The observations made during the Carstensz Expedition give the following impression of the geological structure of the Nassau mountains: 1. Possibly Lower Palaeozoic, Upper Palaeozoic, Mesozoic and Tertiary (Tertiary c, d, e and f) rocks were found. 2. A granodioritic intrusion occurs with a contact zone rich in metasomatic hydrothermal ores (copper, gold). The age of this intrusion is Upper Tertiary, probably even younger than the folding of the mountains. 3. The simplest explanation of the tectonical structure is to assume that these mountains are a big overthrusted mass, moved towards the South over the continuation of the Australian continent. The upper parts (Tertiary) of this mass are folded; otherwise, only North dips were observed. 4. The foreland of this tectonical unit probably for the greater part is covered by unfolded Nassau-molasse deposits. 5. The moraines of a rather important Pleistocene glacier are present.