In the Eastern United States, Precambrian rocks are exposed in the Adirondack massif and in the Appalachian orogen.Rocks dated at 1,300-1,000 m.y.occur as outliers of the Grenville province of Canada.These rocks constitute a western basement to the Appalachian orogen and appear in the Adirondack massif, in anticlinoria along the Blue-Green-Long axis, and in gneiss domes farther east.The Chain Lakes massif in Maine, about 1,500 m.y.old, may represent a different block of continental crust.Rifting of the Grenville continental mass, accompanied by anorogenic igneous activity, began about 820 m.y.ago.An eastern basement (750-650 m.y.old) has been identified in the Avalonian zone of the Appalachian orogen in southeastern New England.It is intruded by calc-alkaline granitic rocks roughly 600-640 m.y.old that are nonconformably overlain by Lower and Middle Cambrian strata.Extensive Precambrian stratified rocks (mostly younger than 610 m.y.but possibly including rocks as old as 725 m.y.) crop out in the Carolina volcanic slate belt and probably in the higher grade metamorphic belt east of that.These rocks include large volumes of felsic volcanic rocks and probably were deposited on a continental crust.The eastern terranes were sutured to ancestral North America during the Appalachian orogenic events that closed the lapetus Ocean basin.The location of the suture is uncertain, but it probably lies just east of the gneiss domes containing rocks older than 1 b.y.'Synthesized stratigraphic columns to which the isotopic ages are tied are identified by circled capital letters on pi. 1. Related stratigraphic columns are indicated by subscripts.The generalized geographic locality of every constructed or synthesized stratigraphic column is shown on pi. 2 but because of space constraints not all are shown on pi. 1.