SAHARA is the great desert region which stretches across the continent of Africa eastwards from the Atlantic for a considerable distance on both sides of the Tropic of Cancer, and is generally distinguished by aridity of soil, absence of running water, dryness of atmosphere, and comparative scarcity of vegetable and animal life. The physical limits of this region are in some directions marked with great precision, as in part of Morocco and Algeria, where the southern edge of the Atlas range looks out on what has almost the appearance of a boundless sea, and forms, as it were, a bold coast-line, whose sheltered bays and commanding promontories are occupied by a series of towns and villagesTizgi, Figig, Laghouat, &c. In other directions the boundaries are vague, conventional, and disputed. This is especially the case towards the south, where the desert sometimes comes to a close as suddenly as if it had been cut off with a knife, but at other times merges gradually and irregularly into the well-watered and fertile lands of the Sudan (Soudan). While towards the east the valley of the Nile at first sight seems to afford a natural frontier, the characteristics of what is usually called the Nubian or Arabian desert are so identical in most respects with those of the Sahara proper that some authorities extend this designation over the whole country to the shores of the Red Sea. The desert, indeed, does not end with Africa, but is prolonged eastwards through Arabia towards the desert of Sind. As the Nubian region has been described under the heading NUBIA (vol. xvii. p. 610), attention will in the present article be confined to the desert country west of the Nile valley. Even as thus defined the Sahara is estimated to have an area of 3,565,565 square miles, or nearly as much as all Europe minus the Scandinavian peninsula and Iceland; but, while Europe supports a population of 327,000,000, the Sahara probably does not contain more than 2,500,000,a figure, however, which is sufficiently startling to those who think of it as an uninhabitable expanse of sand. The sea-like aspect of certain portions of the Sahara has given rise to much popular misconception, and has even affected the ideas and phraseology of scientific writers. Instead of being a boundless plain broken only by wave-like mounds of sand hardly more stable and little less dangerous than the waves of ocean, the Sahara is a region of the most varied surface and irregular relief, ranging in altitude from 100 feet below to some 5000 or 6000 or even it may be 8000 feet above the sea-level, and, besides sand-dunes and oases, containing rocky plateaus, vast tracts of loose stones and pebbles, ranges of hills of the most dissimilar types, and valleys through which abundant watercourses must once have flowed.
The culminating points of the Sahara are probably the summits of the Ahaggar (Hoggar), a great mountain plateau, not inferior to the Alps in the area which it covers, crossing the Tropic of Cancer about 5° and 6° E. long., almost midway between the Atlantic and the valley of the Nile. In its central mass rise with red steep cliffs two peaks, Watellen and Hikena, which Duveyrier believes to be volcanic like those of Auvergne. The height of this country has not been ascertained by direct European observation, but may be gathered from the fact that according to the Tuareg the snow lies for three months of the year, from December to March. To the northwest, and separated from the Atakor-'n-Ahaggar by a wide plain, rises the Muydir plateau, lying nearly east and west for a distance of about 200 miles. Its northeastern extremity is extended towards Timassinin by the Irawen Mountains, which in their turn are separated by a narrow valley from the Tasili plateau (strictly Tasili of the Asjer or Asgar). This great plateau stretches south-east for 300 miles parallel with the Atakor-'n-Ahaggar (from which it is separated by the Amadghor and Adamar plains), and then the line of elevation is continued by low ridges to the Tummo or War Mountains, and so onwards to the highland country of Tibesti or Tu, whose highest point, Tusidde, is 7880 feet above the sea-level, while its south-eastern eminences gradually die away in the direction of Wadai and Darfor (Darfur). About midway between Tibesti and the Niger rises the isolated mountain mass of Air or Asben, in which Dr Erwin von Bary1 discovered the distinct volcanic crater of Teginjir with a vast lava-bed down its eastern side. By some this country is assigned to the Sudan, as it lies within the limit of the tropical rains ; but the districts farther south have all the characteristics of the desert. The low but extensive plateau of Adghagh lies between Air and the Niger. Away to the north-east, in the country of FEZZAN (q.v.), are the dark mountains of Jebel es-S6da, which are continued south-east towards Kufra by the similar range of the Haruj ; and in the extreme south-west at no great distance from the Atlantic is the hilly country of Adrar (Aderer).
Nearly all the rest of the Sahara consists in the main of undulating surfaces of rock (distinguished as hammada), vast tracts of water-worn pebbles (serir), and regions of sandy dunes (variously called maghter, erg or areg, igidi and in the east rliari), which, according to M. Pomel, occupy about one-ninth or one-tenth of the total area. The following is the general distribution of the dunes. From the Atlantic coast to the south of Cape Blanco a broad belt extends north-east for a distance of about 1300 miles, with a breadth varying from 50 to 300 miles. This is usually called the Igidi or Gidi, from the Berber word for dunes. Eastward it is continued to the south of Algeria and Tunis by the Western Erg and the Eastern Erg, separated by a narrow belt at Golea. To the south of the Eastern Erg (which extends as far north as the neighbourhood of the Lesser Syrtis) the continuity of the sandy tract is completely broken by the Hammada al-Homra (or Red Rock Plateau), but to the south of this region lie the dunes of Edeyen, which, with slight interruptions, extend to Murzuk (Morzuk). To the south of the hammada of Murzuk the dunes of Murzuk stretch away south-east. Looked at in its entirety, this series of tracts may be called the northern zone; it forms a kind of bow, with its extremities respectively at the Atlantic and the Libyan Desert and its apex in the south of Tunis. In the south are the Juf, covering a vast area to the south-east of the middle portion of the Igidi, another area between the Adghagh plateau and the Tasili wan Ahaggar, and a third between Air and Tibesti. Away to the east in the Libyan Desert is a vast region of dunes of unascer-tained limits. It must be borne in mind that the sands do not entirely cover the areas assigned to them in the ordinary maps, which are of too small a scale to show the interchange of different kinds of surface. In the Eastern Erg especially the dunes lie in long lines in a north-north-west and south-south-east direction, presenting a gradual slope to windward and an abrupt descent to leeward. There they are generally about 60 or 70 feet high, but in other parts of the Sahara they are said to attain a height of upwards of 300 feet. The true dune sand is remarkable for the uniformity of its composition and the geometrical regularity of its grains, which measure less than '03937 inch. While individually these appear crystalline or reddish yellow (from the presence of iron), they have in the mass a rich golden hue. According to M. Tissandier's examination, animal organisms, such as the microscopic shells of Hhizopoda, so abundant in sea-sand, are strik-ingly absent. Under the influence of the wind the surface of the dunes is subject to continual change, but in the mass they have attained such a state of comparative equilibrium that their topographic distribution may be considered as permanent, and some of them, such as Gern (Peak) al-Shuf and Gern Abd-al-Kader, to the south of Golea, have names of their own. The popular stories about caravans and armies being engulfed in the moving sands are quite apocryphal, but there is abundant evidence against the theory of M. Vatonne as to the dunes having been formed in situ. To understand their origin it is necessary to glance at the general geology of the Sahara, which, however, in this aspect, is only known in detail to the south of Algeria and along the routes of the Rohlfs expedition (1873-74, Dr Zittel) and that of Dr Lenz (1880).
Granite, which, along with gneiss and mica schists, seems to be the prevailing rock in the highlands of Air (Von Bary), comes to the surface more or less sporadically in the neighbourhood of Al-Eglab and in the Adrar districts in the south-west. Gneiss and mica schists are probably the main materials of the Ahaggar plateau. Volcanic rocks (basalt, &c.) form the mountain masses of Jebel es-S6da and the Haruj ; in Air they break through the granite and other rocks in a very erratic fashion. Slates and quartz-ite (possibly Silurian, according to Lenz), which play so great a part in Senegambia, appear to the north of the Senegal, along the edge of the desert, and crop out again in Adrar, on the eastern borders of the Juf, and to the east of Wady Sus. An immense tract from Adrar north-east to the borders of Algeria seems to be occupied by Devonian and Carboniferous formations, the character-istic fossils of which frequently show on the surface ; farther east these rocks are covered by Cretaceous and Quaternary deposits, though they again appear in the Muydir and Tasili plateaus (M. Roche's report ). The development of the Cretaceous system is altogether one of the most striking features of Saharan geology, its extreme limits being the coasts of the Atlantic and the Red Sea, and the area occupied by it in the Algerian Sahara alone being equal to the whole of France. In the Algerian Sahara the Cretaceous rocks_ are covered by no later sediments, with the exception of certain Quaternary deposits, hut in the Libyan Desert Tertiary deposits are abundant, though, according to Zittel, there is no sharp distinction between Cretaceous and Tertiary, the one seem-ing (certain palseological characteristics apart) to pass gradually into the other. Eocene limestones, rich in nummulites and operculums, stretch south and east from the oasis of Siwa and are well seen in the cliffs enclosing the depressed oasal areas which sink down to the Cretaceous rocks. To the south of Farafreh extends a vast tract of Nubian sandstone.
In all parts of the Sahara there is evidence of denudation carried out on a scale of unusual magnitude. The present surface of the desert has been exposed to the protracted wear and tear of the elements. But to determine the exact method by which the ele-ments have done their work has hitherto proved beyond the power of science. The superficial observer is at once tempted to accept the theory of submarine denudation : the Sahara is still the " dried bed of a sea " in even such text-books as Professor Huxley's Physiography and Stanford's Compendium of Geography. The sand-dunes, the salt efflorescence and deposits, and the local occurrence of certain modern marine molluscs all go to help the hypothesis of a diluvial sea. But a more extensive acquaintance with Saharan character-istics shows that such a sea for the Sahara as a whole is impossible. The denudation must probably be explained as due to the combined action of fresh water and atmospheric agencies. Even at present the Sahara is not so destitute as has been supposed of fresh water. Though rain is one of the rarest phenomena of the lowlands, the mountains on its northern borders and the central highlands are both regions of precipitation, and discharge their surplus waters into the hollows. A glance at a good physical map of the Sahara shows in fact the skeleton of a regular river-system. From the north side of the Atakor-'n-Ahaggar, for instance, begins Vfady Igharghar, which, running northwards between the Tasili plateau and the Irawen Mountains, appears to lose itself in the sands of the Eastern Erg, but can be distinctly traced northwards for hundreds of miles. Its bed contains rolled fragments of lava and freshwater shells (Cyrena and Planorbis). In a line almost parallel to Wady Igharghar Wady Mya descends from the plateau of Tademayt, and shows the importance of its ancient current by deep erosion of the Cretaceous rocks, in which a large number of left-hand tributaries have also left their mark. Away in the far east of the Libyan Desert Dr Zittel discovered stalactite caves in the limestone. The question arises, What has become of the abundant water-supply which filled the wadies and hollowed out the caves ? Recent discoveries in the Algerian Sahara suggest that part of the water circulation has become subterranean. The streams from the Atlas which seem to be absorbed in the sands of the desert evidently find a series of underground reservoirs or basins capable of being tapped by artesian wells over very extensive areas. As Olympio-dorus (quoted by Photius) mentions that the inhabitants of the Sahara used to make excavations from 100 to 120 feet deep, out of which jets of pure water rose in columns, it is clear that this state of matters is (historically) of ancient date. Since 1856 the French engineers have carried on a series of borings which have resulted in the fertilizing of extensive tracts ; between 1856 and 1879 155 wells were bored in the province of Constantine alone. In Wady Rir', which runs for 80 miles towards the south-west of the Shott Melrir (comp. infra), the water-bearing stratum is among permeable sands, which are covered to a depth of 200 feet by impermeable marls, by which the water is kept under pressure. The wells, varying much in their discharge and "head," give a total of 3'5 cubic metres per second at an average temperature of 25°'l Fahr. A similar artesian zone exists between Negussa and Wargla. Connexions probably exist with subterranean water-sup-plies in the mountains to the north. That in some way the water in the artesian reservoirs is kept aerated is shown by the existence below ground of fishes, crabs, and freshwater molluscs, all of which were ejected by the well called Mezer in Wady Rir'. Hitherto those subterranean basins have been verified only in a comparatively limited area (the whole expanse of the Sahara being considered) ; but the same phenomena are probably repeated to some extent in other regions. The oases are of course proofs of the presence of a steady supply of underground moisture, for vegetation under the Saharan climate is exceptionally thirsty.
Everything considered, it may therefore be assumed that the desert formerly possessed a surface circulation of water capable of aiding in the processes of disintegration, removal, and deposition. Since the water disappeared other agencies have been at work. The surface of the rocks, heated by the sun and suddenly chilled by rapid radiation over night, gets fractured and crumbled ; elsewhere the cliffs have been scored and the sand thus formed is at once turned by the wind into an active instrument of abrasion. In many places it has planed the flat rocks of the hammada as smooth as ice. Elsewhere it has scored the vertical faces of the cliffs with curious imitations of glacial striation, and helped to undercut the pillar- or table-like eminences which, under the name of gurs or "witnesses," are among the most familiar products of Saharan erosion. The softer quartz rocks of the Quaternary and Cretaceous series (and according to Zittel especially the Nubian sandstone) have been made to yield the sand which, drifted and sifted by the winds, has taken on the form of dunes. The slightest breeze is enough to make the surface "smoke" with dust; and at times the weird singing of the sands, waxing louder and louder, tells the scientific traveller that the motion is not confined to the superficial particles. How important a part the winds may play in the redistribution of the lighter particles is probably shown by the clouds of red dust which were noticed by Edrisi as frequently obscuring the Atlantic sky between Cape Verd and the American coast, and which have recently been referred by Dr Gustav Hellemann to the African Sahara, whence Professor Tacehini also derives the similar clouds of dust observed in many parts of Italy (comp. Tchihatchef).
But even such a river-system as that supposed combined with all conceivable atmospheric agencies would only account for the minor phenomena of erosion. Dr Zittel in dealing with the Libyan Desert finds it necessary to assume violent freshwater floods proceeding from the south, though, as he confesses, this only shifts the difficulty a stage further back, as it involves an enormous change of climate. To render such a change of climate a probable hypothesis various recent speculations combine ; and Dr Theobald Fischer and Dr Oscar Fraas agree in believing that the desiccation has markedly increased in historic times. Evidence derived from ancient monu-ments combined with the statements of Herodotus and Pliny are held to prove that the elephant, the rhinoceros, and the crocodile existed in North African regions where the environment is now utterly alien, and on the other hand that the camel is a late intro-duction. Humboldt sought to attribute the desiccation of the desert region of Asia and Africa to the effects of the north-east trade-wind ; but Dr Lenz, who points out that in North Africa the wind seldom blows from the north-east but generally from the north or north-west (the latter of course from the Atlantic, in the western parts, but farther east from the European regions of precipitation), argues that one of the principal causes has been the destruction of the forests on the highlands. The dry winds from the Sahara are known in Europe as the Scirocco and the Fbhn or Fbn.
Botanically the Sahara is the meeting-ground of representatives of the "Mediterranean" and the "Tropical" floras which have managed to accommodate themselves to the peculiar climatic conditions. The line of demarcation between the two floral areas, almost coin-ciding in the west with the Tropic of Cancer and in the east dipping south towards the meridian of Lake Tchad, assigns by far the greater portion of the area to "Mediterranean" influences. Uniformity, in spite of differences of altitude and soil, is a general characteristic of the vegetation, which outside of the oases consists mainly of plants with a tufty dry stiff habit of growth. The oases are the special home of the date-palm, of which there are about 4,000,000 in the Algerian oases alone. In company with this tree, without which life in the Sahara would be practically impossible, are grown apples, peaches, oranges, citrons, figs, grapes, pomegranates, &c. During the months from December to March wheat, barley, and other northern grain crops are successfully cultivated and in the hotter season rice, dukhn, durra, and other tropical products. Altogether the oasal flora has considerable variety; thirty-nine species are known from the .Kufra group, forty-eight from the Aujila group.
Zoologically the Sahara is also a debatable territory, partly Mediterranean, partly Tropical. Apart from the domestic animals (camels, asses, &c., and very noticeably a black breed of cattle in Adrar), the list of fifteen mammals comprises the jerboa, the fennek or fox, the jackal, the sand rat (Psammomys obesus), the hare, the wild ass, and three species of antelope. In Borku, Air, &c., baboons, hyaenas, and mountain sheep are not uncommon. Without count-ing migratory visitants, about eighty species of birds have been registeredthe ostrich, the Certhilauda deserti or desert-lark (which often surprises the traveller with its song), Emberiza Saharae, three species of Dromolea, &c. Tortoises, lizards, chame-leons, geckos, skinks, &c, of fifteen different species were collected by the single Rohlfs expedition of 1873-74 ; the serpents comprise the horned viper, Psammophis sibilans, Ccelopeltis lacertina, the python, and several other species. The edible frog also occurs. Cyprinodon dispar, a fish not unlike Oyprinodon calaritanus, is found in all the brackish waters of north Sahara and swarms in the lake of the Siwa oasis. The brine-shrimp has been described in the article FEZZAN.
The present population of the Sahara consists almost exclusively of Arabs, Berbers, and Negro tribes. The Berbers (Tuareg or Tuarik, &c.) occupy the west central region almost exclusively, appear sporadically in the western, and stretch northwards into Morocco and Algeria ; the Negro tribes form a compact block in the east central region northwards and north-eastwards from Lake Tchad ; and the Arabs are in possession of all the rest of the country. Politically the Sahara belongs partly to Morocco (Tafilet, &c), partly to Algeria and Tunis (and thus to France), and partly to the Turkish empire (Tripolis, Egypt, &c. ). France especially has been steadily pushing south with the purpose of forming a junction ultimately with her colony on the Senegal. The spirit of independ-ence among the Mohammedan populations has been crystallized and stimulated by the remarkable confraternity of Sidi Mohammed ben 'Ali es-Senusi, founded about 1837, and now possessing about 120 convents or zawiga (mostly in the Saharan region), with its headquarters at Jerabub. With this organization the French have already come into conflict in their southward progress. To estab-lish their influence they propose the construction of a trans-Saharan railway and the opening up of the region to the south of Algeria and Tunis by the construction of an inland sea. According to M. Roudaire, the author and protagonist of this scheme, which is familiarly but deceptively styled the "flooding of the Sahara," it is possible by proper engineering works to create an inland sea to the south of Algeria and Tunis with an average depth of 78 feet and an area of 3100 square miles, or about fourteen times the size of the Lake of Geneva. A Government commission decided that the excavation of the necessary canal would not be difficult, and that, in spite of silting-up processes, the work would at least last 1000 to 1500 years. M. de Lesseps, M. Roudaire's principal sup-porter, visited the district in 1883 and reported that the canal would cost five years' labour and 150,000,000 francs. The scheme, which has met with persistent hostility on the part of M. Cosson and others, is based on the following facts. The Gulf of Gabes is separated by a ridge 13 miles across and 150 feet high from Shott al-Fejej, a depression which extends south-west into the Shott Jerid, which in its turn is separated from the Shott Rharsa only by a still narrower ridge. Shott Rharsa is succeeded westwards by a series of smaller depressions and beyond them lies the Shott Melrir, whose north-west end is not far from the town of Biskra. What we know about such inland seas as the Caspian and the Aral seems to cast serious doubt on the probability of any increase of the rainfall in the Sahara by the formation of Roudaire's sea.
The commerce of the Sahara is not inconsiderable. Among the more important trade routes are(1) from Morocco to Cairo b'r Insalah and Ghadames, which is followed by the pilgrims or' Western Africa bound for Mecca ; (2) from Kuka to Murzuk and Tripolis ; (3) from the Sudan to Tripolis by Air and Ghat ; (4) from Timbuktu to Insalah, Ghadames, and Tripolis ; (5) from Timbuktu to Insalah and thence to Algeria and Tunis ; (6) from Timbuktu to Morocco. The two great products are dates and salt. Full details of the date trade will be found in Fischer's Die Dattelpalmn, 1881. The principal sources of salt are the rock-salt deposits of the Juf (especially Taudeni), the lakes of Kufra, and the rock-salt and brine of Kawar (Bilma).
See, besides the works already quoted, Vatonne, Mission de Ghadames, 1863 ; Duveyrier, Les Touaregs du Nord, 1864 ; Ville, Explor. géologique du Mzab, &e., 1867 ; Pomel, Le Sahara, 1872 ; Rohlfs, Quer durch Afrika (1874), Drei Monate im libyschen Wüste (1875), and Kufra (1881) ; Largeau, Le pays de Birha-Ouargla, 1879; Nachtigal, Sahara und Sudan, 2 vols., 1879; Rolland, "Le Crétacé du Sahara Septentrional" (with geological map of the Central Sahara), in Bull. de la Soc. Géol. de France, 1881 ; Roudaire, Rapport sur la dernière exped. des Cholts, 1881 (and other reports by the same author) ; Tchihatchef, "The Deserts of Africa and Asia," in British Association Reports (Southampton, 1882) ; Derrecagaix, "Explor. du Sahara: Les deux missions du Lieut.-Colonel Flatters," in Bull. de la Soc. de Géogr., 1882 ; Lenz, Timbuktu: Reise durch Marokko, &c., 1884 ; and Reclus, Nouv. Géographie Univ., xi., 1886, which contains an admirable resume. (H. A. W.)
1 Zeitschrift für Erdkunde, 1880.
The above article was written by: H. A. Webster.