Geology Reference
In-Depth Information
In the case of the Whin Sill it was the geological age that was
correct and the radiometric date that was seriously in error
because of the dating system used. However, at the time there
was no way of knowing this so William Urry wasted five pre-
cious research years working on a method that was never going
to produce accurate results. But science progresses like that -
it is often a gradual process of elimination that eventually
reveals the truth after many years of assiduous hard work.
Unfortunately, it is sometimes easy to get discouraged along the
way when results are not forthcoming, and by 1938, after the
failure of Urry's five-year e¬ort to develop a scale based on his
extremely disappointing helium results, no university anywhere
in the world retained a programme designed to build a geolog-
ical time scale.
The di~culty was still due to the same things - ages could
still only be obtained from igneous rocks that contained unusu-
ally high amounts of lead, in itself a problem due to the scarcity
of these rocks, and when rocks suitable for radiometric dating
were found, poor geological control meant uncertainty as to
what geological age they were. The nature of igneous rocks is
that they tend to cut across or intrude into the very sedimentary
rocks that contain the fossils used for determining the geolog-
ical age of the igneous rocks that cut them. The arguments
become somewhat circular. As a result very often all that could
be said of the geological age of an igneous rock that intruded a
sedimentary rock was that the igneous rock must be the
younger.
But there was light at the end of the tunnel, and once again
it was a physicist who was holding the torch. With the recogni-
tion by Rutherford back in 1929 that there were
two
isotopes of
uranium that each decayed to a di¬erent lead isotope it was soon
realised that the relationship between uranium and lead
contained not one, but
two
geological clocks that could be used
to verify each other - the decay of uranium 238 to lead 206, and
the decay of uranium 235 to lead 207. Not only that, but because
