Approximately 4 kb tectonic overpressure was required, according to one theory, for the formation of jadeite-aragonite-bearing rocks of the Franciscan. Strength of two common Franciscan rocks—massive graywacke, and thin-bedded shale and graywacke—was determined to see if this tectonic overpressure could have been generated under the conditions of metamorphism; that is, 4 kb confining pressure, 200 to 300° C, in the presence of aqueous pore fluid. The strength was found to depend markedly on both pore and total pressures. The required tectonic overpressure could only have been generated in massive graywacke, and only when pore pressure was less than hydrostatic. With even moderate amounts of interbedded shale, tectonic overpressure could not have exceeded 1 kb.