Geology Reference
In-Depth Information
resulting in values that were much too low. But the criticism of
Joly was done in a gentlemanly manner and Holmes even ticked
o¬ another reviewer for having been discourteous to Joly - not
everyone was as patient as Holmes.
At the other end of the scale Henry Russell, an American
Professor of Astronomy, had determined that the age of the
Earth lay between 2000 and 8000 million years by assuming that
the total amount of lead in the Earth's crust was all produced
from the decay of uranium. Clearly this was not so. Revising
Russell's figures with what Holmes considered to be more real-
istic values for the amount of lead and uranium in the Earth's
crust, Holmes brought Russell's age down to 3200 million but
then himself turned reactionary. He stated emphatically
'it is
clear that the Earth . . . cannot have existed for so long as
3,200 million years'
because, he argued, all the ages from the
oldest minerals so far dated fell largely in the 1000-1100 million
years bracket. The oldest was only 1525 million years, therefore
'the frequently quoted age of the Earth, 1600 million years,
thus appears to be of the right order.'
It was problems like those posed by Joly at one end of the
scale and Russell at the other end that caused Holmes to turn
once again to his dream of constructing a geological time scale.
If only radiometric dates could be determined on common rocks
of a known geological age then all this uncertainty would dis-
appear. Once a framework was established it would only be
necessary to fill in the gaps as more and more data became
available. But how to do it? What technique would prove reliable
and easy enough to become routine? Minerals in the common
rocks contained so little uranium that chemical determination
of the tiny amounts of lead produced by radioactive decay was
impossible. But even if it had been possible, the necessity for
atomic weight determinations to then distinguish between the
various lead isotopes was still so laborious that it was a major
deterrent to the progress of uranium-lead dating. The method
continued to languish.
