Petrologic, geochemical, and geophysical studies of cores from DSDP sites 417 and 418 show that the Cretaceous eruptive crust was formed at a stable slow-spreading axis similar to the present Mid-Atlantic Ridge, probably not close to major transform fractures. Interpretations of whole-rock and glass chemical variation suggest that magma fractionation occurred in temporally and spatially transient storage reservoirs at various depths between the zone of magma segregation and the ridge crest. We suggest that melt generation, uprise, and eruption are part of a related episode of activity involving rapid upward propagation of fractures. Eruptive episodes are recorded by stratigraphic sequences of contiguous, mostly cogenetic, lavas, separated by breccias, sediment, and/or extreme alteration zones suggestive of lengthly quiescent intervals. The section drilled at 417D comprises three such episodes (in 365 m of basement). whereas seven are recorded in 418A (in 540 m). Patterns of downhole changes in magnetic inclinations and polarities strongly support the suggestion that iterative burial of successive lava episodes occurs with crustal spreading from the axis. This model predicts that the eruptive layer is imbricated and that it shows increasing complexity of lithology, chemical compositions, and magnetic character with depth. Studies of crack distribution and laboratory measurements of core physical properties indicate preferential zones of water circulation which decrease with depth. Permeability of the lava pile is largely controlled by the imbricate eruptive structure.