Geology Reference
In-Depth Information
Unknown to them at the time was the fact that uranium itself
also has three isotopes, and although more than 99% is uranium
238, uranium 235 is also significant because it decays much faster
than 238. And yes, the end product of uranium 235 is yet
another stable isotope of lead! So, when the Viennese team were
measuring what they thought was 'ordinary' lead, it turns out that
what they were actually measuring was the lead produced from
the decay of uranium 235, which had not yet been 'discovered'.
Thus when Holmes corrected his uranium-lead ages for the pres-
ence of thorium-derived lead and what he thought was 'ordinary'
lead, in fact the 'ordinary' lead was still unaccounted for.
And so science progresses. Three steps forward, one step
back, but each little bit adds to the bigger picture. With this new
understanding of lead and its isotopes, and the ability to adjust
previously determined ages to apparently account for the pres-
ence of 'ordinary' lead, the oldest rocks from Mozambique were
now judged to be 1500 million years old. And because the Earth
had to be older than the oldest rock it contained, the age of the
Earth was deemed to be at least 1600 million years. But not
everyone was convinced.
In March 1915 Henry Shelton, school teacher and journalist
on 'scientific and philosophical subjects', gave a talk to the
Geological Society which examined 'The Radioactive Methods
of Determining Geological Time'. Shelton was clearly not a
convert and as a consequence, hardly an authority on the sub-
ject. Phrases such as '
The attempt to assess exact, or even approximate
times by means of lead ratios is premature
and
entirely invalid'
and
'radioactive experiments . . . are not sufficiently important in themselves
to be authoritative against the balance of the evidence derived from other
lines of investigation
' were bound to antagonise Holmes. Although
generally a quiet, reserved and rather shy individual, Holmes
never had any compunction when it came to standing up and
defending his cause. With his deep knowledge of the subject he
was driven by a conviction that no matter what ever anyone else
said, the ages he had determined were at least within the right
