X-ray diffraction and chemical analyses of well samples from clay beds along Long Island's south shore indicate that clays from marine, nonmarine, and transitional depositional environments can be distinguished from one another on the basis of clay-mineral and chemical content. The depositional environments of the samples used were previously identified through other criteria such as microfossil content and gross lithology. The comparisons were done to evaluate the usefulness of mineral and chemical analysis in stratigraphic correlation. Marine clays of Pleistocene and Late Cretaceous age generally contained a full suite of clay minerals, illite, kaolinite, chlorite, and mixed-layer clays, whereas nonmarine and transitional Upper Cretaceous clays were rich in kaolinite but contained only small amounts of illite. The mineral composition of a clay bed is determined by differential flocculation and size sorting of clay particles; these processes in turn are controlled by the salinity of the water. The predominance of kaolinite in some marine Upper Cretaceous clay beds and the presence of mixed-layer clays in transitional beds are attributed to differences in salinity. Chemical differences among samples merely reflect the composition of the clay-mineral assemblages. Results of earlier studies in the northeastern Atlantic Coastal Plain agree with the results obtained in this study, which suggest that chemical-mineral analysis is a valid technique for correlation of clay beds on Long Island, New York.