The ages of biotite, muscovite, and potassium feldspar in granitic rocks from 27 localities in Nova Scotia, as determined by A/K and Sr/Rb methods, fall in the range 300 to 400 m.y. At two localities, where nearby intrusive contacts with upper Lower Devonian fossiliferous sedimentary rocks are established, the best estimate of the age of the intrusive rocks is 365 ± 20 m.y. Biotite and potassium feldspar in granite from Boisdale Hills (Cape Breton Island), associated with nearby Middle Cambrian sedimentary rocks, have an estimated age of 500 ± 20 m.y. A single age analysis of biotite in the metamorphosed Meguma series gives 340 m.y. Radiation-damage ratios for zircon at nine localities where the age has been determined show one concordant result and eight which are lower than the A/K and Sr/Rb ages by 16 to 33 per cent.

The evidence of the mineral ages in the granitic rocks investigated thus far in Nova Scotia strongly suggests a protracted period of intrusion in the Devonian, with the upper Lower Devonian older than 365 m.y. Middle Cambrian is tentatively believed to be older than 500 m.y.

Holmes' B time scale, with Cambrian limits of 430 and 510 m.y. and Devonian limits of 255 and 313 m.y., would require an extension of 10-25 per cent to meet the Nova Scotia data.

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