Studies using radiometric age determinations on Precambrian rocks in the Appalachians have located rocks that are more than 1,000 m.y. old. A Rb-Sr whole-rock isochron study of granitic gneiss in the Manhattan Prong of southeastern New York shows that this rock is at least 1,350 m.y. old. Rb-Sr data also show that this granitic gneiss developed strontium isotopic homogenization over outcrop-sized volumes (10 m in diam) at about 1,100 m.y. ago.