Sedimentary rocks intruded by Tertiary mafic dikes on the Colorado Plateau typically display systematic dike-parallel joints. These closely spaced joints occur only near dikes, their spacing commonly increasing with distance from the dike contacts. Igneous breccias along some of these joints indicate that the joints are not younger than the dikes and, therefore, did not form during cooling. Field relations are best explained if the joints form in host rocks beyond the dike tip, becoming juxtaposed against the dike with continued propagation: an interpretation supported by previously reported descriptions of ground surface cracks formed near eruptive fissures and of microcracks formed near the tips of larger extension cracks during laboratory experiments. Tensile stress generated by magmatic pressure is sufficient to fracture host rocks beyond dike tips. The tensile maxima are located on either side of the dike plane, beyond the tip. The stresses increase in magnitude closer to the tip, thus explaining the greater abundance of joints near the dike plane. Noting that dikes may parallel regional joint sets, we contrast (1) emplacement along older joints oriented arbitrarily with respect to the principal stress directions acting at the time of intrusion and (2) emplacement along self-generated fractures propagated in a plane perpendicular to the least compressive stress direction. Magma can invade along older joints if magmatic pressure exceeds the horizontal stress acting across the joint plane. This situation is most common if the horizontal principal stress difference is small compared to the magmatic driving pressure or if joints are nearly perpendicular to the direction of least compressive regional stress. Magma must advance by filling self-generated fractures if older, suitably oriented, joints are absent.