A detailed study of the petrographical characteristics of the acid palaeovolcanic formations in the Armorican Massif and in part of the Central Massif (France) reveals important ignimbritic provinces ranging from Precambrian to Carboniferous age. Close comparisons show that the ignimbrites, though varied in the details, form a whole which has constant characteristics. They can thus be related or opposed to ignimbrites of more recent origin. Field data, essential for the identification of recent ignimbrites, are of little practical help when dealing with old ones. But the study of macroscopic samples proves highly effective: heterogeneity, pseudo-fluidality, spindle-shaped structures, etc…, are among the relevant features. Only a microscopic study, however, can show vitroclastic structures and reveal the striking similarity between spindle-shaped structures and the « fiammae » of more recent ignimbrites.