The important Mesoamerican obsidian source El Chayal has been carefully explored and sampled. Forty new source specimens have been subjected to neutron activation analysis, and the composition patterns are recorded and compared. The westernmost of the known Guatemalan obsidian sources, Tajamulco, has also been studied. Precise analytical data are also reported for the important Ixtepeque and Rio Pixcaya sources. In addition, the complete analytical data for a group of artifacts in Belize excavated by Norman Hammond are reported and compared with the source compositions. The establishment in the literature of a library or collection of precise and intercomparable composition data for the sources of this general region is suggested, and the present paper is offered as a first contribution toward this objective.

Neutron activation analysis was used to determine the chemical composition of sources of obsidian artifacts from the Paleoindian site of Los Tapiales in Guatemala with an occupation date of around 10,700 radiocarbon years ago. Three different sources were detected, all between 50 and 75 km from Los Tapiales. The predominant source represented in our ten samples was in the Rio Pixcayá area, and another was in the Tajumulco area. A third source, identified by comparison with other work, appears to be in the area of San Bartolomé Milpas Altas.