This volume covers Leg 48 of the cruises of the Drilling Vessel Glomar Challenger. Unlike many cruises of the Deep Sea Drilling Project, Leg 48 was scientifically planned to drill a number of sites to systematically address the problems of passive margin evolution. Sites drilled during Leg 48 were selected on the continental margin of the Bay of Biscay and on the Rockall Plateau. Both these margins are characterized by reduced Tertiary progradation thus enabling safe penetration of the synrift and prerift sediments at shallow depths. The two margins also offered a contrast in both age and structure. The margin of the Bay of Biscay apparently formed by rifting at about 130 m.y. and is structured into a series of rifted blocks and half-grabens overlain by a thin Cretaceous and Tertiary cover. The southwest Rockall Plateau which was rifted at about 60 m.y. B.P. does not show a prominent horst and graben structure but, instead, consists of a thick deltaic (?) sequence closely similar to that observed beneath the outer Voring Plateau off Norway. Leg 48 of the International Phase of Ocean Drilling of the Deep Sea Drilling Project began on 12 May 1976 and ended 61.86 days later in Aberdeen, Scotland, on 13 July 1976. During this leg, Glomar Challenger traveled 1935.4 nautical miles and drilled 10 holes at eight sites.