This volume covers Leg 12 of the cruises of the Drilling Vessel Glomar Challenger. A principle scientific objective of Leg 12 was to sample the sediments immediately above the basement at a number of sites, to determine the age of the crust, and hence to provide some key data in the evolutionary history of the North Atlantic. Sites were chosen in the Labrador Sea, the Reykjanes Ridge, the Iceland Basin, Rockall Trough and the Bay of Biscay with these objectives in view. A second objective of the leg was to drill into two features, Orphan Knoll and Rockall Plateau, which were believed, on geophysical grounds, to be fragments of sunken continent. It was expected, if it was found that they were continental in origin, that the sediments would contain evidence of the subsidence history which might relate to the North Atlantic evolution. D.V. Glomar Challenger sailed from Boston, Massachusetts, U.S.A., on June 19th 1970 and arrived at Lisbon, Portugal on August 11th. Thirteen holes were drilled and cored at nine sites.