These Saharan rivers have not contained perennial surface water for long ages. In places they have been covered by more recent sand-dune formations of great extension, but they date from the present geological period. Associated with the desiccation of these valleys is the characteristic of extreme dryness which is one of the few features more or less in accord with popular conceptions of the Sahara. The barrenness of the Sahara is less due to the inherent sterility of the ground than to climatic conditions; desiccation has been intensified in the course of centuries by the purely mechanical processes attendant upon an extremely continental climate and excessively high day temperatures. The latter combined with the extraordinary dryness of the air have contributed to the decay of vegetable, and consequently of animal, life wherever man has not been sufficiently powerful, in numbers or energy, to stay the process. Sterility and desiccation are interacting causes and effects. There is no reason to believe that any sudden change of climate has taken place in the Sahara since the neolithic period, or that it is very much drier now than two thousand years ago. Maximum and minimum temperatures, both average and absolute, have a very wide range seasonally and within the period of twenty-four hours. Temperatures of over 100° F. in the shade are common at all seasons of the year during the day: the thermometer frequently falls to freezing point at night during the winter. Ice is not unknown in the mountains of Tibesti, Air and Ahaggar. The rainfall is irregular except within the belt of summer rains which are so characteristic of Equatorial Africa. In Tibesti the cycle of good rains seems to recur once in thirteen years: in many years both here and elsewhere in the Sahara no rain falls at all. But with these adverse climatic conditions the surprising fact remains, not that the Sahara is so barren, but that it is so relatively well-favoured and capable of supporting different races of people in such comparatively large numbers.[2] The Air mountains, like the Desert steppes, are only sparsely inhabited. The hill-sides are too wind-swept and rocky to support forests or pastures of any value. Many of the valleys are capable of being cultivated, but in practice are only gardened here and there. In certain districts there are groves of date palms which have been imported from the north. Air is in reality a great Saharan oasis divided from the Equatorial belt by a zone of desert and steppe. It differs from the south in its flora and general conditions, though by its position within the belt of tropical summer rains it belongs climatically to the Sudan. [Illustration: TRADE ROADS F. R. del. Emery Walker Ltd. sc.]