This was quoted in that same link
Holmes published The Age of the Earth, an Introduction to Geological Ideas in 1927 in which he presented a range of 1.6 to 3.0 billion years.
Its all Guesstimation,
History repeats itself, every generation thinks they have the right answer until a new technology disproves it. Here's another outline on how it kept changing
How old is Earth? (scientific methods)
1.In 1897, Lord Kelvin assumed that the Earth was originally molten and calculated a date based on cooling through conduction and radiation. The age of Earth was calculated to be about 24-40 million years based on the laws of thermodynamics.
Unknown at the time was that the Earth has an internal heat source (radioactive decay)
2. In 1899, John Joly (Irish) calculated the rate of delivery of salt to the ocean. River water has only a small concentration of salts. Rivers flow to the sea, therefore, evaporative concentration of salts can be calculated.
By this method, the age of oceans will equal the total salt in oceans (in grams) divided by rate of salt added (grams per year)
Age of Earth was calculated to be 90-100 million years.
Problems: no way to account for recycled salt, salt incorporated into clay minerals, salt deposits.
3. In 1860, the thickness of total sedimentary record is divided by average sedimentation rates (in mm/yr) and calculated to be about 3 million years old. In 1910, the same measurement yields about 1.6 billion years old.
Early measurements of maximum thickness of sediment ranged from 25,000 m to 112,000 m. With more recent mapping, thickness of fossiliferous rocks is at least 150,000 m. The average sedimentation rates are about 0.3 m/1000 years. At this rate, the age of the first fossiliferous rocks is about 500 million years.
Problems: This calculation does not account for past erosion or differences in sedimentation rates; also ancient sedimentary rocks are metamorphosed or melted.
4. Charles Lyell (1800's) compared amount of evolution shown by marine mollusks in the various series of the Tertiary System with the amount that had occurred since the beginning of the Pleistocene. He estimated about 80 million years for the Cenozoic Era alone.
5. Radioactivity is discovered by Henri Becquerel in 1896. In 1905, Rutherford and Boltwood used radioactive decay to measure the age of rocks and minerals. Uranium decay produces He, leading to a date of 500 million years for the oldest rocks.
In 1907, Boltwood suspected that lead was the stable end product of the decay of uranium and published the age of a sample of urananite based on Uranium-Lead dating to be 1.64 billion years.
Mass spectrograph was used after WWI (1918). Led to the discovery of over 200 isotopes. Many radioactive elements can be used as geologic clocks since each element decays at its own nearly constant rate. Once this decay rate is known, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent and the amount of stable daughter elements.
So far, oldest dated Earth rocks are 3.96 billion years. Older rocks include meteorites and moon rocks, where Moon rocks from the Lunar highland are about 4.5 billion years old, mare basalt rocks are 3.2 - 3.8 billion years old. Meteorites are all older than 4.5 billion years.
