The two monsonitic syenites from the Oslo area, larvikite and tonsbergite, differ mainly in the colour of the feldspar minerals. The blue schiller which is so predominant in the larvikite varieties is normally completely masked by the red colour of the tonsbergite feldspars. The nature of the blue schiller and of the red colour has not been revealed by ordinary microscopic investigations. The blue · schiller is mainly seen on the plane (100), and the assumed cryptoperthites are supposed to be oriented parallel to (801). The red colour of the tonsbergite feldspars is supposed to be due to same metasomatic unmixing of the iron content in the feldspar phase (Chr. Oftendahl 1948 and I. Th. Rosenqvist 1951).