An application of the electron probe x-ray microanalyzer to the analysis of extremely minute inclusions in minerals is described. Strongly birefringent grains about 0.03 mm. in size, which resemble what has been termed valleriite, occur in chalcopyrite from the Mackinaw mine, Snohomish County, Washington. These grains were found by the electron probe analysis method to contain about 55 per cent iron and less than 5 per cent copper; the composition was inferred to be approximately FeS. These results suggest that the mineral previously generally termed valleriite in high-temperature copper ores is distinct from the type of valleriite from Kaveltorp, Sweden, and is probably an undescribed sulfide.