The rocks studied consist of calcite-quartz-diopside-graphite assemblages in metamorphosed fossiliferous limestones of the batholith of Southern California. The carbonate rocks are minor members of a sequence of pelitic, arenaceous, and carbonaceous metasediments which are observed in the field in every stage of transition to oligoclase-quartz-gneiss and oligoclase-biotite-gneiss. Light-colored diopside-quartz-plagioclase skarns are also members of the metasedimentary sequence. The calcites of the limestones are very low in MgCO3 which is in thermodynamic agreement with the absence of dolomite. Assuming equilibrium, the diposide-graphite assemblage enables limits to be placed on the fugacity of oxygen during crystallization. This fugacity range corresponds to the low side of the univariant reaction: The skarns are interpreted as dolomitic beds which have been changed to diopside skarn by silica metasomatism. Some evidence is also presented for the derivation of graphite from the breakdown of carbonate, but this is regarded as only a local phenomenon which can be attributed to the buffering action of the original high carbon content of the rocks. In addition, the stability limits of coexisting iron oxides and graphite are discussed.