Abstract
About half the rock samples representing earlier stages in the history of Earth's crust are reversely magnetized (Jacobs 1984, pp. 47-48), i.e., the molecular-domain-sized "compass needles" in them point southward, rather than northward as they would if unrestrained at the present time. The surprising implication is that the geomagnetic field has reversed at some time, or times, in the past. A sequence of 26 reversals has been recognized for rocks extending from the Upper Miocene to the present, representing only 5.5 million years of the conventional Phanerozoic spread of nearly 600 million years from the lowest metazoan fossils to modern conditions (Merrill and McElhinny 1983, p. 140).