The typical clayey silt muds of the Hudson River channel contain between 5 and 6 percent of humic and other easily oxidizable organic matter, and about 50 percent water. The dominant clay minerals are chlorite and illite in both fresh and brackish water muds, but a great variety of other finely-fragmented minerals are also present. Man-made detrital pollutants in the sediments, including fly ash, tar, coal, slag, brick and steel, are largely confined to the upper inch of sediment. Silt-sized fragments of these materials, presumably worked downward by worms, are detectable down to the base of 4-foot cores. Cation-exchange capacities of these clayey silts are in the order of 30 to 40 milliequivalents per 100/gms. Three quarters of the exchange capacity is accounted for by the organic fraction of the sediments. H (super +) ions (rather than Ca (super ++) , Na (super +) , and others) apparently occupy nearly three quarters of the available ion exchange sites (the sediments are / "unsaturated" with the common metallic cations). In the estuarine muds (daily exposed to diluted sea water) exchangeable Na (super +) ions are not abundant; and nearly as much exchangeable manganese as sodium is present. Organic coatings seem to hold, and interfere with the exchange ability of, many of the Ca (super ++) and Mg (super +) ions. Cation exchange capacity correlates directly with organic content of the sediments.