The Middle Ordovician Ammonoosuc Volcanics of the Bronson Hill anticlinorium in the upper Connecticut River valley, New Hampshire–Vermont, is composed of amphibolite, greenstone, and felsic schists. Major-element analyses of rocks believed to have been basaltic flows, such as pillowed greenstones and dense amphibolites, reveal that they are tholeiitic in composition. The felsic schists do not have igneous compositions, therefore indicating contamination by sedimentary detritus. Regional metamorphism appears to have been isochemical. However, sea-floor alteration prior to regional metamorphism probably depleted the basalts in MgO and slightly enriched them in SiO2 and P2O5. On the basis of trace-element discrimination diagrams (Ti-Zr, Ti-Zr-Y, Ti-Zr-Sr), two distinct basaltic populations exist, suggesting an abyssal oceanic affinity for one group and an island-arc affinity for the other. It is proposed that the opening of the proto–Atlantic Ocean (Iapetus) in early Paleozoic time and its subsequent closure during the Taconic orogeny would explain the interfingering of Cambrian to Middle Ordovician abyssal tholeiite, island-arc tholeiite, and eugeosynclincal metasedimentary rocks in the northern Appalachians.