Zones of abnormal subsurface pressures, both above and below hydrostatic, have been described in many areas. The very abrupt changes in pressures and salinities, together with the undercompacted nature of the high-pressure zones, indicate that they are effectively isolated from their surroundings. If this isolation occurred at a shallower depth than the present one, the isolated volume would have been subjected to increasing temperatures as it moved downward. The P-T-density diagram for water shows that, for any geothermal gradient greater than about 15°C/km, the pressure in an isolated volume increases with increasing temperature more rapidly than that in the surrounding fluids. This mechanism for producing excess pressures will operate in addition to most of the ot er processes that have been suggested, but the overall influence in any given area will depend on how well the system remains isolated. If a normally pressured system becomes isolated and is then subjected to a decrease in temperature (for example, if erosion removes considerable quantities of overburden) the pressure in the system will fall below the external hydrostatic pressure. This may have happened in some areas which now have abnormally low pressures.