The region under study, the Thônes syncline (Haute-Savoie, France), is situated about 40 km SE of Geneva. This syncline is occupied mainly by Helvetic flysch formations overlain by ultrahelvetic and penninic klippes. The detrital flysch series of Eocene to Oligocene age is characterized by volcanic greywackes known in the Western Alps as "grès de Taveyanne", "grès du Champsaur" and "grès du val d'IIliez". The volcanic elements of the Helvetic flysch originate from two different series of rocks: diabases (pillow lavas) and andésites s.l.. The "grès de Taveyanne" greywackes, situated in the lower part of the Helvetic flysch formation contain up to 80% "andesitic" elements which must originate from a unit higher than the Prealpine "nappe des Gets". The name proposed for this units is "unité des andésites". The andésites s.l.—a rock suite ranging from basalts to andésites s.s. and dacites—originate most likely from an ancient andesitic island arc, probably related to an underlying subduction zone. The age of these rocks lies between Upper Jurassic and Upper Eocene. Andesitic rocks have been found recently in place in the alpine arc. Most of them may have disappeared partly due to subduction and partly due to erosion of the "nappe des andésites" during the alpine orogeny. The "grès du val d'Illiez" greywackes, which form the upper part of the Helvetic flysch sequence contain "andesitic" as well as diabasic elements, but never more than 40% alltogether. The petrographic study of the "grès du val d'Illiez" conglomerates indicates that a great number of pebbles (diabases, gabbros, alcaline granites) originated by erosion from the "nappe des Gets" (nappe with Ligurian elements). It could be established that at least the diabases and the gabbros were metamorphosed before the "mise en place" of this nappe and before the burial metamorphism of the Helvetic realm. The diabases can be related to the oceanic basaltic volcanism of the Piemontais- Ligurian paleogeographic realm of Upper Jurassic to Cretaceous age. The flysch complex in the Thônes syncline was subdivided into six different types on the basis of composition and abundance of the volcanic components. It is possible to establish a detailed stratigraphy based on this petrographic classification and consequently to unravel the structural style of the whole complex. In regard to the intensity of burial metamorphism, the flysch in the Thônes syncline falls in the laumontite zone of the zeolite facies corresponding to the zone of deep diagenesis of some authors. The presence of corrensite, a clay mineral, formed in basic volcanic greywackes and in shales, is also characteristic of this zone. Corrensite is widespread in the zeolite-bearing rocks as well as in rocks with carbonate cement in which zeolites are not present, their formation in this chemical environment being hampered by the high CCL partial pressure. Laumontite and coriensite are typical for a temperature of formation of 100 °C to 200 °C. With a normal geothermal gradient of 30 °C/km and a surface temperature of 20 °C the maximum overburden of prealpine units is equivalent to an ancient burial depth of 2,7 km and 6,0 km and to pressures of 0,7 kbar and 1,5 kbar respectively. The intensity of metamorphism decreases slightly in the SW part of the syncline where laumontite is also associated occasionally with heulandite-clinoptilolite. Since laumontite is associated with corrensite, the laumontite facies should be called more correctly «laumontite-corrensite facies» in accordance with KÜBLER'S (1973b) classification of diagenetic and low metamorphic rocks.