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A time-scale for polarity reversals of the geomagnetic field has been established for the last 4.1 m.y., using the potassium-argon method of dating rocks. The data for this time-scale consist of polarity- and age-determinations for about 240 rocks made by a number of workers; these are summarized in tabular form. At least 17 geomagnetic reversals occurred during the period covered, and intervals of constant polarity ranged in length from 700 000 years to about 50 000 years or possibly less. Two possible correlations of this time-scale with the Pliocene-Pleistocene boundary are discussed. In several parts of the world glacial deposits have been shown to be as old as 3 m.y., in some cases by application of the reversal time-scale either on land or in deep-sea sediment cores.
