Geology Reference
In-Depth Information
order of magnitude. The Earth was not less than a hundred mil-
lion years old as suggested by Shelton, but at least a thousand
million years old.
Unhappily, all radioactive procedures were su¬ering from the
damage inflicted on them by the helium method, which Holmes
had shown gave ages almost half those measured by the
uranium method, so it was against this background that he
stood up to defend his case for an ancient planet. Always polite,
he at first welcomed the opportunity to discuss the matter,
'But
find I cannot agree with Professor Shelton's generalisation that
the radioactive methods were not authoritative in their own
right. If that were the case, it would surely be necessary to
give up any serious attempt to construct a geological time-
scale'.
It obviously never entered his head that other people
might think it was not necessary to construct a time scale!
Advocating a positive approach to the helium set-back, he even
turned it round to have a dig at his opponents:
We must not moan over the apparent diffculties with which
the geologist has been faced since the advent of radium. If at
present some of our ideas are mutually incompatible, the
discrepancies do not demand a wholesale rejection of the
facts, but simply a re-interpretation of the fundamental
hypotheses on which so many of our doctrines seem to hang.
But his reasoning fell on deaf ears and his opponents con-
tinued to dismiss all radioactive results as simply too unreliable,
this time citing inconsistencies within the uranium-lead
method itself. Feeling more and more backed into a corner
Holmes replied:
When inconsistent ratios are obtained there is either evi-
dence of extreme alteration of the minerals or it has been con-
taminated due to the presence of 'ordinary' lead. However,
with the new knowledge of isotopes from atomic weight deter-
minations, corrections to anomalous results can now be made.
Shelton was dismissive:
I consider the problem lies in the unreliability of determining minute
