A variety of igneous and metamorphic rocks are exposed in and near the Rio Mamoni Valley in eastern Panama. The floor of the valley exposes rocks ranging from quartz monzonite through diorite to gabbro. These are part of a Cretaceous batholith that forms the core of the western San Blas mountains in eastern Panama. The Rio Mamoni cuts diagonally across a narrow portion of this batholith. Steep east-west faults separate the plutonic rocks from igneous and metamorphosed sedimentary rocks, interpreted as ophiolitic, that occur to the north and to the south of the batholith. Major element geochemistry of this suite of rocks shows a strong positive correlation of K₂O and Na₂O with SiO₂; Al₂O₃, MgO, MnO, FeO, TiO₂, P₂O₅ and CaO show a negative correlation with SiO₂. These variation diagrams show relatively well developed trends. An AFM diagram shows a strong tholeiitic trend. This is compatible with either a primitive island arc or an oceanic rift environment for these igneous rocks. Phenix City gneiss amphibolites show similar positive and negative major element correlations relative to SiO₂ as well as tholeiitic tendencies. Fault contacts between the batholith and the ophiolitic rocks are well exposed in some places. Additional geochemistry is planned to test the hypothesis that the rocks that make up the Cretaceous batholith are consanguenous and that the batholith was a source of magma for the nearby ophiolitic rocks. Alternatively, the pluton may pre-date the surrounding ophiolites. This study was supported in part by NSF Grant EAR-9820701 to Thomas B. Hanley.