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The attempt to reconstitute the pluvial and arid periods of Saharan Africa finds, in the Hoggar region. vestiges of what may be humid and dry periods in alternation. It is apparent that the humid periods for the lowland country derive from the extension northward of tropical conditions but that, on the other hand, the humid periods in the highlands are related to the southward spread of mediterranean climatic conditions. Evidence is presented that the Quaternary opens with a warm and humid episode during which tropical fauna extend northward and reach to fairly high elevations, which period may possibly be identified with Lower Villafranchian in the archeologic scale. A drying-out period was followed by a cooler humid period during which the higher mountains show the spread of mediterranean mountain flora (Upper Villafranchian), terminating in at least one period of wet-cool conditions making for solifluction and frost-cleaving. Middle Quaternary conditions suggest an arid period of sand dune accumulation followed by a more moist period in which a tropical fauna again extends northward (Upper Acheulian). Upper Quaternary shows an arid period followed by a short moist period in which savanna flora and fauna spread at the lower elevations and a more xerophilous mediterranean flora and fauna spread though the highlands (Moustero-Aterian). The alternation of climatic periods is not fully regular, and there are many problems remaining in the interpretation of climatic, floral, faunal, and human occupance sequences, but a progressive drying-out does seem indicated. [ABSTRACT FROM AUTHOR]