INTRODUCTION The Peruvian part of the Geologic Map of South America is a reasonably sound indication of the general distribution of rocks of the various systems, of regional structural trends, and of larger areas of intrusive, extrusive, and metamorphic rocks. Since careful reconnaissance mapping has been done only in limited regions, however, much of the map is a compilation and interpretation of data from diverse sources. Data available up to the end of 1947, much of which has been published subsequently, was used. Regions of greatest geologic control are northwestern coastal Peru, central Peru around Oroya, and southern Peru. Northwestern Peru, particularly the coastal region around the oil fields, has long been the subject of detailed geologic study leading to geologic and paleontologic publications, some of them accompanied by maps. The work here of Bosworth, Iddings, Olsson, and Petersen has revealed a wealth of geological information. The geologic map of this region is based upon the work of these writers, together with unpublished data gathered by the geologists of the International Petroleum Company, Ltd., up to 1946. It was compiled by A. Lyndon Bell, Chief Geologist of the Company. Other reliable work is that of the geologists of the Cerro de Pasco Corporation in the highlands of central Peru. A summary of early work, without map, was published by D. H. McLaughlin in 1924. In recent years several projects leading to the publication of reconnaissance geologic maps of larger areas have been completed in widely separated parts of the country. Since