A diabase dike about 140 km long (the Shelburne dike) cuts in a northeasterly direction across the southwestern part of Nova Scotia. The dike, recently dated at 201 Ma, forms part of a major Appalachian system of diabase dikes and basaltic flows of early Mesozoic age, emplaced during the first stages of opening of the present Atlantic Ocean.The Shelburne dike is tholeiitic and quartz normative. Its chemistry resembles that of the Palisade sill of New Jersey, but differs substantially from the more primitive magnesian composition of a similar dike on the Avalon Peninsula of Newfoundland. A more advanced stage of differentiation is reflected also in the composition of its main minerals (augite, pigeonite, zoned orthopyroxene, calcic plagioclase). Such chemical variations among the roughly contemporaneous diabase dikes of the northern Appalachians complicate the existing petrogenetic and tectonic models of the development of the Appalachian dike swarm.