Abstract The Mid-Atlantic Ridge in the central North Atlantic was the source of the first deep-sea basement rocks to be studied in detail (Nicholls and others, 1964; Muir and Tilley, 1966; Miyashiro and others, 1969). The historical background and compositional details of basalts recovered from the modern Mid-Atlantic Ridge are discussed by Schilling and by Melson and O’Heara in other chapters in this volume. Some of the first recoveries of Old oceanic basement rocks were obtained by drilling in the western North Atlantic (Ayuso and others, 1976; Bryan and others, 1977). In this paper we summarize (1) the characteristics of pre-1 Ma lithosphere in the western North Atlantic and (2) inferences about its geologic evolution during the past 200 m.y. Such a review is subject to some serious limitations. Very large areas of the North Atlantic remain unsampled by drilling, and many crustal rocks that have been recovered may be too altered to yield definitive information about their magmatic geochemistry. Also, the deepest crustal penetrations in the North Atlantic have failed to reach the base of layer 2, which is composed almost entirely of extrusive basalts. The nature of the deeper levels of layer 2 and of the (presumably) gabbroic layer 3 must be inferred from samples recovered from a few holes apparently drilled in anomalously thin layer 2, from dredged samples recovered in fracture zones, or by extrapolation of observations at site 504 which was drilled into the deeper levels of layer 2 near the