The stratigraphy of the Eocene rocks of the Toa Baja drillhole is dominated by volcaniclastic sediments which are interbedded with marly pelagic limestones, especially in the lower part of the hole, and lava flows. Petrological, geochemical and paleontological evidence suggest that the site of deposition was a deep marine basin and the source of the volcanics were subaerial or shallow submarine island arc volcanoes. ¿American Geophysical Union 1991