Geology Reference
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fossil-bearing rocks. In this way many basalts acquire well-
defined fossilised brackets below and above the flows which can
be used to assign a quite precise geological age to the basalt.
Working at the Massachusetts Institute for Technology
(MIT), Urry took nearly four years to analyse the thirty-nine sam-
ples initially selected, but when Holmes finally saw the results
he was over-joyed. He was just writing the third edition of his
book on The Age of the Earth and in it he proudly compared Urry's
helium ages with established uranium-lead ages. The corres-
pondence was remarkable. Another wild miracle seemed to have
occurred. Holmes wrote euphorically in his book:
. . . it is remarkable how consistently the age estimates fall
into appropriate positions. That this stringent test of internal
consistency is satisfactorily met must be regarded as the final
proof that the ages calculated from lead and helium ratios are
at least of the right order and that no serious error is any-
where involved.
Remember three steps forward, one step back? This time it
was two steps back. As Holmes' book was going to press a prob-
lem was noticed by an assistant professor of physics, Robley
Evans, who had recently come to MIT to develop yet another new
method for determining helium ages. When Evans' new tech-
nique was su~ciently well established and giving consistent
results, ages were determined on some of the same samples that
Urry had used and the two sets were compared. Urry's ages were
found to be significantly higher than those determined by
Evans. After exhaustive investigations the problem was traced
to an error in Urry's equipment which resulted in all his ages
being consistently too high, and fortuitously in line with the
uranium-lead ages. It was a devastating moment as they realised
that five years of hard work would have to be discarded, and it
was back to the drawing board. Helium was still leaking, even
from these barely radioactive samples. Evans summed up the
position: 'These general inconsistencies in helium age ratios
indicated quite clearly that there was some fundamental failure
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