Abstract The lower 1100 feet of the 2300-foot John Day Formation near Mitchell, Oregon, consists largely of massive tuffaceous claystones and vitric tuffs. These deposits, of early Miocene to late Oligocene age, are an air-laid accumulation of silicic (dominantly dacitic) ash on a land surface that was initially hilly, but later became almost level. Much of the vitric ash weathered to montmorillonite before burial; nearly all the unweathered ash was subsequently altered to mont-morillonite and clinoptilolite. The principal pyrogenic minerals are at least locally altered in the lower 350 feet of the formation, and clinoptilolite, orthoclase, vermiculite, kaolinite, fluorapatite, cristobalite, and quartz are among the authigenic minerals formed within these beds. Authigenic orthoclase has been noted within the John Day Formation over an area of about 600 square miles, and, with one exception, it has not been observed in overlying and underlying volcanic rocks. The orthoclase is a relatively pure K feldspar; it occurs as a pseudomorph after plagioclase and forms between a trace and 8 per cent of the claystones. A potassium-argon age of 22 m.y. was obtained from authigenic orthoclase in a bed 65 feet above the base of the formation, and seemingly reliable ages of 31 and 24–25 m.y., respectively, were obtained from pyrogenic materials from beds 165 and 1110 feet above the base. Authigenic orthoclase was probably formed at burial depths between 400 and 2200 feet, and at a temperature of 20–40°C. K and Si needed to form orthoclase were supplied by the alteration of glass. Laboratory studies suggest that a K+/H+ ratio of 109 may be necessary to form orthoclase within its stability field at temperatures of 20°–40°C, but the geologic data suggest that the K + /H + ratio in the subsurface water was probably no more than 2.5 × 10 6.5 at the time of formation of the authigenic orthoclase of the John Day Formation.