Geology Reference
In-Depth Information
considered to be highly probable that the earth was originally
gaseous and that the period of consolidation, up to the time
of formation of a solid crust
[Holmes called this the granitic
layer],
was relatively short. Jeffreys, for example, estimates
that “the earth probably became solid within 15,000 years of
its ejection from the sun”. Even if this estimate were wrong
by a factor of a thousand, the age of the granitic layer would
not be appreciably different from the age of the earth
[in fact
it would be equivalent to the 'odd fifteen' he had suggested they
neglect when writing to Nier!].
Accordingly the age I have
determined refers to the time when the granitic layer sepa-
rated from average earth material during the consolidation of
the globe . . . and that, for all practical purposes, is indistin-
guishable from the age of the earth.'
Having thus established the age of the Earth by assuming the
Ivigtut galena to represent primeval lead, Holmes had then
projected his ideas backwards in time to a point at which
uranium
'first began to deteriorate'
and when the lead ratios
were essentially set to zero. He deduced this
'age of uranium'
to be 4460 million years ago. By this reckoning, if the age of the
Earth was 3000 million years, then the 'short' period Holmes
mentions between consolidation from the gas cloud and for-
mation of the granitic crust must have been preceded by a very
'long' period of more than a billion years while uranium decayed
within the gas cloud, a fact that does not seem to have inter-
ested him at all. But, as Kuenen pointed out, concepts regard-
ing the birth of the solar system were 'rather vague', so Holmes
did not concern himself unduly with events that occurred before
the Earth consolidated. Ten years later he must have kicked him-
self when he realised just how close he had got to the right
answer for the age of the Earth, which he had interpreted as 'the
age of uranium'.
A few months after making these initial calculations, with his
Marchant calculating machine red hot from a revised estimate
based on no fewer than 1419 solutions for
'the time since the
