Page:Africa by Élisée Reclus, Volume 1.djvu/363

 GEOLOGICAL FORMATIOXS. 2S7 out the open plain. They arc all in the form of a crescent, the boms turning to the south under the influence of the north wind. Geoixkjical Formations. Nearly all the sandstone rocks and dunes are destitute of vegetation ; but few shrubs are found 6n tbe slopes of the crystalline mountains, which are embellished by these verdant thickets. Hitherto no fossils of animals have been discovered in the sands of the desert of Korosko, but only some petrified trees, like those in the Bayuda stepi)e, in Egypt, and several other countries of Eastern Africa. According to Russegger, these Nubian sandstones have been deposited since the chalk period. One of the most extraordinary products of this geological formation consists of spheroidal stones of all sizes, resembling bulls, shot, and bullets. They are so thickly scattered over the soil that travellers had seriously proposed to Mohammed AH that he should supply his artillery parks from this source. These stone bullets, similar to those found in Hungary in the mountains near Koloszvar, are formed of concentric beds of variously coloured sands, hollow in the middle, or else filled with loose sand, and with a very hard ferruginous exterior. The cir- cumference of the stone is frequently marked by a ridge similar to that which the moulds leave on the bullets at their point of contact. The great caravan route which traverses the Nubian desert, to the east of the Nile, from AbCl-Hamed to Korosko, extends over a space of about 300 miles, which comprises some of the most remarkable localities, offering examples of all the geological formations of the country. This region is specially termed aiinur, a name probably of Berber origin, for in the language of the Tuaregs iemum means a " tract of country." After having ascended the trachyte-crested hills, and surmounted the granite escarpments, the caravan route winds from breach to breach between the sandstone hills, and even crosses a plain which, according to the Arabs, is an ancient lacustrine basin, the Bahr-bel&-m&, or " Waterless River." Nevertheless there are no indications which point to the presence of running or still waters having ever been in this place. One well only, that of Morad, yields a scanty supply of fresh water to travellers crossing the atmur. But there are regions in this desert where the sand contains abundance of saline substances which doubtless proceed from ancient evaporated lakes. In the vicinity of the river the natives extract this salt and sell it to the caravans. The largest of the dry valleys which wind through the desert of Nubia is that of Wady-Allaki. Taking its origin in the mountains of the Etbai, it follows a north-westerly course and falls into the Nile below Korosko ; its basin is more than 10,000 square miles in extent. It has occasionally happened that the Wady-Allaki, suddenly filled by heavy showers, has for some hours suddenly become a powerful affluent of the Nile, the force of its current completely barring the main-stream. But the valley of the wady and the tributar}* gorges are nearly always dry ; nevertheless, the concealed moisture is revealed by the trees, under which the Bisharin tribes arc acoustomed to encamp.