During the Royal Society Expedition to southern Chile in 1958 geological observations were made at Chepu and on the San Pedro tableland in the island of Chiloe. The island is composed of schist, Tertiary sediments, fluvioglacial deposits and volcanics. The schist, of uncertain age, forming the basement at Chiloe, is an extension of the schist of the coastal range of southern Chile, and falls in the chlorite zone (greenschist facies). It includes greenschists and altered gabbroic rock as well as normal quartzofeldspathic schist, flat lying at San Pedro but strongly crumpled and folded on the west coast. Tertiary or Quaternary volcanic rocks, of limited distribution, include banded rhyolite, partly spherulitic, and altered vesicular hypersthene andesite. Upper Tertiary sandstone unconformably overlying the schist on the coast south of Chepu contains abundant fossil invertebrates (mainly Mollusca) attributed to the Lower Pliocene. The Chepu assemblage is intermediate in character and apparently in age between the Miocene (Navidad and Ranquil) faunas and the Middle Pliocene (Coquimbo) fauna of Northern and Central Chile, containing persistent Miocene elements together with Pliocene immigrant elements that do not occur together in the better known faunas of more northerly districts. Two new species of Ocenebra and a new subspecies of Acanthina crassilabrum Lamarck are described.