The density of particles is a property which has not received adequate attention in recent studies of sediments. The distribution of densities in about 100 samples (chiefly sediments) was studied. Care was taken not to change the material by chemical means. The distribution are specific and reproducible for a given material. They confirm the evidence obtained microscopically that many sediments are made up of compound particles. In the samples studied very few particles with density greater than 3.4 or less than 1.4 were encountered. The method enables one to differentiate between younger and older sediments, as, during the aging of a sediment, organic matter combines (probably largely by physical forces) with mineral constituents, chiefly Fe compounds. Below a particle size of 400 microns there seems to be no pronounced correlation between particle size and composition. The surface charges which bind the components into a composite aggregate may change on oxidation or on reduction, as shown by the change in density distribution. The method provides a useful extension to the existing approach to the study of sediments and shows that particle size and particle shape may not be the most important characteristics of a sediment.