It is a drama consisting of a prologue and three acts, complex characters, and no clear heroes or villains.
We, of course, know the final outcome, but we should not let that influence our appreciation of the story as it unfolds.
The third act sees the entry of a newly discovered set of physical laws—those governing radioactivity.Radioactivity offered not only a resolution to the puzzle of the earth’s energy supply but also a chronology independent of questionable geologic assumptions and a depth of time more than adequate for the processes of evolution.Florian Cajori, author of the 1908 article “The Age of the Sun and the Earth,” was a historian of science and, especially, of mathematics, and Ray Lankester, whom he quotes, was a zoologist. The first act consists in a direct attack, led by Lord Kelvin, on the extreme uniformitarianism of those such as Charles Lyell, who regarded the earth as indefinitely old and who, with great foresight (or great naivety, depending on your point of view: see the third installment of the 1900 “The Age of the Earth” article by W. Sollas), assumed that physical processes would eventually be discovered to power the great engine of erosion and uplift.The second act of the drama sees a prolonged attempt by a new generation of geologists to estimate the age of the earth from observational evidence, to come up with an answer that would satisfy the demands of newly dominant evolutionary thinking, and to reconcile this answer with the constraints imposed by thermodynamics.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.
That is the background to the intellectual drama being played out in this series of papers.
The third, which by the end was the most acute, presented a problem that outlasted the controversy itself.
Thus, when in 1919 Shapley stated that for him the radiometric timescale was fully established, he acknowledged that there was as yet no explanation for the sun’s energy. In 1920 Sir Arthur Eddington came up with the answer: the fusion of hydrogen into helium.) In reply to Lord Kelvin’s attacks, the geologists used two principal lines of reasoning.
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.
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.
He inferred that where the layers are not horizontal, they must have been tilted since their deposition and noted that different strata contain different kinds of fossil.