Geology Reference
In-Depth Information
tude, and that the age of the Earth was to be measured in
thousands rather than hundreds of millions of years. It was now
up to exponents of the hour-glass methods to reconcile their
ages with the radiometric dates, rather than the other way
round. However, there were still many problems and a few
individuals who persisted in sticking to the old methods. John
Joly was one.
In 1925 Joly published a book on
The Geological Age of the Earth
,
which favoured an age of between 160 and 240 million years to
the Base Cambrian. This he deduced from his own method for
measuring the age of formation of the oceans, which he still cal-
culated to be 80-100 million years, and which he now supported
by radiometric dates determined from thorium-bearing minerals
rather than uranium minerals. Having been forced to abandon
the helium dates he had once so favoured, and having lost the
argument that uranium had decayed more rapidly in the geo-
logical past (and therefore the Earth was not as old as uranium
dates indicated), he now homed in on thorium dates because
they were often lower than those determined from uranium and
so gave ages more in keeping with his age of the oceans.
Although Joly was an established and well respected member of
the geological community, Holmes did not hesitate to show him
the error of his ways. In a polite but damning review of Joly's
book Holmes again disposed of the sodium method - as he had
done more than twelve years previously - by reviewing the evi-
dence, or lack of it, and stating with some authority that:
'At
least it is certain that the sodium method cannot at present
be considered as making any serious contributions to the
problem'.
He then went on to show how inconsistent the thorium
results were. Unlike those from uranium which showed a
consistent correlation between lead and uranium, thorium-lead
ratios were all over the place. Holmes thought that some of the
lead isotope resulting from the decay of thorium must be
escaping and, like the helium results two decades earlier, were
