A detailed study of the mineralogy and paragenesis of the Zn-Pb ores of northwestern Illinois reveals that conditions of ore deposition were the same in the stratigraphically higher "crevice" deposits as they were in the deeper "flat-and-pitch" deposits. Except for an increase of galena relative to sphalerite in the crevice deposits, the minerals, their habits, and their relation to one another are the same in the 2 classes of deposits. In addition, spectrographic analyses show no differences in trace element content between samples of the same mineral from the different stratigraphic horizons. It is proposed that the 2 classes of deposits do not represent zones of different conditions of deposition. Two possible causes of the increase of galena in the stratigraphically higher deposits are discussed.