More than 100 independent isotopic ages have been determined for minerals from an area in northern Michigan about 35 miles square. Granites, pegmatites, and metamorphosed sedimentary and volcanic rocks have yielded Rb-Sr ages for feldspar, muscovite, and biotite, K-Ar ages for hornblende, muscovite, biotite, and feldspar, and U-Pb and Th-Pb ages for zircon. It was anticipated that we would learn from the measurements both the intrusive and metamorphic history of the area and would be able to place limits on the age of the Precambrian sediments in this area. The conclusions may be summarized as follows: (1) Granites and pegmatites with approximate ages of 2,700 and 1,900 m.y. have been found. (2) The major mineral-forming metamorphic event occurred between 1,800 and 2,000 m.y. ago. (3) The Precambrian sedimentary rocks called Animikie are older than 1, 900 m.y. and younger than 2,700 m.y. (4) Biotite Rb-Sr and K-Ar ages and muscovite K-Ar ages were strongly modified by the equivalent of a rise in temperature approximately 1,350 m.y. ago, although no mineral-forming event in this interval has been observed. (5) The K-Ar system was further affected by a thermal rise at about 1,100 m.y. This later event is probably recorded geologically by a few diabase dikes.