THE structure and nature of the uppermost oceanic mantle and crust has been, and still is, the subject of a major international research effort . Yet the direct field evidence known to us on this subject, whether obtained by drilling in the I.P.O.D. oceanic crust programme² or by submersible surveying in the Famous programme , has been either very incomplete or limited to the upper part of layer 2. As a result, the problem of the comparison between ophiolites and oceanic crust is still debated . We report here the preliminary results of an in situ geological survey in which we have sampled and observed what we believe to be an almost complete geological section starting a few kilometres below the Moho discontinuity and ending in the upper part of layer 2 (ref.4). This section outcrops on the flanks of Gorringe Bank, which is a ridge 200 km long and 60 km wide between Tagus and Horseshoe abyssal plains in the Atlantic ocean, South West of Cape Saint Vincent, Portugal (Fig. 1) (Fig. 1). [NOT CONTROLLED OCR]