The upper part of the T’immersoi, where it is called the Tafassasset, is also the northern boundary of Air. The wells of In Azawa[35] and Asiu in this valley may be regarded as the point where the main roads from the north enter the extreme limits of the country. Further east on another road between Air and Ghat, von Bary fixed the boundary at the Wadi Immidir, which is in the same latitude as In Azawa.[36]
The eastern boundary of Air runs along the line where the last rocks of the group disappear below the sand of the steppe and desert, which extends from north to south between the mountains of the Fezzan and the fringe of Equatorial Africa, and from west to east between the mountains of Air and those of Tibesti with its adjacent massifs. This vast area is crossed by a few roads only, the most important ones being (a) the road from Murzuk along the Kawar depression to Agadem and Lake Chad, (b) and (c), the two principal tracks from Air eastwards to Bilma by Ashegur and Fashi respectively, and (d) the road from Zinder by Termit to Fashi and Kawar. Watering-points are very few, and the habitable oases can be numbered on the fingers of two hands; pasturage is everywhere scarce. This great waste is one of the most unknown parts of North Africa; its eastern portion along the Tibesti mountains as far north as the Fezzan may be said to be absolutely unknown except for two tracks to the mountains whither occasional camel patrols have passed.
Kawar and the other oases along the Chad road appear to be closed basins of the Eastern Saharan type. They seem to have no outlet towards the south either into the Chad or into the Niger systems. The desert east of Air, therefore, contains the eastern watershed of the T’immersoi basin, for the valleys of Eastern Air do not run into the desert as Chudeau has suggested,[37] but turn southwards on leaving the hills, in ill-defined depressions or folds which join the Tagedufat valley or one of the other channels flowing westwards in Tegama or Damergu. One valley to the south of Air, probably the Tagedufat itself, is stated to run all the way from Fashi across the desert.
The southern limits of Air may be placed along the Tagedufat basin, where the rocks of Air disappear below the sand dunes and downs of Tegama and Azawagh steppe desert. The valley is of some size and flows roughly N.E. and S.W. towards T’immersoi, but whether it actually joins this system or the Gulbi n’Kaba, which finds its way into Sokoto Emirate under the name of the Gulbi n’Maradi and thence into the Niger, is not certain. The former hypothesis seems more probable, but I was unable to follow the Tagedufat sufficiently far west to verify it, nor could I discover any data on the French maps;[38] local reports substantiate my supposition. Both systems in any case are in the Niger basin. Air is not on the watershed between Niger and Chad. The choice of the Tagedufat valley as the southern boundary of Air is made on geographical grounds. What may be termed the political boundary is rather further north along the line of the River of Agades.
PLATE 3
DESERT AND HILLS FROM TERMIT PEAK
Commencing within 50 km. of the In Azawa wells, Air is a low plateau of Silurian formation with islands of Archean rock. Through the plateau-plain a number of separate formations have been extruded by, in many cases, apparently quite recent volcanic action. The northernmost massifs of Taghazit and Zelim lie in about latitude 20°. The volcanic period was of considerable duration, but all the recognisable volcanoes and derived phenomena are post-Eocene.[39] Some of the basalt flows, more especially those from Mount Dogam near Auderas in Central Air, are not old, while the Teginjir lava flow appeared to me so fresh as probably to have come into existence during the historical period. The volcanic phenomena take the form of cinder cones with steep sides as at Teginjir (Mount Gheshwa), cumulo-volcanoes, as in the T’imia and probably Bagezan massifs, domes as in the case of Mount Dogam, and basalt flows in various parts, notably in the T’imia valleys.[40] Aggata[40] appears to be another volcanic peak, but the serrated crest of Ighzan is a phenomenon of the rapid cooling of an igneous extrusion rather than an example of erosion. There are numerous volcanic massifs distinct from each other all over Air, more especially in the centre and north; they are nearly all granitic and very rugged. The Auderas basin is of basalt and cinerite.[41] The plateau, which is in the main horizontal, rises in the centre to a step some few hundred feet higher than the north and south and forms a pedestal for the Bagezan and other massifs some 1500 to 3000 feet higher again. The peaks are as much as 4500 feet[42] above the plateau, which varies from 1500 feet above sea level in the south along the River of Agades, to 2000 feet in the Ighazar in North Air. Round Auderas the plateau may be taken as about 2500 feet above the sea, while to the east of the Bagezan massif the plateau is about 3000 feet, sloping gradually away to the south and east. Between Agades and Auderas there is an abrupt ascent on to the central step of the plateau of some 2000 feet; a corresponding descent of about 150 feet takes place near Assada.
The effect of these massifs rising sharply out of the plateau is curious. The Archean or Silurian plain and the volcanic mountain groups are phenomena which have not yet had time to become correlated. The result is that the broad and very gentle valleys of the plateau-plain wander in and out among the disconnected massifs and are fed by deep torrents draining the slopes of impermeable rock. Water erosion has not yet had time to widen or deepen the ravines, while the broad valleys have wide sandy bottoms, where pebbles only rarely occur; their sides are well wooded with pasture on the plains between the beds, except where masses of round basalt boulders, the product of the volcanic disturbances, cover the surface. The massifs have hardly been affected by erosion. The broad valleys between them are the corridors of communication in the country. “Cette superimposition à une vieille pénéplaine usée,” says Chudeau,[43] “de massifs éruptifs jeunes, donne a l’Air un aspect surprenant, presque paradoxal.” And this is the charm of the country that has been called by travellers the Saharan Alps. There is contrast everywhere, but nothing is perhaps more striking than the black patina which the red rocks have assumed. The wind-borne sand has polished them till they shine with a dark metallic gleam, while the sheltered rifts and ravines retain their pink and red surfaces. It is a land of lurid colour, except at midday, when the African sun dominates everything in one blinding glare.
[1]The name “Sudan” is used throughout to indicate the country referred to by the Arab and early European geographers under this name, that is to say, the country inhabited by negroid people north of the purely negro zone and south of the Saharan deserts. The “Anglo-Egyptian Sudan” is more correctly described as the “Nilotic Sudan.”