the Thiviers-Payzac Unit. The quartzite shows similarities to the Puy-de-Cornut-Arkose of the Génis Unit. Even a relationship with the grès armoricain in Brittany is taken into consideration. Its age is therefore most likely Ordovician (Tremadocian). Engastine mafics The underlying Engastine Mafics are a complex of mafic, magmatic rocks. They likewise appear only in the western and central Thiviers-Payzac Unit. In a 500 Meter wide strip they follow immediately to the south of the Puy-des-Âges quartzite. Near Juillac this strip widens to about 2 kilometers. The age of the maximally 500 meter thick mafic rocks is taken to be Cambrian. They consist of alternating greenschists and amphibolites in which are intercalated several layers of metadolerites and metagabbros. The very fine grained greenschist of light green to dark green colour contains as major constituents the minerals plagioclase (oligoclase or andesine), amphibole (hornblende) and epidote (clinozoisite). Biotite is a minor constituent and quartz, calcite and opaques are accessories. The greenschist represents ancient subalkaline basalts. The metadolerites and the metagabbros are much more coarse-grained and consist mainly of hornblende and basic plagioclase that has undergone saussuritisation. Donzenac Schist Below the Engastine mafic complex follows the epizonal (low-grade) Donzenac Schist. The schist crops out in a 3 kilometer wide, slightly curved band that starts at the type locality Donzenac and extends to Lanouaille. Here the band is cut off by the left-lateral Dussac Fault, a major strike-slip fault, and offset by several kilometers to the southwest. The band then follows through to just northeast of Thiviers, where it ends. The Donzenac schist is also included in the Cambrian. The schist has silky hues of grey and is mostly made up of phyllosilicates like muscovite and biotite or muscovite and chlorite. The phyllosilicates are accompanied by quartz, acid plagioclase and garnet of the almandine zone. The schist sometimes reveals relatively fine-grained, decimeter-sized, dark interlayers of arenitic composition, most likely ancient greywackes. The arenitic interlayers show clasts of quartz, plagioclase and epidote surrounded by newly formed minerals like phyllosilicates, quartz and very fine-grained albite. Thiviers sandstone The Thiviers sandstone is the lowermost formation in the Thiviers-Payzac Unit appearing at the surface, taking up about two thirds of its total surface area. The formation is a detrital, volcanic succession of late Neoproterozoic and Cambrian age. It can be subdivided into four different facies: medium- to coarse-grained greywackes. sandy schists and rhyodacitic tuffs. polygenetic and intraformational conglomerates. quartzites formed