A petrographic study of the siderolithic formation with pisolithic iron filling the karsts of the urgonian horst of the Mormont and the stratified siderolithic rocks of the Talent. The predominant argilaceous mineral is kaolinite, accompanied by illite and by a secondary montmorillonite. The predominant iron oxide is goethite. An outline of the relations among these different minerals is includet at the end, as well as hypothesis about the origin of iron pisoliths.