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This paper sketches out developments in North American paleomagnetism that have markedly affected geological thought, to the exclusion of those related mostly to the past character and properties of the earth’s magnetic field or to magnetic properties of rocks. Early work (1948) at the Carnegie Institution’s Department of Terrestrial Magnetism was mainly directed, as was also that of P.M.S. Blackett in England, at elucidating the past behavior of the geomagnetic field and its origin; proving or disprov-ing continental drift was not a primary objective, and the concept of sea-floor spreading was, of course, still unheard of. The first surprise came with the discovery of numerous reversely magnetized igneous and sedimentary rocks. It took about 10 years to establish that most of these reversed rocks reflected field reversals rather than self-reversal con-tingent on mineralogical composition. The development of a magnetic time scale based on the chronology of field reversals was of paramount geological significance as it led to confirmation of the Vine-Matthews hypothesis and to plate tectonics; it is now widely applied to worldwide stratigraphie correlations. Unexpected “abnormal” directions of magnetization in pre-Tertiary rocks were first interpreted in terms of polar wander, or, in the case of deformed rocks, in terms of magnetostriction and stress-induced anisotropy; they became accepted by British paleomagnetists as evidence for continental drift a few years before their American colleagues reached the same conclusion. Paleomagnetism has further affected geological thinking by establishing, through determination of the earth’s paleoradius, the improbability of Carey’s expansionist views and, most

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