Geological field relationships and previously reported isotopic age determinations throughout the Appalachians provided evidence for a pattern of Lower Paleozoic tectogenesis. The regional isotopic age pattern indicates that intrusion of ultramafic and mafic igneous materials and major episodes of metamorphic recrystallization throughout the Appalachians were completed before the end of the Silurian Period.New K–Ar ages of metamorphic rocks from Vermont and adjacent regions in the Northern Appalachians are reported and confirm this pattern. Whole-rock ages of slates from the Taconic Klippe (460–445 m.y.) indicate that metamorphism and initial formation of slaty cleavage occurred during Middle to Late Ordovician times. Mica ages of Paleozoic schists exposed along the Green Mountain Anticlinorium (425–375 m.y.) are indicative of prolonged post-metamorphic uplift and cooling of this structure during the Silurian Period and Early Devonian Epoch. Local recrystallization and consequent loss of argon from the Taconic slates also occurred during this time. Ages obtained from a variety of metamorphic rocks from eastern Vermont fall in a narrow range (345 ± 7 m.y), indicating that renewed uplift and rapid cooling of this area, east of a line of ultramafic intrusions, occurred during Late Devonian time.Comparison of isotopic age data from the Appalachian and British Caledonian fold belts indicates that a broadly synchronous sequence of Lower Paleozoic tectonic and magmatic activity, lasting approximately 100 m.y., occurred in both regions.