The P-wave velocity structure of the outermost 200 km of the outer core (the E' region) is analyzed using a suite of differential travel-times for SKS, SKKS and S-waves bottoming below within, and above this depth range. Core-penetrating phases with paths beneath Alaska exhibit similar SKS-SKKS differential travel-time patterns to phases sampling the core beneath the central Pacific, but very different SKS-S differential travel-times. These patterns can be explained by a core velocity model in which the outermost 50 to 100 km has 1--2% slower P-wave velocities than standard Earth models, perhaps due to a stably-stratified chemical boundary layer, combined with 3% lateral variations in S-wave velocity in the lowermost mantle. ¿American Geophysical Union 1990