The lower Paleozoic Browns Mountain Group of volcanic and sedimentary rocks underlies much of the Antigonish Highlands on the northern mainland of Nova Scotia. The rocks are apparently unfossiliferous and pre-Lower Silurian in age. Volcanic rocks belonging to the Keppoch Formation give a Rb–Sr whole-rock isochron age of 528 ± 40 m.y.; the indicated value for the initial ratio 87Sr/86Sr is 0.7032 ± 0.0020. The apparent stratigraphic age of the lower part of the Browns Mountain Group then is Cambrian with a middle Cambrian age favored.