Geochronology - Four Revolutions in the Earth Sciences - page 42

After reviewing the claims of Joly and others, Holmes came to his conclusion:
no independent evidence exists to show that the rates of radioactive decay have
varied. Therefore, “the discordance between the time-estimates drawn from the
rates of geological and radioactive changes cannot be held to constitute a sufficient
reason for rejecting current opinions unless it is conclusively demonstrated that the
geological estimates are beyond question.” 20 He then went on to show, as Barrell
would reiterate a few years later, that the geological estimates were not beyond
question. Indeed, since “the modern hour-glass [of erosion] is running at two-and-
a-half to four times its average rate,” those estimates from geological uniformity
are too low by the same factor. “In the geological evidence,” Holmes concluded,
“there is nothing impossibly at variance with the dictates of the radioactive miner-
als” (176).
Billions, Not Millions
Soon after the First World War, the British Association and the American Philo-
sophical Society held conferences to consider the age of the Earth. By this time,
scientists had measured many ages in the hundreds of millions of years and some
in the billions. Both conferences concluded that the Earth is from 1.3 to 1.5 bil-
lion years old. In the second edition of his topic, published in 1927, Holmes parted
company with the geological hourglass methods. They are “incapable of providing
exact results because the assumption of uniform rates [of erosion] throughout the
past cannot be granted,” he wrote. “If the present rates are five times greater than
the average, then geological time must be of the order of 1,500 million years.” 21
In 1926, the U.S. National Research Council, an arm of the National Academy
of Sciences, convened a large committee to prepare a report titled Physics of the
Earth . The purpose was to “give the reader, presumably a scientist but not a spe-
cialist in the subject, an idea of its present status together with a forward-looking
summary of its outstanding problems.” The subcommittee on the age of the Earth
included a physicist, an astronomer, and four geologists, one of whom was Ar-
thur Holmes. It produced a 1931 topic of 487 pages, over two-thirds of them writ-
ten by Holmes in a section—really a topic in its own right—titled “Radioactivity
and Geologic Time.” 22 The most striking thing to one reading this section today is
what a wealth of knowledge scientists had produced in the twenty-five years since
Rutherford had pulled that specimen of pitchblende from his pocket.
In one section of his chapter, Holmes calculated the age of the Earth assuming
that all the lead in average igneous rocks had come from radioactive decay; in oth-
er words, that none was original. This computation gave 3,000 million years as the