The Barndoor diabase in north-central Connecticut cuts the New Haven Arkose, the oldest formation in the Newark Group of Triassic age in Connecticut. The intrusions are dikes in part and slightly transgressive sills in part. The rocks are typical diabases, with unusual features such as pigeonite with semispherical fractures and pale-green pyroxene. The most calcic augite occurs in interior parts, requiring crystallization in source chambers followed by intrusion of a crystal-bearing magma. Five feeder conduits from two source chambers are proposed. Extensive differentiation occurred in source chambers, and minor differentiation occurred during emplacement and cooling. Plagioclase composition ranges from An70 to An34 and augite from Wo47En43Fs10 to Wo35En40Fs25. Chill-zone rocks are chemically similar to Talcott Basalt, which is the lowest of three basalt flows in the Newark Group in Connecticut, and indicate contemporaneity of intrusive and extrusive events.