Three conclusions are supported by the investigation outlined in this paper. (1) The two hydrous aluminum silicates, montmorillonite and smectite, appear to be identical. (2) Both may be prominent as constituents of bentonite and fuller's earth. (3) Fuller's earth from one or more long recognized European localities has been formed by the alteration of volcanic ash to smectite (montmorillonite). This is a type of origin generally relied upon to determine bentonite.