The mechanisms of dike propagation are analyzed through detailed examination of small structures associated with the dike intrusion. Exposures in the Ramon area, Israel, offer an opportunity to evaluate theoretical models of tensile fracture propagation and to better understand the factors governing the dike geometry. Dike-related structures are studied in two Jurassic units: the Ardon Formation, a well-stratified sequence of shales and dolomites, and the Inmar Formation, which is composed of porous, massive sandstones with a few shale lenses. It is shown that for the different host rocks the propagation mechanisms and structures differ significantly. Dikes in the Ardon Formation are dominated by 1-20 m wide segments, confined to distinct layers. The mechanisms of segment containment are determined by calculating the stress intensity factor for an extension crack approaching a mechanical interface. It is apparent that dike segmentation and containment are controlled mainly by in situ stress and shear moduli differences between adjacent layers and partly by bedding plane slippage. Dikes in the Inmar Formation are dominated by smaller segments and by 1-10 cm wide and 10-100 cm long fingers with intermittent smooth patches and slickensides. Opposite dike walls display exact mirror images of these features. Dikes propagate in alternating stages of viscous flow and brittle deformation. The viscous stages include fluidization of the sandstone by the dike-related fluids and increase of the pore pressure in the rock. A viscous instability develops between the highly viscous fluidized sandstone and the less viscous dike-related fluids, forming a zone of viscous fingering in front of the dike. Stages of brittle fracturing occur when the pore pressure is released and the flux of fluids is insufficient to fluidize the rock. ¿American Geophysical Union 1991