Kaolinite development usually requires rainfall of 120 cm per year under conditions of extensive leaching. Kaolinite formation from the La Posta Quartz was studied in a road cut 763 m east of the intersection of La Posta Road and Old Highway 80, San Diego County, California, where the average rainfall was 38 cm to 50 cm per year. The kaolinite was developed in fault gouge resulting from nearby oblique faulting in Thing Valley, California. Rock alteration sequences, developed perpendicular to the trend of the faults, included four distinct zones: Zone 1, least altered grussified quartz diorite (making up the majority of the road cut); Zone 2, red altered grussified quartz diorite; Zone 3, white kaolinite clay seam; and Zone 4, narrow central red clay seam. Mineralogical and elemental analyses along these profiles indicate the original minerals weathered in the following order: hornblende, biotite, and oligoclase. Hornblende and biotite altered to montmorillonite in the least altered and red altered zones. The transformation of oligoclase to kaolinite began with the oligoclase Ca-rich core in the red altered zone and concluded with the Na-rich seams of oligoclase in the white clay zone where pseudomorphs of kaolinite after oligoclase were abundant. In a few cases, oligoclase altered to prehnite at the boundary between the red altered zone and white clay zone. The presence of laumontite in the red altered zone was indicated by x-ray diffraction analysis. Mass balance equations were made by utilizing the chemical and mineralogical analyses of the original rock and the extracted pore waters to determine whether kaolinite is still forming under present-day conditions. Two mass balance calculations were made between (1) the original minerals (biotite and oligoclase) reacting to form kaolinite, and (2) montmorillonite reacting to form kaolinite. Both sets of calculations indicate insufficient acid is present in the pore waters for the formation of kaolinite today. Montmorillonite should be the dominant residual weathering mineral indicated by the measured SiO2 and (HCO3)- contents in the pore waters following the criterion of I. Barshad in 1966. Montmorillonite is the clay mineral found in soil profiles nearby. The identification of the zeolitic facies minerals, prehnite and laumontite, developed from the alteration of oligoclase in addition to the presence of kaolinite in the fault gouge indicate hydrothermal alteration. Fault gouge is necessary for the permeability to be sufficiently great to produce extensive kaolinitization. To meet the conditions of prehnite and kaolinite synthesis, moderate hydrothermal solution temperatures of 100°C to 150°C must have existed sometime in the past. Present hot springs are located at Aqua Caliente 32 km north of the study area.