Geology Reference
In-Depth Information
present day values - which would essentially give the age of the
Earth.
Unfortunately, Nier considered that the majority of his
twenty-five samples contained a mixture of both primeval lead
and radiogenic lead, the latter having contaminated the
fossilised primeval value as uranium and thorium continued to
decay within the Earth's crust. But there was one sample, a
very ancient galena (lead ore) from Ivigtut in Greenland (where
some of the oldest rocks in the world are now found), that
contained very low lead ratios and no uranium or thorium.
These factors suggested that it might just be a relict of primeval
lead that had become trapped in the Greenland rocks at the same
time as formation of the Earth. So, making the assumption that
the Ivigtut galena did indeed represent primeval lead, and
using Nier's other twenty-four samples to represent the isotopic
constitution of lead as is was at various ages throughout
Earth history, Holmes once again set out to date the age of the
Earth.
The calculations were complex and immensely time-
consuming to do by hand, so he applied to the University for a
grant with which to purchase a Marchant calculating machine.
A whole year passed before it was sitting on his desk. Silent and
speedy, it was £74 8/- well spent!
On the 16th of February 1946 Holmes wrote again to Nier to
tell him some exciting news:
Ever since your isotopic analyses of ore-leads was pub-
lished I have hoped that it would be possible to calculate from
the results the time that has elapsed since the Earth's primeval
lead began to be contaminated by radiogenic lead. The acqui-
sition of a calculating machine a few months ago has now
made possible the somewhat formidable calculations and I
have just completed the work. The age works out at about
3,000 million years by various sets of solutions . . . the aver-
age of the best set of solutions being 3015. We can however,
afford to neglect the odd 15! This looks like being the first
