The weathering of the chalk in the Basin of Paris during the period of emersion of the final Cretaceous induced the formation of soils the witness of which are found again ; erosion of these soils provided the material of some sediments in the Lower Paleogene. These soils and sediments contain the same clay minerals : kaolinites, smectites and (7-14Sm) mixed layers. The kaolinites have always a more or less disordered structure. The smectites are principally diocta-hedral, alumino-ferriferous ; they belong essentially to the mixed beidellites. The (7-14Sm) mixed layers are more aluminous than smectites. The different structural and chemical characters studied show that these mixed layers constitute a complex group, intermediary between kaolinites and smectites with which they are almost mixed at the boundaries of the group. They do not correspond to a well-defined mineral species, but to a population of regular structures with diverse sequences or of irregular ones. Beidellites, relatively resisting in alkaline environment, transform into (7-14Sm) mixed layers, as leaching increases. Montmorillonites, less stable, are hydrolyzed and give way to kaolinites. Thus, there are two parallel itineraries in the weathering of Cretaceous series according to the intensity of leaching : montmorillonite - beidellite - (7-14Sm) mixed layers montmorillonite - kaolinite These two itineraries are distinct and mixed layers do not appear like a stage of transition between smectites and kaolinites. However, if leaching is prolonged resulting in an acid environment, mixed layers in their turn are hydrolyzed, and this is why kaolinite only is found in sandy materials. se mixed layers constitute a complex group, intermediary between kaolinites and smectites with which they are almost mixed at the boundaries of the group. They do not correspond to a well-defined mineral species, but to a population of regular structures with diverse sequences or of irregular ones. Beidellites, relatively resisting in alkaline environment, transform into (7-14Sm) mixed layers, as leaching increases. Montmorillonites, less stable, are hydrolyzed and give way to kaolinites. Thus, there are two parallel itineraries in the weathering of Cretaceous series according to the intensity of leaching : montmorillonite - beidellite - (7-14Sm) mixed layers montmorillonite - kaolinite These two itineraries are distinct and mixed layers do not appear like a stage of transition between smectites and kaolinites. However, if leaching is prolonged resulting in an acid environment, mixed layers in their turn are hydrolyzed, and this is why kaolinite only is found in sandy materials.