Volcanics from the Arisaig-Antigonish district of northeastern Nova Scotia underlie fossiliferous Lower Silurian (Lower Llandovery) sediments. The minimum geologic age of the volcanics is Early Silurian, and the maximum geologic age probably is post-Early Ordovician. Twelve whole-rock samples of the volcanics were analyzed for rubidium, strontium, and strontium isotopic composition. The best estimate of the age of these twelve samples is 430 ± 15 m.y.; this is a maximum age or close to a maximum age for the Ordovician–Silurian boundary. This age plus the authors' results for the Silurian-Devonian boundary indicate that the Silurian Period in northeastern North America may have been shorter than generally has been estimated. Additional Silurian-age volcanics are being analyzed.

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