Half a lifetime had gone by since Arthur Holmes had lain in his tent in Mozambique, racked with fever, dreaming of developing a geological time scale and wondering how he could reconcile the age of the Earth as determined by radioactivity with that calculated by the old established methods of sedimentation rates. While progress on a geological time scale had been made over the following years, it had largely been in the physics arena: improved understanding about the atom; the discovery of isotopes; development of the mass spectrometer; and recognition of the four stable isotopes of lead. The geological side, however, lagged far behind. A rock assigned an age of 300 million years, for example, still could not be classified as ‘Carboniferous’ with any confidence because it was still not known how long, in geological time, the Carboniferous ranged. So, as we saw with the helium results from the Whin Sill, extreme errors could be accepted as reasonable values because no limits could be placed on the extent of the Carboniferous. Clearly, what was needed was a time scale that said ‘the Carboniferous starts here at this age and ends there at that age, therefore any age in between must be Carboniferous’. But that was still a long way off.