Unconformities that can be traced for distances of many kilometers permit identification of chronologic units and interpretation of the geologic history of the Precambrian in the Ahaggar mountains of the central Sahara (Algeria). The Pharusian is represented by sedimentary and volcanic material filling huge tectonic troughs between asymmetric Suggarian blocks. The Suggarian and Pharusian correspond to two major orogenic cycles, in each of which general metamorphism preceded the major phases of folding. Migmatization developed toward the close of the folding phases, and progressed toward formation of increasingly more discordant granites and even toward rhyolitic volcanism. The structure as a whole is dominated by huge fractures dating from the end of the Suggarian cycle, which were subsequently rejuvenated and later controlled the deposition and evolution of the sedimentary cover.