The structures of North Arm Mountain ophiolitic basalt and diabase indicate that these formed as a frozen lid to an underlying magma chamber in a system analogous to present-day mid-ocean spreading systems. Volcanics consist of pillow lavas, flows, and breccias. Volcanics grade downward into sheeted dikes by increase in dike frequency across a transition commonly less than 50 m thick. Sheeted dikes, subparallel throughout, dip away from the paleospreading axis, located to the west of present ophiolite position as indicated by chilled margin bias, implying rotation of the lid down and away from the axis. One-way chilling statistics, dike thickness, and dike intrusion sequence analyses all point toward a relatively narrow but complex zone of dike intrusion. The mean measured dike thickness is 0.90 m. Diabase contains zones of fracturing subparallel to dike trends, interpreted to reflect oceanic fissuring and faulting. These commonly terminate downward in foliated gabbro and (or) lineated amphibolite. Diabase grades into gabbros across a complex transition wherein (1) dikes grade into gabbro by gradual decrease in dike frequency, (2) dikes grade abruptly into gabbro, and (3) gabbro intrusions truncate and stope overlying diabase.