History of the Age of the Earth
In 1799 William Smith, an English surveyor working for a canal company, noticed that rocks generally appeared in layers. Also, fossils contained within them always appeared in the same order. From then on, geologists were able to compare the relative ages of rocks no matter where they were from.
Smith's discovery was useful, but still didn't give an age for the earth. In 1867, British geologist Charles Lyell estimated that 240 million years was probably enough time for the evolution of the life to have reached its present form (based on Charles Darwin's theories of natural selection).
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| William Smith [1769-1839] |
Sir Charles Lyell [1797-1875] |
Bertram Boltwood [1870-1927] |
In 1905, US chemist/physicist Bertram Boltwood discovered that rocks containing uranium always contained lead. Furthermore rocks from any single part of the earth always had a constant ratio between the two. He suggested that lead is the end product of the radioactive decay of uranium:
Once the half-life of Uranium (and the intermediate nuclei) was determined, estimating the age of uranium-bearing rocks became much easier.
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