Nine whole-rock samples of the Skala (post-Ludlow, pre-Gedinnian) age, volcanic rocks from the Eastport Formation, Eastport, Maine, and nine whole-rock samples of Lower Devonian (New Scotland age) volcanic rocks from the Hedgehog formation, Presque Isle, Maine, were analyzed for their rubidium, strontium, and strontium isotopic composition. Analyses of the Eastport Formation and Hedgehog formation volcanic rocks produced, respectively, isochron ages of 412 ± 5 m.y. (λRb87 = 1.39 × 10−11 year−1) with an initial ratio of 0.707 ± 0.001and 413 ± 10 m.y. with an initial ratio of 0.706 ± 0.002. The combined results give an age of 413 ± 5 m.y. for the Silurian-Devonian boundary; this age is based on the average of the age values of the individual samples, and the precision assigned is two standard deviations of the mean of the individual ages. This age is a precise measurement of the Silurian-Devonian boundary in northeastern North America and is in reasonable agreement with recent time-scale estimates considering the uncertainty of the Rb87 decay constant.

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