The U.S. Geological Survey has adopted an interim scheme for subdivision of Precambrian time. Four time units are established with boundaries at 800, 1,600, and 2,500 m.y. The units are given simple letter designations: Precambrian W (oldest), Precambrian X, Precambrian Y, and Precambrian Z (youngest). The scheme is intended for use within the United States; it is not a proposal for worldwide classification of the Precambrian.

The main guiding principles used are (1) that the purpose of subdivision is to provide the most convenient units for depiction and analysis of the Precambrian history of the United States, and (2) that the boundaries of the time units will not correspond intentionally to any defined geologic event, such as orogeny or plutonism--they are defined entirely by geochronologic numbers, chosen so as to split as few of the known episodes of sedimentation, orogeny, or plutonism as possible.