from recrystallised sandstones. Innumerable, meter-sized doleritic dikes cut through the formation inducing local contact metamorphism. The term “sandstone” is somewhat misleading, because the formation is clearly dominated by the rhyodacitic tuffs of volcanic origin, all the other facies merely being alteration products. The once sodium-rich rhyodacitic tuffs have now become dark, massive and thickly bedded rocks. Lodged within a fine-grained matrix of chlorite, white mica, quartz and albite are millimeter-sized clasts of quartz, plagioclase (albite or oligoclase) and epidote. The following observations underline the explosive character of the volcanic rocks: fragmented quartz crystals with sharp, pointed edges and tips. broken, pointed plagioclase. Rock fragments of once albite-rich, leucocratic lava. The greywackes are mineralogically very similar, but richer in quartz phenocrysts and their matrix is enriched in phyllosilicates. They are probably derived from the rhyodacites. Likewise the chemical composition of the quartzites, which is almost identical to the ryhodacites! The underlying formations of the Thiviers sandstone – plagioclase-rich paragneisses and micaschists – are nowhere exposed. Intrusive rocks As already mentioned before the Thiviers-Payzac Unit houses three different magmatic bodies: Saut-du-Saumon orthogneiss. Corgnac granite. Estivaux granite. Saut-du-Saumon orthogneiss The prekinematic Saut-du-Saumon orthogneiss is a 15 kilometer long, very drawn out, squid-like, NW-SE-striking granitic body, whose tail starts at Donzenac in the south and whose tentacles end in the Orgnac region near the river Loyre. The orthogneiss is made up of eye-shaped, subcentimeter-sized alkali feldspar porphyroclasts and is therefore an augengneiss. Its matrix consists of quartz and feldspars. The biotite has aligned itself parallel to sigmoidal shear bands. The porphyroclasts have been broken and sheared left-laterally along parallel fractures. The entire body of the orthogneiss is intensively foliated in a NW-SE fashion. The foliation planes are close to vertical and contain horizontal lineations also striking NW-SE. The rock is therefore a S-L tectonite. Along its borders with the enclosing formations (Thiviers sandstone and Donzenac schist) mylonites and ultramylonites with sinistral shear sense have formed. In its interior dextral and sinistral shearing interfere in a non-coaxial fashion. The protolith of the orthogneiss was once a porphyritic granitoid which was later on (during the Variscan orogeny) deformed plastically. Its original cooling age has been determined as middle Ordovician (Acadian phase). According to Bernard-Griffiths (1977) the mylonitic deformations took place close to the Devonian/Carboniferous boundary about 361 million years ago. Corgnac granite The Corgnac granite is a 6.5 kilometer long massif within the Thiviers sandstone. It