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By the early 1960s, most of the major radiometric dating techniques now in use had been tested and their general limitations were known.They observed that every rock formation, no matter how ancient, appeared to be formed from still older rocks.Comparing these rocks with the products of present erosion, sedimentation, and earth movements, these earliest geologists soon concluded that the time required to form and sculpt the present Earth was immeasurably longer than had previously been thought.
Bishop James Ussher, a 17th-century Irish cleric, for example, calculated that creation occurred in 4004 B. There were many other such estimates, but they invariably resulted in an Earth only a few thousand years old.By the late 18th century, some naturalists had begun to look closely at the ancient rocks of the Earth.The discovery of radioactivity in 1896 by Henri Becquerel, the isolation of radium by Marie Curie shortly thereafter, the discovery of the radioactive decay laws in 1902 by Ernest Rutherford and Frederick Soddy, the discovery of isotopes in 1910 by Soddy, and the development of the quantitative mass spectrograph in 1914 by J. Thomson all formed the foundation of modern isotopic dating methods.But it was not until the late 1950s that all the pieces were in place; by then the phenomenon of radioactivity was understood, most of the naturally occurring isotopes had been identified and their abundance determined, instrumentation of the necessary sensitivity had been developed, isotopic tracers were available in the required quantities and purity, and the half-lives of the long-lived radioactive isotopes were reasonably well known.Lord Kelvin and Clarence King calculated the length of time required for the Earth to cool from a white-hot liquid state; they eventually settled on 24 million years.
James Joly calculated that the Earth’s age was 89 million years on the basis of the time required for salt to accumulate in the oceans.