Geochronology - Four Revolutions in the Earth Sciences - page 41

After evaluating Boltwood's uranium-lead ages and his own measurements,
which included an age of 370 million years for an igneous rock from Norway,
Holmes concluded that “wherever the geological evidence is clear, it is in agree-
ment with that derived from lead as an index of age. Where it is obscure . . . the
evidence does not, at least, contradict the ages put forward.” 14
In 1913 Holmes published the first of several topics, this one titled The Age of
the Earth . After carefully reviewing the debates of the preceding fifty years, he
consigned Kelvin's calculations to the dustbin of history:
With these discoveries the long controversy was finally buried, and Kelvin's treatment of the
problem was proved to have been fallacious. The discovery of radium did not only destroy the
validity of the older thermal arguments; but also, it led directly to the elaboration of a new and
more refined method . . . every radioactive mineral can be regarded as a chronometer register-
ing its own age with exquisite accuracy. Indeed, if our interpretation is correct, some of the
oldest Archean rocks must date back 1600 million years. 15
Holmes then turned to Joly's salt clock, noting that it incorporated two funda-
mental and unprovable assumptions: “That all the sodium liberated from igneous
rocks is contained in the ocean, and that all the sodium carried annually to the
ocean has been liberated from such rocks for the first time.” 16 Holmes wrote that
for Joly's estimates to tally up, igneous and sedimentary rocks “would be obliged
to lose nearly twice as much sodium as they actually contain!” (72). He concluded
that deductions using the sodium method “must be regarded as being purely provi-
sional” (75). In 1926, Holmes would go further, noting that “many geologists have
rejected [the sodium method] as worthless.” 17
The final chapter of the 1913 topic set out the issue:
Of the various methods which have been devised to solve the problem of the earth's age, only
two, the geological and the radioactive, have successfully withstood the force of destructive
criticism. From the mists of controversy which for half a century have hung over the sub-
ject, the two hour-glass methods alone emerge, and the final issue must be fought out between
them. 18
Holmes regretted that
many geologists feel it impossible to accept what they consider the excessive periods of time
which seem to be inferred. That there exists a serious discrepancy obviously points to a flaw
in the underlying assumptions of one or the other or both of the methods. The fundamental
assumptions on which the arguments are based cannot both be right. One of them must be re-
jected.
Uniformity . . . is involved equally in both calculations. If we favour the uniformity of geo-
logical processes—a well-worn doctrine which has done good service—then we must reject
uniformity of radioactive disintegration. 19