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Part 1: Introduction |
Department of Geology and Mineralogy, Parks Road, Oxford
The great age of the Earth was first scientifically argued by Hutton in the eighteenth century. Next, a world-wide time-order scale of geological events was established using the biostratigraphic methods of William Smith. In the second half of the last century ‘hour-glass’ methods, based on salinity of the oceans and the maximum thickness of sediment, provided a framework for a rough time-scale, measured in years. The development of a reasonably detailed and satisfactory time-scale for geology is, however, an achievement of the present century and has been accomplished by using as an hour-glass method the rate of radioactive decay of certain elements present in rocks and minerals of known position in the time-order scale.
In 1913, from tentative radiometrically established ages, Holmes produced a quantitative Phanerozoic time-scale, and he and others have gradually refined it. In linking the isolated dates obtained by radiometric methods with the biostratigraphic time-scale, use has usually also been made of the maximum thicknesses of sediments accumulated in the various intervals of the time-order scale, and this is believed to be still the most useful method.
