An extensive body of pelitic hornfelses ranging in composition from andalusite+biotite +quartz+muscovite to sillimanite+cordierite+perthite+biotite+quartz±almandine occurs in a metamorphic septum near the southern end of the Sierra Nevada batholith east of Isabella, California. Field and laboratory investigations suggest the hornfelses developed at high temperatures over a prolonged period of time. Chemical analyses of some constituent minerals show that the stability field of biotite+andalusite diminishes and biotite and cordierite become richer in Fe2+ relative to Mg with increasing temperature. Fe appears to have been reduced from a somewhat oxidized state during prograde thermal metamorphism, suggesting the metamorphic body was “open” to oxygen. The development of garnet with accompanying cordierite, biotite, and sillimanite may be related to excess Ca in some rocks, but this is not definite.