Concentrations of Mn, Ni, Cr, Fe, Mg, and Ti in biotites separated from 28 samples of tuffs from the Green River Formation of Utah have been used in this study as correlation tools. Concentrations were determined by XRF methods. Comparisons of various element ratios using F tests, Tucky tests, diagrammatic tests, and discriminant analysis using density functions show that the three ratios Mn/Fe + Mg, Mn/Ti, and FeO/MgO are the most easily determined and useful element ratios for correlating tuffs. These ratios can be demonstrated to provide a better means of discriminating among individual tuff layers than the methods proposed by Desborough et al. (1973).