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were of course central to confirmation of the reality of the field reversals and this is seen by the much greater emphasis given to this situation in Table 4.1. The three cases of disagreement reported in the table refer to measurements made before the advent of magnetic cleaning techniques in paleomagnetic studies and are probably in error. Today the hypothesis of field reversal is so well accepted that baked contact studies are now used not so much as a check on the reality of field reversals but as a check on the age of the magnetization of igneous rocks (see §3.3.4). This is especially true in studies of old Precambrian rocks in which the importance of evidence relating to the age of the magnetization being measured is paramount.

Wilson (1962a) undertook a study into the magnetic record in a doubly-baked rock that was particularly convincing in the field-reversal versus self reversal debate. The situation is illustrated in Fig. 4.2. A lava flow, reversely magnetized, had heated an underlying laterite. The baked zone of the laterite had a direction of magnetization the same as that of the lava - reverse. Subsequently, both lava and baked laterite were intruded by a dike whose magnetization was also reverse

Fig. 4.2. Evidence for field reversal from the magnetic record in a doubly baked rock. A lava flow originally heated the laterite horizon. Subsequently a dike intruded both, reheating both lava flow and laterite in its adjacent region. In this region the laterite was thus first reheated by the lava and then reheated a second time by the dike. From Merrill and McElhinny (1983).

Fig. 4.2. Evidence for field reversal from the magnetic record in a doubly baked rock. A lava flow originally heated the laterite horizon. Subsequently a dike intruded both, reheating both lava flow and laterite in its adjacent region. In this region the laterite was thus first reheated by the lava and then reheated a second time by the dike. From Merrill and McElhinny (1983).

but whose direction differed by about 25° from that of the lava and baked laterite. A study of the zone of second heating of the laterite caused by the dike showed a gradual change from the dike direction, adjacent to the dike, to the lava direction at a distance where the second heating had produced no effect. Thus, in the same region both superimposed magnetizations were of reverse polarity. It is difficult to explain this observation by any reasonable mechanism other than that the Earth's magnetic field had changed polarity and was reverse during both heating episodes.

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