The Bright Angel group of the southern Great Basin region is defined as consisting of the predominantly argillaceous strata which lie between the underlying Prospect Mountain quartzite and overlying Middle Cambrian limestones. This lithogenetic unit is shown to range in age from partly pre-Cambrian in the Nopah Range of southeastern California to entirely Middle Cambrian in the Grand Canyon of Arizona. The Birght Angel group illustrates the fact that the problems of stratigraphic classification are four-dimensional and, as such, are not amenable to treatment by the conventional dual system of stratigraphic nomenclature.
The fact that rock units and unconformities may vary in age from place to place is determined as the sole factor demanding (1) a three-fold nomenclatural system, and (2) abandonment of the concept that erosional breaks may serve as time-stratigraphic boundaries. This variation in age of lithogenetic units is recognized as a fundamental truth in stratigraphy, nearly equal in significance to the laws of superposition and faunal succession, and is appropriately designated therefore as the principle of temporal transgression.