The Baca geothermal area is located in the Valles caldera in the Jemez Mountains of north-central New Mexico. Union Geothermal Company drilled a number of exploration wells to supply steam for a proposed electric generating plant. Drill cuttings from one of these wells, Baca 22 (see Figure 1), were studied with a petrographic microscope and by x-ray diffraction to determine the nature of the original rocks and of the hydrothermal alteration. The hydrothermal alteration will be used to determine the temperatures of alteration which can then be compared with borehole temperatures to determine if the mineral assemblages are compatible with present day temperatures. It will be shown that there is evidence indicating that the upper 2000 feet of borehole is cooler now than it has been in the past