The predominant clay minerals found in the waters of rivers in the area of Chesapeake Bay consist of a well-formed to weathered illite with minor amounts of kaolin and a trace of weathered chlorite. Mixed-layer clays were observed in the estuaries and are related to and difficult to distinguish from differentially hydrated monomineralic clays when these clays are in a weathered condition. A regularly stacked mixed-layer illite-chlorite was observed at one locality in the James Estuary. A chlorite-like and vermiculite clay are forming in estuaries along the Atlantic Coast. Thermal stability of the diagenic chlorite increases with increasing salinity of the environment and to less extent with depth in the sediment. The chloritic material arises principally from the diagenesis of weathered illite in the Atlantic Coastal environment and probably passes through a vermiculite stage. Montmorillonoid, and to less extent illite, are altered to a chloritic material in the Gulf of Mexico and along the Pacific Coast. The first phase planned for the study of the geochemical relation of chloride, sodium, calcium, potassium, and magnesium in the clays of an estuary has been completed. The chemical data lend support to the hypothesis that a chloritic-vermiculitic clay mineral is being constructed by the alteration of weathered illite and montmorillonoid, and the reconstitution of a trace amount of chlorite.