In the S.-central Adirondacks, garnet porphyroblasts are found in rocks of various compositions and origins. However, they are abundant only in basic rocks. Their shape, habit, size, and composition depend on the nature of the host rock. They are commonly surrounded by rims or shells whose composition also depends on the nature of the host rock. On the other hand, garnet porphyroblasts are not found in basic rocks that have kept relics of their magmatic texture and mineralogy. Such rocks contain garnet only as coronas. These observations show that garnet porphyroblasts have grown in situ after a period of intense componental movements. The available chemical data suggest that the formation of the Barton mines dark ore results from a process of metamorphic differentiation taking place during a period of rising temperature. There is no reason to suppose that metasomatic changes have been important.