Geochronology and the geological record

Some history

By about the middle of the nineteeth century natural scientists had come to the conclusion that geological time was to be measured in hundreds of millions of years. Darwin in the first edition of The Origin of Species published in 1859 estimated that 300 million years had elapsed since the end of the Mesozoic Era; Lyell concluded in 1867 that about 240 million years had elapsed since the beginning of the Ordovician Period. Despite the temptation to point out the obvious discrepancy in these particular estimates and to suggest that they were no more than educated guesses it must be remembered that they reflected the most careful marshalling of a vast number of verifiable observations. Indeed they can now be seen to be remarkably realistic and in marked contrast to the estimates based on more theoretical considerations such as ocean salinity, sediment thickness, the cooling of the Earth or the Sun's energy source.

Towards the end of the nineteenth century the geologists' views regarding the age of the Earth came under particularly strong attack from W. Thomson (later Lord Kelvin) whose arguments were based on the energy source of the Sun and the cooling history of the Earth, and who concluded that the Earth was probably younger than about 100 million years. That geologists found Thomson's estimates thoroughly unpalatable is very clear from the literature of the time but his calculations were apparently irrefutable. However, the impasse was soon to be resolved following the discovery of the phenomenon of radioactivity