Geochronology

In summary, it now seems possible to obtain accurate ages for many of the common rock types. When rocks have been affected by metamorphic episodes after the time of crystallization, it is possible to recognize this from the age values and, in some cases, to date both the time of crystallization and the time of metamorphism. Work is under way that will make it possible to draw accurate geochronological maps of the continents. An impressive regularity is already evident in the map for North America. Knowledge of the temporal and areal distribution of ages should have an important bearing on theories of the origin of continents and of mountain-building processes (50).