The 238U-234U-230Th-232Th system has been investigated in 84 silicic crystalline rocks obtained from drill cores, surface, and near-surface samples in California, Wyoming, Colorado, and Illinois. Results of these analyses displayed on ternary diagrams with apexes for 238U, 234U, and 230Th indicate five predominant geochemical processes that affected uranium in the rock: (1) bulk uranium leaching where 238U and 234U were removed with little or no fractionation; (2) preferential 234U leaching by alpha recoil displacement (234U recoil loss) with lesser 238U loss; (3) 234U recoil loss with little or no 238U loss; (4) uranium assimilation where both 238U and 234U were added with present-day 234U/238U activity ratios varying from 0.8 to 1.2; and (5) addition of 234U and 230Th by daughter emplacement processes (234U+230Th recoil gain). Evidence for the existence of 234U and 230Th recoil gain in rocks is the most important finding of this investigation. Radioactive disequilibrium occurs in the majority of rocks analyzed where 234U recoil loss is the predominant process associated with incipient weathering; U assimilation and 234U+230Th recoil gain occur under conditions of substantial water penetration along fractures and into weathered zones in the rocks. Relatively unfractured and petrographically fresh rocks from the UPH-3 drill hole in northern Illinois are closest to being in radioactive equilibrium for any suite of rocks included in this study and they demonstrate that equilibrium during the last 0.5 m.y. can be maintained over a substantial vertical distance where there has been little or no movement of water in the basement rock. The 238U-234U-230Th system can be a sensitive indicator of geologically recent U mobility and rock/water interaction both in petrographically fresh core samples and in 'sealed' fracture zones.