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Leg 81 drilling results on the west margin of Rockall Plateau, combined with available geophysical data, provide the first transect of the "dipping reflector" type of passive margin. Unlike passive margins characterized by large tilted fault blocks, this type is characterized by an oceanward dipping suite of reflectors and may be the predominant type of rifted margin.The "dipping reflector" margin can be divided into four structural zones: the ocean crust (Zone I), an outer high (Zone II), the area of dipping reflectors (Zone III), and a "landward" zone (Zone IV) of subhorizontal reflectors. The Leg 81 transect sampled Zones II, III, and IV at four sites.The dipping reflectors were penetrated at two sites, 553 and 555. At these sites, the dipping reflectors consist mainly of subaerial tholeiitic flow basalts and minor interbedded sediments; high acoustic impedance contrasts between flows/ packets of flows and sediments may cause the reflections. The flow basalts making up much of the sequence were very probably erupted entirely within the Anomaly 24B-25 reversed polarity interval and possibly a good deal less. Effusive basalt eruption was succeeded by a major phase of pyroclastic volcanism that ceased just prior to Anomaly -24B and is recorded as a widespread ash-fall deposit throughout the NE Atlantic.Two hypotheses have been proposed to explain the dipping reflectors. Mutter et al. (1982) consider that they are formed by some kind of subaerial seafloor spreading. By contrast, Roberts et al. (1979) andHinz (1981) propose that they have formed by voluminous eruption on stretching continental crust. The Leg 81 results neigher prove nor disprove that the dipping reflectors are underlain by oceanic or continental crust, although preliminary gravity interpretation favors the latter. However, the Leg 81 results provide data and in turn constraints on reasonable models for the formation of this type of margin.

Ten sites were drilled on Leg 32 and seven sampled chert and Porcellanite of Cretaceous age. Only Site 313 had Cenozoic (Eocene) chert. The lack of Eocene chert is explained by the sites being north of the high-productivity equatorial zone at that time. Sites 305, 306, 310, and 313 contain nodular and bedded chert and Porcellanite associated mainly with carbonate. Sites 303, 304, and 307 contain mainly bedded cherts associated with pelagic clay; minor carbonate occurs with chert immediately above basement. The porcellanites with carbonate contain initially both opal-CT and quartz as the silica phase. The conversion of opal-CT to quartz is complete in younger rocks where it is associated with carbonate rather than clays. More quartz is precipitated directly if the ratio of foraminifera to nannofossils is high in the sediment being replaced. The presence of clay in a microenvironment favors precipitation of opal-CT. Porcellanites associated with noncarbonate sediments are mainly composed of opal-CT. Sites 303 and 304 have the oldest opal-CT: Hauterivian to Valanginian age (~ 118 to 130 m.y.).Biogenous opal is the silica source for most cherts. Composition and texture of the host sediment determines where chert will form. Bedded calcareous porcellanites may take 40 to 50 m.y. to form, but chert nodules seem to form earlier. Some chert of Barremian to Hauterivian age associated with volcanic glass, clinoptilolite, and montmorillonite may have a volcanic origin.