The upper crustal seismic structure of the slow spreading Mid-Atlantic Ridge is studied using a genetic algorithm-based waveform inversion of multichannel streamer data. Four single-ship multichannel profiles from 35¿N are analyzed: one in the rift valley and three in the rift mountains along 0.7, 1.6, and 1.9 Ma crust. A layer 2A horizon is continuously imaged along three profiles and is associated with a sharp velocity increase from extrusives to dikes. Its depth and regularity in the rift valley indicate that most of the extrusive section is built on the inner valley floor through a pattern of deposition and fault-bounded uplift into the rift mountains. Its variability along one line, however, shows that this process is disrupted during tectonically dominated periods. A thickening of layer 2A toward the Oceanographer fracture zone may be the result of along-axis magma transport. The interval of rapid velocity increase at the base of layer 2A thins with age, a possible response to enhanced hydrothermal mineralization within the zone of mixed dikes and extrusives. Transition zone (~200 m) and off-axis layer 2A thicknesses (350--600 m) are similar to those at other spreading centers. This indicates that equivalent extrusive volumes are produced at all spreading rates along a relatively narrow zone of dike emplacement. However, differences in on-axis layer 2A thickness between this area and fast spreading ridges suggest that the exact pattern of thickening varies between spreading regimes. Relative to fast spreading ridges, the moderate velocity increase with age recorded in the upper crust (from 2.3 to >2.7 km s-1 within ~2 Myr) may be due to a less active hydrothermal system and hence slower porosity reduction.