The area is in the San Andreas Rift Valley of southern California. The geothermal reservoir is in an arkosic sandstone more than 2,000 feet thick and below an insulating shale 2,000 to 3,000 feet deep. Interstitial concentrated NaCl-CaCl 2 -KCl brine occurs; no steam is present. At 3,000 feet, the temperature exceeds 300 degrees C and is 360 degrees C at 7,000 feet. Mineral reactions and alterations are described. Below the shale, the temperature-depth profiles approach adiabatic gradients. Thermal convection of the pore fluids is the primary mechanism of heat transfer. The temperature gradient is equated to salinity with changes in depth. Various other physical features are evaluated. The brine formed by the evaporation of Colorado River water trapped in the reservoir sands.