The investigated area is located in the southwest of Costa Rica on the edge of the "Rainforest of the Austrians". On the basis of geological mapping and X-ray diffraction analyses the following lithologies (Golfito Terrane, late Cretaceous-Paleocene) can be distinguished: the majority of the investigated area is formed by green-coloured, fine-grained tuffites. There are also true basic tuffs and clastic sediments. All of these rocks have considerable amounts of volcanoclastic components, which have been transformed into diagenetically formed smectite and sometimes zeolithe too. The basic character of these rocks is partly proven by relatively high amounts of pyroxene and plagioclase. In additon to these sediments, quartz-free volcanic rocks and volcanic breccias can be observed. The chemical analyses classify them as basaltic rocks. A regionally occurring iron-rich talc mineral is evidence for hydrothermal activity. The weathering products (=laterites) formed from these rocks can be distinguished by their quartz content. The laterites overlying the green tuffites show considerable amounts of quartz whereas those overlying the tuffs/volcanic rocks are more or less quartz-free. Both laterites have kaolinite-smectite (sometimes pure kaolinite too), goethite and hematite as well as partly gibbsite. Consequently, the primary lithologies can partly be distinguished by the laterites developed from them.