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The pattern of geomagnetic reversals for the past 150 m.y. has been well established from potassium-argon and polarity measurements on lava sequences, polarity measurements on continuous sequences in deep-sea sediment cores, and the interpretation of marine magnetic anomalies. Interest in the wider use of magnetostratigraphic methods has increased through the development of the extremely sensitive SQUID magnetometer, capable of fast measurement of virtually all rock types. The frequency of reversals appears generally to have been lower in pre-Cenozoic times when the magnetic field exhibited a cyclic behavior in polarity bias. Analysis of land-based paleomagnetic data suggests the existence of quiet (no reversals) and disturbed (many reversals) intervals throughout the Phanerozoic. The delineation of the quiet intervals offers the most encouraging prospect for their use in intercontinental correlation. The discovery of a quiet interval just below the PrecambrianCambrian boundary suggests a possible method for arriving at a definition of this boundary.

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