Radiometric dating of basement rocks in South America by the M.I.T. geochronology laboratory has indicated the following thermo-tectonic episodes. The gneisses of the Imataca Complex of the Venezuela Guayana Shield are about 3000 m.y.; they have been intruded by igneous rocks of 2000 m.y. age. Ancient rocks between these age values have been found in the Bação Complex in Minas Gerais. The principal cratonic rocks of southeastern Venezuela, the Guianas, northern Brazil in the Amapa district, the coast region between Belém and São Luis, the region west of the Tocantins River, and in the São Francisco area near Salvador all show ages in the range 2000 ± m.y. The region in northeast Brazil represented by the Cariri orogenic belt shows an average Rb–Sr whole-rock isochron age value of 640 m.y. K–Ar age values in the Belo Horizonte district, Minas Gerais, show a metamoprhic overprint value of 500 m.y. A similar value is found in Colombia, east of the Macarena mountains.