80] because it contains no running water, and very little in wells. Far from being destitute of vegetation, it is covered with grass and creeping plants, and there are large patches of bushes, and even trees. It is remarkably flat; and prodigious herds of antelopes roam over its trackless plains. The soil is composed in general of a fine soft sand, lightly coloured--that is, of a nearly pure silica. In the ancient beds of dried-up rivers lie immense patches of alluvial soil, which, hardened by the sun, form great reservoirs, retaining the rain-water for several months of the year. The quantity of grass flourishing in this region is remarkable. It grows generally in thick tufts, occasionally intermingled with spaces where the earth is naked or closely overgrown with creeping plants. These, deeply rooted in the soil, suffer but little from the effects of the excessive heat. Most of them have tubercular roots, and are so organized as to furnish both food and liquid during the long droughts--an epoch when one vainly seeks elsewhere anything which can appease one's hunger or one's thirst. The rich vegetation of the Kalihari is due to its geological constitution. It consists of a great valley, or rather of a vast basin, whose bottom is formed of a diluvial earth, and which is encircled by a belt of rocks, cloven at several places. It follows that where the rain is abundant, the slope of the hills directs it towards the centre of the basin, and this rain filters and deposits itself beneath the surface of the soil. And it appears to be a proof of this statement, that on digging in the sand cisterns are formed, or "sucking-places," which are filled with water supplied by subterraneous conduits.