Age Studies of Zircon and Feldspar Concentrates from the Franconia Sandstone

We were unable to use unequivocally the convergence idea of Pettijohn and Barth for a determination of the time of onset of significant continent formation. Analyses of detrital zircon mixtures indicate that some of the oldest zircons have ages of about 3,200 m.y. Chronometric data in peripheral regions of the North American continent suggest that the intensity of continent-forming processes diminished greatly after the Grenville. The continents appear to have been formed mainly during a 2-b.y. interval occurring in the middle of the earth's 4 1/2-b.y. existence. Isotope data were obtained for authigenic orthoclase rims enclosing detrital K-feldspar grains and they require that the rims form in partially or nearly closed systems, thus favoring epigenetic, non-hydrothermal modes of formation.