Grains of clastic organic matter similar to coal occur scattered among the mineral grains in most shale and sandstone; they are called phytoclasts because many of them have relict plant structures. The phytoclasts that have been least altered in a given sample can be used to determine the degree of metamorphism of the host rock. As the rank of phytoclasts increases after burial—mainly in response to temperature increase—rank can be used to indicate past rock temperature in the range of about 80° to 200°C for exposure times longer than those associated with contact metamorphism. The proportions of different phytoclasts vary greatly in different samples, making bulk chemical analysis unreliable for determining rank. Optical properties of phytoclasts can be used as parameters of rank, and measurement of reflectance of polished grain mounts under the microscope is the technique applicable to the greatest variety of samples and best suited to selection of the least altered grains in a sample. Reflectance of phytoclasts from drill-hole cores, the duration of phytoclast burial in the host rocks, and the present-day temperatures of the rocks are used to construct a rank-time-temperature model of organic metamorphism. The model is used for the Great Valley sequence and Franciscan assemblage, thick upper Mesozoic strata in California. Reflectance of phytoclasts from rocks of the Great Valley sequence that are now exposed increases with interpreted former burial depth, but the deduced past temperature gradient was unusually low, about 5°C per km (0.17°C per 100 ft). This low gradient value also appears to apply to the Franciscan assemblage where it is in thrust contact underneath the Great Valley strata. The past temperature, thermal gradient, and burial depth of Franciscan rocks, interpreted from the gradient of phytoclast alteration, corroborate studies of mineral paragenesis in these rocks, which are in the zeolite and blueschist metamorphic facies. The former low thermal gradient and the structural position of these strata support the concept of crustal tectonics in which the Franciscan assemblage moved under coeval Great Valley sediments, and both subsequently rose rapidly and were dissected by erosion.