When heavy minerals are liberated by weathering of their parent rock, resistant species show relative concentration and less resistant species decrease in relative abundance or disappear completely. The change in relative abundance of the various minerals from fresh rock to the weathered products is a measure of their comparative resistance to weathering. Samples of fresh and weathered Wissahickon schist from Pennsylvania and Maryland were analyzed in this way. Zircon proved to be the most resistant mineral considered, garnet the most readily destroyed in weathering. Other common heavy minerals show considerable range in resistance. The results of this study, particularly the data on weathering of garnet, were used in interpreting the origin of nearby Lower Cretaceous sediments. Applications of the method and results to other fields of geology and to soil science are suggested.