The two principal suggested modes of facilitating overthrust faulting are (1) lubrication at the sole by evaporite beds or micaceous shales and (2) flotation due to anomalously high (> hydrostatic) pore-water pressures. Past rapid sedimentation and tectonic compression have been suggested as important causes of anomalously high water pressure (Hubbert and Rubey, 1959). We suggest osmosis as another important possibility. Field data on shale beds and experimental studies on compacted clays show that such material can act as semipermeable membranes that greatly retard passage of dissolved electrolytes relative to H₂O. Equilibrium osmotic pressure, π, across an ideal membrane is given by (a, activity; V_H2O⁰₁, molar volume of distilled water). At 80°C, π is 470 bars between saturated halite solution and distilled water; it is 360 bars between saturated and 10 per cent solutions. At 25°C the values are 20 per cent lower. Other dissolved components, if present in similar proportions, will enhance the effect. Anomalous water pressures of at least 400 bars above hydrostatic have been measured in oil wells; many of these wells penetrate evaporites and/or shales which separate formation waters of differing salt concentrations. These pressures are explicable by assuming osmotic equilibrium across a membrane which separates saturated halite solution from solutions up to 10 weight per cent NaCl. Thus, osmotic equilibrium may be an important mechanism for floating thrust sheets. Lubrication of thrust sheets by shales or evaporites and flotation by anomalously high pressures may be simply different manifestations of the same geologic milieu. © 1965, The Geological Society of America, Inc.