This volume covers Leg 70 of the cruises of the Drilling Vessel Glomar Challenger. The major objectives of the first part of Leg 70 were to collect data and sediment representative of the hydrothermal mounds and ridges located about 20 km south of the Galapagos spreading axis. Undisturbed, continuous stratigraphic sequences of sediments and basement rocks from the mounds and from off-mound areas were required to answer the following specific questions: 1) How does the stratigraphy on a mound compare to that off a mound? Similarly, how do the pore water chemistry, physical and paleomagnetic properties, mineralogy, and paleontology of the sediments compare between these two areas? 2) How does the heat flow measured over the entire sediment column compare to that measured in the upper few meters of sediment? 3) Do the two sub-bottom reflectors correspond to lithologic boundaries or to physical property changes in sediments? 4) How variable is sediment stratigraphy of mounds within the same area? 5) Is there a difference between the alteration of basement rock directly beneath the mounds and away from them? 6) Are the chemistry, mineralogy, and physical and magnetic properties of the basement rock affected by the hydrothermal processes which produced the mounds sediments? 7) Are magnetic properties of the basement rocks compatible with the regional magnetic anomaly? 8) Over what period of time were the mounds formed, and are they presently active? The D/V Glomar Challenger departed Balboa, Panama on 11 November 1979, heading to the Galapagos hydrothermal mounds area to drill 5 sites (506 – 510). The cruise ended in Callao, Peru in December 1979.