A highly plastic montmorillonitic breccia rock from Fortun in Sogn is investigated by means of X-ray and chemical methods. The X-ray diagrams shows, a remarkably, strong 001 reflex and assymetric hk-bands. The montmorillonite is considered dioctahedric. The ion-exchange capacity is found to be 128 m.eq./100 grams. Probably, the rock originated through hydro-thermal alterations from anorthositic inclusions below the upper Jotun thrust plane.