Geochronology - Four Revolutions in the Earth Sciences - page 38

Assumptions
In a lengthy 1911 article titled “The Age of the Earth,” Joly correctly stressed that
the hourglasses of the geologists were not the only ones to require assumptions:
so did the hourglass of radioactivity. He identified three suppositions on which the
new methods depended. 6 First was that at the time the mineral being investigated
had formed, it had contained neither helium nor lead. If a mineral had contained
original daughter atoms, it would appear older than its true age. This possibility
suited Joly since the ages inferred from radioactive decay were older than those
from his salt clock. Second, the methods based on radioactivity assumed that the
specimen had neither gained nor lost daughter atoms, which in either case would
have rendered the calculated ages meaningless. Again, because salt is removed
from the ocean and deposited in beds, the salt clock suffered from the same prob-
lem. The third assumption was that the rate of decay of the parent element—its
half-life—remains constant. Joly took pains to avoid suggesting that changes in
heat and pressure might have altered the rate of decay, no doubt aware that both
Madame CurieandRutherfordhadtriedbutfailed toinducesuchchanges.Instead,
Joly wondered whether variations in the rate of decay might have arisen “intrins-
ically, ultimately due possibly to conditions of origin.” The decay of uranium, he
wrote, “150,000,000 years ago may have been many times what it is now.” 7 Given
thenumberofsurprisingfindingsinchemistryandphysicsthathadfollowedRönt-
gen's discovery and how little was known about the atom at the time, who could
have said that Joly's skepticism was unjustified?
Hisfundamentalpointwasthatiftheagesofseveralhundredmillionyears,even
billions, as measured from radioactivity are correct, the hourglasses of sedimenta-
tion and salt would have to be off by a factor of ten or more. Since he and the other
hourglass calculators did not see room for an error of that size in their estimates
of sediment accumulation, the mistake would have to lie in their assumed rate of
sedimentation. But it seemed absurd that the estimated rates could be wrong by a
factor of ten.
Having spent a decade refining his figures and adopting improvements sugges-
ted by others, Joly found it impossible that the ages calculated from geological
uniformity and his salt clock could be so far off. Even though he trained as a phys-
icist, not a geologist, and even though he understood radioactivity well enough to
found the Irish Radium Institute in 1914, Joly evidently saw no good reason why
geology should once again submit to physics.
Even before Joly had voiced his doubts, George F. Becker (1847-1919) of the
U.S. Geological Survey anticipated them. In a 1908 paper Becker redid the calcu-