Conventional stratigraphic mapping of parts of the Icelandic flood basalt succession and regional studies of the palaeomagnetic stratigraphy suggest that in at least two areas the basalt pile is composed of large lenticular shield-like lava units. Each unit is related spatially to its own feeding dyke swarm and is the result of a protracted period of dominantly fissure volcanism from a single fissure zone. The geophysical evidence suggests that the most important seismic discontinuity is the boundary between layers 2 and 3: P seismic velocities 5.1 and 6.3 km/s respectively. This seismic discontinuity has been mapped over large areas of Iceland by Palmason who has shown that it is generally at a depth of between 2 and 5 km below sea level. In eastern Iceland the discontinuity is approximately horizontal and markedly discordant with the observed dip of the individual lavas at sea level. It is suggested that, under Iceland, layer 3 is composed of intrusive dykes and gabbroic masses, whereas layer 2 is made up of extrusives cut by dykes and smaller intrusions. The observed relationships of the lava lenses constituting layer 2 are compatible with a crustal spreading model. The drift away from the axial zone, largely accommodated by dyke injection, appears to be at about 1 cm/year, a rate comparable to that observed on the adjacent Reykjanes Ridge.