Twenty-two Upper Cretaceous (Brazeau Formation) and 9 Paleocene (Paskapoo Formation) bentonites from the Foothills of the Rocky Mountains in western Alberta were chemically analyzed by X-ray fluorescence. Petrographic modal analyses were made of the coarse fractions of 11 of the Upper Cretaceous bentonites to compare chemical and mineralogical compositions.Following modal classification schemes of igneous petrologists, the original ashes were predominantly rhyodacites but ranged from dacites to rhyolites. A trend toward increasing acidity is indicated in Late Cretaceous time, reaching a maximum near the Cretaceous-Tertiary boundary.The elemental composition of the bentonites shows no close quantitative relationship to the composition of the coarse crystal fraction. Alteration of the ashes to bentonites has resulted in preferential loss of silica over alumina, and of sodium over potassium and calcium. The chemical composition of the bentonite as a whole is controlled largely by the composition of the dominant clay mineral, montmorillonite, but original differences in composition between ashes are preserved to some degree.