The walls of St Catherine's Church are slightly pink. That's because they are made of rocks of Eskdale granite which has a lot of feldspar in it. It's called the Eskdale pluton. Pluto was the god of the underworld and this describes the way the molten rock intrudes from below, never reaching the surface but eventually solidifying. I looked up information on the rock
http://jncc.defra.gov.uk/pdf/gcrdb/GCRsiteaccount297.pdf and was interested to read about the method used to date it. The rock contains crystals of zircon, which is zirconium silicate. This can trap uranium atoms. There are two possible ways for isotopes of uranium to decay into isotopes of lead but there is no way for lead to be ordinarily part of a zircon crystal. Any lead in there must have come from decay of uranium. If you count the atoms of uranium and lead (this could be done with a mass spectrometer) then you can use an equation linking the ratio to the age of the rock.
https://en.wikipedia.org/wiki/Uranium-lead_dating Since long decay chains are involved, I wondered whether some of the uranium would have decayed but not yet become lead. Most of the decays are very fast. A couple of the intermediate isotopes have half-lives of the order of tens of thousands of years, which must be why the technique is said to be valid only for rocks older than 1 million years. And for the record, the rock is dated as 452 +- 4 million years.
