The middle Pleistocene volcanic complex between Bessan and Saint Thibery (France) is composed of basanite volcanic material. Three main formations are recognized: tuffs, cones, and flows. Petrographic analysis of the material shows that the basanite is slightly deficient in silica as indicated by the presence of leucite. The basanite contains phenocrysts, microlites, and xenoliths. Scorias were the first products emitted, dikes the last. Structural analysis of the flows is based on their internal and external characteristics which were affected by the mode of diffusion of the lava, the behavior of the gas, and the cooling. The frontal and lateral borders of the flows are marked by coarse breccias which distinguish the original limit of the flow from the erosional limit. The upper borders of the flows are eroded, the lower borders are marked by scoriaceous breccias.

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