Editing Age of Earth (section)

===Arthur Holmes establishes radiometric dating===
Although Boltwood published his paper in a prominent geological journal, the geological community had little interest in radioactivity.{{citation needed|date=March 2015}} Boltwood gave up work on radiometric dating and went on to investigate other decay series. Rutherford remained mildly curious about the issue of the age of Earth but did little work on it.

[[Robert Strutt, 4th Baron Rayleigh|Robert Strutt]] tinkered with Rutherford's helium method until 1910 and then ceased. However, Strutt's student [[Arthur Holmes]] became interested in radiometric dating and continued to work on it after everyone else had given up. Holmes focused on lead dating because he regarded the helium method as unpromising. He performed measurements on rock samples and concluded in 1911 that the oldest (a sample from [[Ceylon]]) was about 1.6 billion years old.<ref>Dalrymple (1994) p. 74</ref> These calculations were not particularly trustworthy. For example, he assumed that the samples had contained only uranium and no lead when they were formed.

More important research was published in 1913. It showed that elements generally exist in multiple variants with different masses, or "[[isotope]]s". In the 1930s, isotopes would be shown to have nuclei with differing numbers of the neutral particles known as "[[neutrons]]". In that same year, other research was published establishing the rules for radioactive decay, allowing more precise identification of decay series.

Many geologists felt these new discoveries made radiometric dating so complicated as to be worthless.{{citation needed|date=March 2015}} Holmes felt that they gave him tools to improve his techniques, and he plodded ahead with his research, publishing before and after the First World War. His work was generally ignored until the 1920s, though in 1917 [[Joseph Barrell]], a professor of geology at Yale, redrew geological history as it was understood at the time to conform to Holmes's findings in radiometric dating. Barrell's research determined that the layers of strata had not all been laid down at the same rate, and so current rates of geological change could not be used to provide accurate timelines of the history of Earth.{{citation needed|date=March 2015}}

Holmes' persistence finally began to pay off in 1921, when the speakers at the yearly meeting of the [[British Association for the Advancement of Science]] came to a rough consensus that Earth was a few billion years old and that radiometric dating was credible. Holmes published ''The Age of the Earth, an Introduction to Geological Ideas'' in 1927 in which he presented a range of 1.6 to 3.0 billion years. No great push to embrace radiometric dating followed, however, and the die-hards in the geological community stubbornly resisted. They had never cared for attempts by physicists to intrude in their domain, and had successfully ignored them so far.<ref>[http://www.es.ucsc.edu/~rcoe/eart206/Badash_AgeEarthDebate_SciAmer89.pdf The Age of the Earth Debate Badash, L ''Scientific American'' 1989 esp p95] {{webarchive|url=https://web.archive.org/web/20161105173355/http://www.es.ucsc.edu/~rcoe/eart206/Badash_AgeEarthDebate_SciAmer89.pdf |date=2016-11-05 }}</ref> The growing weight of evidence finally tilted the balance in 1931, when the [[United States National Research Council|National Research Council]] of the US [[United States National Academy of Sciences|National Academy of Sciences]] decided to resolve the question of the age of Earth by appointing a committee to investigate. 

Holmes, being one of the few people who was trained in radiometric dating techniques, was a committee member and in fact wrote most of the final report.<ref name="Dal7778">Dalrymple (1994) pp. 77–78</ref> Thus, Holmes' report concluded that radioactive dating was the only reliable means of pinning down a [[geologic time scale]]. Questions of bias were deflected by the great and exacting detail of the report. It described the methods used, the care with which measurements were made, and their error bars and limitations.{{citation needed|date=March 2015}}