Geology and Structure.—Exact geological information about many parts of the Sahara is not known, but the main structure is now fairly well understood. In the central Tuareg massif we find Archaean gneisses, mica schists, granites, etc., which are flanked by folded Silurian rocks upon which rest Devonian and, in some places, Carboniferous strata. Permian, Trias, Jurassic and Lower Cretaceous rocks are apparently missing in the central Sahara, although the Jurassic crops out in south-west Algeria and along the Atlas borders, but a large area is covered by horizontally bedded Middle and Upper Cretaceous rocks, at the base of which is a variegated clay with gypsum. This latter, a typical lagoon formation, indicates the incoming of changed conditions which caused a widespread transgression in the Cenomanian epoch (lower chalk of Britain) and established an epi-continental sea on the stable mass of northern Africa. Miocene, Pliocene and Quaternary deposits occur locally, the Nummulitic limestone being important in the Libyan desert and Egypt. The Sahara is quite distinct from the folded Atlas mountains (q.v.), which tectonically belong to Europe and to which North Africa acted as hinterland during the period of Tertiary folding.
The winds of the Sahara are very dry, and in consequence there is little or no precipitation, except in the high land region of the central massif, where there is a fall of a few inches per annum, and also along the coastal fringes. Neverthe less, the desert as a whole is not entirely without water, but possesses a skeleton of a regular river system. Inadequate streams radiate from the central massif, but they are soon lost in the sand of the surrounding country. Similar streams flow down from the Atlas mountains, the Wad Dra'a finally reaching the ocean, whilst in the Atlantic and Mediterranean coastal regions similar streams occur, some reaching the sea, others losing themselves in the desert. Irrigation has been attempted in various parts, and it has been found that in many places, especially south of the Atlas and in Algeria, the geological structure is such that artesian water has been obtained. The water so obtained is remarkably fresh, and by means of it agriculture has been made possible in several districts, but the arid hammada country must be abandoned as hopeless. How far such irrigation works can be extended is very uncertain for the detailed geological structure of the area is not yet sufficiently known.
Several theories have been put forward to explain the enormous quantity of sand in the Sahara. The theory of marine origin is now no longer tenable, for the sand is Quaternary, or at the oldest Pleistocene, in age, and there is no evidence of a marine transgression since pre-Tertiary times.
During the great Ice age, when the southern limit of the ice sheet extended across Central Europe, meteorological conditions were such that much, if not all, of the desert belt lay in a warm, moist, temperate or sub-tropical zone. Under such conditions rock disintegration and soil formation would proceed in the way normal to such regions, the soil being held in position by vegeta tion, as is still the case in parts of Mauritania, the regions of north of Timbuktu and round Lake Chad and the several oases. A mature river system also assisted in the disintegration and transportation of sediments. Later the soil lost its moisture and the winds became dry, consequent upon the changed meteorologi cal conditions brought about by the retreat of the ice, and vege tation disappeared, leaving the dry soil at the mercy of the wind.
The summer heat (June, July and August) is very great and is measured by Augieras in terms of the number of days on which a thermometer not directly exposed to the sun's rays reached 50° C (122° F). In 1913 the number was 33, in 1914 it was 37, in 1915 it was 11 and in 1916 it was 42. During the hottest night in 1916 the temperature did not go below
but the average summer night temperature is about 77°. These figures are for the western Sahara; the central highlands are cooler. The highest recorded temperature for the western Sahara is 56° C (132.8° F). In the cool season the monthly average maxima are about 68°-78°, the minima about 41°-50°. On one or two nights of the year the ground in the west may freeze ; there is more frost in the central highlands. In summer the soil-tem perature may occasionally reach 175°, the relative humidity may be as low as 5% even in the oases; in the cooler season the rela tive humidity may increase to 48%. The winds in many parts are chiefly the north-east trades, but there may be summer centres of low pressure, and in the western Sahara the wind often blows from the west.
The three chief trees are the date palm, Acacia tor tile (on the rocky areas), and Tamarix articulate (in beds of wadis). The small plants belong mostly to the Compositae, Cruciferae, Leguminosae and Gramineae, usually with small, often hard, leaves, and sometimes fleshy stems. Some desert plants have leaves grey with hairs, which immediately suck in any drop of dew, while others have salt incrustation that attracts any mois ture; some plants (colocynth) have very deep roots. Many desert plants retain vitality in spite of years of "resting" while they wait for water ; the rose of Jericho is famous in this con nection.