During Eocene times, a lateritic cover was formed, soon to be destroyed by erosion, and whose last traces are to be found in karstic cavities, down into which the material has been drawn. These laterites are almost entirely composed of kaolinite. At a later stage, the percolation of waters in the karst creates different physico-chemical conditions, and initiates a new evolutionary cycle. The environment becomes neutral or weakly acid, and well drained kaolinite is destroyed through the effect of migration "per descendum". Waters containing silica and alumina allow the local neoformation of kaolinite and halloysite (example : specimen No. 206). Accessorily or rather seasonally, centrifugal migration brings silica and alumina into contact with alcaline and alcali-calcic cations freed through the alteration of the calcareous walls; in this environment of a locally alcaline type, illite, a mixed-layer mineral of the illite-montmorillonite type, and exceptionally vermiculite, may recrystallise.