Cylindrical columnar jointing has been observed in late. Precambrian quartz-dolerite dikes in the Beartooth Mountains of Montana and Wyoming. This jointing extends across the entire width of the dikes, producing cylinders normal to the contacts. Radial fractures are almost invariably developed, but they are not as prominent as cylindrical fractures. There is no apparent correlation between cylindrical jointing and the local rock type, country-rock type, or shape of the dike. Study of these fractures indicates that their origin is probably related to cooling conditions within the dike.