Standard techniques for analysis of U, Th, and Pb, including ion-exhange chemistry and mass spectrometry. Dissolution and iion-exchange chemistry were modified from Krogn (1973). Isotopic ratios were measured on a 12-in. NBS mass spectrometer with on-line digital processing. Lead was measured on a single rhenium filament, using H3PO4 and silica gell, and uranium and thorium were measured by surface emission, using a triple rhenium filament assembly.