A study has been made of the feasibility of applying the Rb/Sr method of age determination directly to sedimentary rocks through the use of the sedimentary accessory mineral glauconite. Seven glauconites of Cambrian to Tertiary age, separated from shales, sandstones, and limestones, were found to have Rb contents of 180 to 310 ppm; 14 samples were found to have non-radiogenic Sr concentrations of 5 to 310 ppm and radiogenic Sr-87 contents of 0.05 to 0.7 ppm. The large variation in Sr content is believed to be due to differences in the proportion of contamination CaCO₃ (containing traces of Sr) remaining in the samples, which were purified by physical means only, and not to variations in the glauconites. The glauconites analyzed may be truly authigenic, as all t e ages computed are reasonable; however, the great present imperfection of knowledge of absolute sedimentary time makes critical evaluation of such data most difficult. Further development of this method is well justified, for the demonstrated analytical feasibility of the determination of both Rb/Sr and K/A ages for glauconites and other authigenic K, Rb minerals may make possible the establishment of a dependable absolute time scale for at least the post-Precambrian sedimentary column.