The Los Ranchos Formation is a Lower Cretaceous volcanic pile that forms the base of the Greater Antilles in central Hispaniola. It has been divided into basal units consisting largely of (locally pillowed) spilite flows (Cotui Member) and keratophyre and quartz keratophyre flows, minor tuffs, and intrusions (Quita Sueno Member), which are overlain by debris flows and other fragmental rocks, finer grained, water-laid sediment (Meladito Member), and unpillowed spilite flows (Platanal and Naviza Members). The Meladito and Platanal Members are cut by the Zambrana Fragmental Member, of uncertain origin, and the Pueblo Viejo Maar-Diatreme Members, which formed during a phreatomagmatic eruption during late Los Ranchos time. The maar contains carbonaceous sediments with abundant plant fossils, indicating that the Los Ranchos Formation became emergent during later phases of volcanism. These rocks have been converted to metamorphic minerals characteristic of the lower greenschist facies by reaction with sea water shortly after deposition. SiO 2 abundances in these rocks, which have not been modified greatly by this sea-water metamorphism, have a strongly bimodal distribution. Confirmation of this bimodal distribution, which can be obtained by comparison of Los Ranchos rock compositions to those for rocks altered experimentally by sea water, shows further that spilite was probably originally basalt but does not indicate whether the original keratophyre was a rhyolite or dacite. The 87 Sr/ 86 Sr ratios of these rocks, which were also modified by reaction with sea water, suggest that the spilites had an initial ratio of no more than 0.7036, which is similar to that of other rock suites in the area, but do not limit the initial ratios of the keratophyres to similarly low values. The lower part of the Los Ranchos Formation probably correlates with the Water Island Formation in the Virgin Islands, and the upper, fragmental part probably correlates with the Louisenhoj Formation. These rocks, as well as similar lithologies in southeastern Puerto Rico, appear to have been emergent during final phases of volcanism. The thickness of these assemblages is not great enough for them to have emerged if they grew from abyssal sea floor, suggesting that they grew from previously thickened crust. The fact that these rock units are compositionally bimodal provides further support for this possibility, because bimodal volcanic assemblages are commonly thought to form by extended differentiation or partial melting of preexisting crust.