The oldest known minerals and rocks

I. I ntroduction . I t is now familiar knowledge that radioactive minerals, such as uraninite and monazite, are geological clocks endowed with an unfailing mechanism for keeping time by steadily ticking out atoms of helium and lead. The method of reading these natural timekeepers is comparatively simple in principle, though the interpretation of the readings may often be difficult and even uncertain in practice. Since we know the rates at which lead is generated from thorium (Th 232 ) and from each of the parental isotopes of uranium (UI or U 238 , and AcU or U 235 ), it is possible to determine the time required for the accumulation of the lead now present in any radioactive mineral which has been analysed for Pb, U and Th. If the mineral was free from lead at the time of its formation, and if it has remained unaltered, except by its own atomic disintegrations, then the time so computed is the absolute age of the mineral. But these rigid conditions are so rarely fulfilled that it is preferable to speak only of the ‶apparent age″ of a mineral, until the effects of original lead and alterations have been suitably assessed and allowed for. Fortunately, thanks to the outstanding success of Professor A. O. Nier in separating the isotopes of lead and measuring their proportions by means of his mass spectrometer, we have recently been furnished with a wealth of accurate data which provides a new and solid basis for making such assessments with reasonable confidence. The purpose of this