On Leg 54 of the Deep Sea Drilling Project a transect of four drill-holes was placed across the Galapagos Mounds Hydrothermal Field. Basement basalts, pelagic sediments, and two types of hydrothermal sediments (Mn-oxides and Fe-rich green clays) were recovered. Stratigraphic examination of the cored sequence provides the following generalized sequence of a hydrothermal mound (from top to bottom): Mn-oxide cap, Fe-rich green clays (nontronite) layers intercalated with pelagic sediments, normal pelagic sediment, and basement basalts. The green clays have textures due to both replacement of previously existing sediment and open space filling. Thus, intercalation of the green clays with pelagic sediment may be indicative of a replacement-growth process during formation of the hydrothermal layers. The x-ray diffraction data, microscopic observations, and general chemistry reveal that the oxides are composed mainly of todorokite, birnessite, and amorphous compounds. Chemical analyses suggest that the oxides are nearly pure combinations of Mn, O, and various amounts of H 2 O, with a conspicuous paucity of Fe and transition elements. The green clays are 10-12 A smectites (nontronite) that are chemically similar to glauconite. Mg probably substitutes for Fe in the clays, and K may occur as an adsorbed, interlayered cation. The smectites are also low in most transition elements. Oxide crusts from the uppermost cores in DSDP holes 424 and 424A contain Mn-oxides intimately mixed with the smectites. These phases were separated and were seen to correspond to chemical end members of a physical mixture in the sediments. A mixing line for hydrothermal material is suggested between these end members. Alteration of the underlying ferrobasalt is limited to "alteration rinds" around cracks, fractures, and flow surfaces. These rinds are formed by the precipitation of smectite, Fe-oxides, and minor amounts of calcite and pyrite is vesicles, microfractures, and primary porosity in the affected basalts. In general, the basalts are fresh and alteration is not pervasive. Umber deposits of the Cyprus ophiolite as described by Robertson (1975) are similar to the hydrothermal mounds under discussion in lack of pervasive basalt alteration and enrichment in Mn-oxides. It is proposed that Mn Deposits previously reported from ophiolite sequences may be analogous to the Mounds Mn-oxides.