Page:Africa by Élisée Reclus, Volume 1.djvu/175

 west, a mean altitude of about 8,000 feet. All the regions attaining or exceeding this height are called dega, a term analogous to the Persian sarhad and Arab nejd. Below the altitude of 6,000 feet, the intermediate valleys and gorges dividing the plateau, excavated by the mountain torrents to various depths, take the name of kwalla, kolla, or kulla, a zone of "hot lands" corresponding to the ghermsir of Persia, or to the tehamas of Arabia. Between these two zones stretches the coina- -dega, or temperate region. In many places the rugged escarpments present a sudden contrast between the degas and the kwallas, the difference of their relief being heightened by that of their climate and vegetation. The cataracts, such as that of Davezut, near Debra-Tabor, fall either in a single sheet or through a suc- cession of rapids from one zone to another. Most of the partial granite or basalt masses of the plateau have outer walls formed of cliffs and superimposed talus, which give the hills the appearance of step-pyramids; but some of these kwallas are little more than fissures or gorges, like the North American cafions. Such chasms appear to be but a stone's throw across; their true size, however, can only

be seen on descending into the abyss, walking for hours on the edge of giddy precipices, crossing the torrents at the bottom, and then scaling their abrupt sides. The defiles are occasionally blocked by masses of rock swept down by the mountain torrents, and presenting serious obstacles to the local traffic. The most remark- able ravines occur along the eastern edge of the plateau, where the total fissure exceeds 6,500 feet, measured from the summit of the degas down to the sea-level. Nowhere else can a more convincing proof be observed of the erosive action of running waters. The two walls of certain gorges, rising nearly vertically within a few feet of each other to a height of some hundreds of feet, represent an erosion of hard rock amounting to at least ten thousand five hundred million cubic feet. Nevertheless, the waters have regulated the fall of the channel, which averages not more than one in forty yards. This incline is easily ascended, but several of the defiles remain blocked for months together by the mountain torrents; every year new paths have to be formed across the debris, while some have had to be entirely abandoned. The route to Kumaili, through which the English army marched to the Abyssinian plateau, had probably not been occupied by a military force since