The Baltimore mafic complex in the central Appalachian Piedmont consists of tectonically emplaced layered ultramafic, mafic cumulate, and volcanic rocks. The general stratigraphy of the layered rocks is basal dunite and chromitite grading upward through lherzolite and websterite to hypersthene gabbro and quartz-gabbro at the top. Cyclicity within the cumulus sequence is shown by the presence of upper-level peridotite and the occurrence of olivine gabbro at higher stratigraphic levels. The cumulus crystallization sequence is olivine plus chrome spinel followed by orthopyroxene, clinopyroxene, and plagioclase. Cryptic variation in the composition of the cumulus minerals is shown by olivine Mg_97–75, orthopyroxene Mg_90–49, clinopyroxene Mg_92–59, and plagioclase An_97–73. Coexisting pyroxenes define a wide miscibility gap reflecting subsolidus reequilibration. Pre- and post-tectonic emplacement recrystallization histories in the metagabbros are shown by textural relationships and compositions of amphiboles. Post-emplacement metamorphic recrystallization and chemical redistribution occurred after tectonic emplacement into the metasedimentary rocks of the Glenarm Series. Strontium and neodymium isotopic data show that the parent magma was contaminated by a continental component prior to extensive crystallization. Association of the plutonic rocks with metavolcanic gneisses characterized by bimodal chemistry suggests that the complex may have formed in a marginal basin originating inland from a continental margin volcanic arc or as a sub-arc plutonic complex.