A number of notable contributions to our knowledge of long-period fluctuations of the geomagnetic field have been made since 1967 through studies of the paleomagnetism of igneous and sedimentary rocks. The highlights of this research include the following: (1) A more accurate radiometric time scale of geomagnetic reversals has been developed for the past 5 m.y., with the result that during this time all polarity intervals with durations longer than 50,000 years have probably been recognized. (2) An intensive study of deep-sea sedimentary cores has served to define this time scale more accurately and to extend it back in time well beyond 5 m.y. (3) Marine magnetic anomalies have been analyzed by using the theory of sea-floor spreading to yield a reversal time scale extending back 70 m.y. (4) Long-period geomagnetic secular variation within individual polarity intervals has been investigated paleomagnetically by studying paleointensities and by measuring variations in angular dispersion of directions of the earth's field as a function of latitude. In the present report all these subjects are reviewed with the exception of marine magnetic anomalies, which are discussed in a separate report. In addition to the references cited specifically in the text, numerous important papers relevant to these subjects are listed in the bibliography.
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