INTRODUCTION Prior to the search for oil in the Ecuadorian Oriente, the geology of this extensive jungle area remained practically unknown. In 1892 Teodor Wolf compiled and reviewed earlier observations on the Oriente by Villavincencio, Orton, Stuebel, and Reiss. In the early 1920’s, the Leonard Exploration Company became interested in the oil prospects of the Oriente, and their geologists, J. H. Sinclair and T. Wasson, made the first geological reconnaissance in the area between Baeza, Coca, Canelos, and Macas (Wasson and Sinclair, 1927). Some of the igneous rocks of the Oriente region were described by R. J. Colony and J. H. Sinclair (1928 R. J. Colony and J. H. Sinclair (1932). Beginning in 1939 and ending in 1950, the entire Oriente was under continuous investigation by the geologists of The Shell Company of Ecuador, Ltd. Some of the first results obtained by these geologists are mentioned briefly by J. C. Granja (1942). More comprehensive summaries have been given by H. J. Tschopp (1948 H. J. Tschopp (1953). STRATIGRAPHY The formations that crop out in eastern Ecuador range from Early Paleozoic, if not from Precambrian, to Recent in age. On Table 1, a total column of between 7600 and 16,700 m of Paleozoic to Recent sediments is considered as overlying metamorphic rocks of early Paleozoic or Precambrian age. METAMORPHIC ROCKS (pKm) The crystalline basement of Eastern Ecuador consists of various types of older granites; ortho- and para-gneisses; mica, garnet-bearing, amphibolitic, sericitic, talcose, and graphitic schists; phyllites; some quartzitic rocks; and calcareous slates. This group of highly metamorphosed rocks is the main