Adsorption on clay minerals greatly affects the reliability of trace elements as environment indicators for ancient shales. Numerous experiments have shown that adsorption and fixation depend on many factors related to the clay minerals in the shales, to the specific trace elements involved, and to numerous characteristics of the depositional environment. It follows that no simple relationship can exist between an element's concentration in shales and a single environmental parameter such as paleosalinity. At the present time no trace element can be considered an ideal indicator; boron seems one of the best but it has only limited value. Further experiments on clay sorption combined with analyses of recent sediments may locate trace elements whose sorption is affected chiefly by a single environmental parameter and which possess other necessary qualifications for environment indices. These elements should provide criteria for determining major variations in an environmental parameter, but they can probably never be used to detect minor variations.