The phenomena that control sand composition are complex, dynamic, and interlinked. Although source-rock lithology is of critical importance in determining the allowable range of sand compositions, chemical weathering may overprint the source-rock signal substantially. In the humid tropical environment of Barro Colorado Island, Panama, source rocks of similar lithological character produce sediments of differing compositions in different physiographic environments. Moreover, sediments derived from source rocks of dissimilar mineralogical composition but produced on similar physiographic terrains may be nearly indistinguishable. Physiography influences the composition of sediments released from the source areas by controlling the duration of chemical weathering. On relatively flat terrains, rates at which loose solids are generated by weathering may exceed rates at which slope processes can remove this material. Soils are exposed to the weathering environment for long periods of time and are strongly leached. Consequently, weathering products are cation poor and contain relatively immobile elements. Incompletely weathered detritus is not abundant and is greatly enriched in chemically resistant phases. On steeper slopes, potential solid erosion rates exceed weathering rates. Residence times of minerals within soils are short, resulting in weathering products that are cation rich and contain an abundance of unstable phases. On Barro Colorado Island, fluvial sediments are dominated by authigenic particles composed of secondary weathering products. Although the stability of many such particles in larger transportational and depositional systems and in the diagenetic environment has yet to be assessed, they may be more important constituents of fluvial sediments in tropical environments than has been previously recognized.