Ultramafic and gabbroic rocks dominate the lower crust of the North Arm Mountain massif of the Bay of Islands ophiolite, Newfoundland. Synkinematic sills (≤50 m thick) make up a large proportion of the crust. There is no evidence for large, long-lived magma chambers. Peridotitic intrusions assimilated and reacted with their hosts. Chromitites formed by incongruent dissolution of pyroxene and feldspar. Pyroxenite formed through pore-scale hybridization between invading pyroxene-saturated magmas and partial melts of gabbroic hosts. Most feldspathic peridotites and olivine gabbros display textures suggesting that they are mixtures of primitive magmas and partially solidified gabbroic cumulates. The crust has acted as a reactive filter: the chemical evolution of primary magmas owes as much to assimilation and reaction with older cumulates as it does to fractional crystallization.