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By 1788 Hutton had formulated a theory of cyclic deposition and uplift, with the earth indefinitely old, showing “no vestige of a beginning—no prospect of an end.” Hutton considered the present to be the key to the past, with geologic processes driven by the same forces as those we can see at work today.

This position came to be known as uniformitarianism, but within it we must distinguish between uniformity of natural law (which nearly all of us would accept) and the increasingly questionable assumptions of uniformity of process, uniformity of rate and uniformity of outcome.

Having come that far, they were initially quite reluctant to accept a further expansion of the geologic timescale by a factor of 10 or more.

And we should resist the temptation to blame them for their resistance. Different methods of measurement (such as the decay of uranium to helium versus its decay to lead) sometimes gave discordant values, and almost a decade passed between the first use of radiometric dating and the discovery of isotopes, let alone the working out of the three separate major decay chains in nature.

Robert Hooke, not long after, suggested that the fossil record would form the basis for a chronology that would “far antedate ...

even the very pyramids.” The 18th century saw the spread of canal building, which led to the discovery of strata correlated over great distances, and James Hutton’s recognition that unconformities between successive layers implied that deposition had been interrupted by enormously long periods of tilt and erosion.

’s free newsletters."data-newsletterpromo-image="https://static.scientificamerican.com/sciam/cache/file/458BF87F-514B-44EE-B87F5D531772CF83_source.png"data-newsletterpromo-button-text="Sign Up"data-newsletterpromo-button-link="https:// origincode=2018_sciam_Article Promo_Newsletter Sign Up"name="article Body" itemprop="article Body" Aristotle thought the earth had existed eternally.

Roman poet Lucretius, intellectual heir to the Greek atomists, believed its formation must have been relatively recent, given that there were no records going back beyond the Trojan War.The constancy of radioactive decay rates was regarded as an independent and questionable assumption because it was not known—and could not be known until the development of modern quantum mechanics—that these rates were fixed by the fundamental constants of physics.It was not until 1926, when (under the influence of Arthur Holmes, whose name recurs throughout this story) the National Academy of Sciences adopted the radiometric timescale, that we can regard the controversy as finally resolved.The third referred to the heat of the sun, particularly the rate at which such heat is being lost, compared with the total amount of energy initially available.The first argument was completely undermined after taking into account the amount of heat generated by radioactive decay.One outstanding feature of this drama is the role played by those who themselves were not, or not exclusively, geologists.

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