Before the development of isotopic dating methods, how did scientists estimate the age of the earth?
primarily by the thickness of sedimentary layers as well as fossil evidence, using the assumptions of uniformitarism and organic evolution.
The age of the earth in 1860 was estimated at 3 million years old.
This estimation was based on the thickness of the sedimentary layers throughout the earth divided by an estimation of the rates of erosion.
In 1913 Luthor Hobbs using the same process of an estimation of the thickness of the geological layers and rates of erosion estimated the age of the earth as 1.6 billion years.
In 1830 Charles Lyell estimated the age of a geological layer in England as 1 million years old. He determined the age of a fossil layer based on the proportion of extinct species of mollusks to the proportion of living species of mollusks. Lyell advocated for a great age of the earth based on his evidence of changes in organisms.
Later in 1841 John Phillips established the first geological column time scale based on traits of fossils. The present time scale used today was frozen in the 1840s.
" Owning to the "irreversibility of evolution they(fossils) offer an unambiguous time scale for relative age determinations and worldwide correlations of rocks" American Journal of Science Vol 255 June 1957. The time scale is based on the assumption of organic ( Darwinian) evolution.
Other methods used to estimate the age of the earth have been.
1897 age 24-40 million years. Lord Kelvin based on cooling of the earth according to the second law of thermodynamics
1899 age 90-100 million years. John Joly based on the sality of the ocean divided by the rate of erosion.
1905 age 500 million Rutherford based on radioactivity of rocks
The basic ideas of the relative ages of fossils was fixed by 1840 based on the sedimentation, uniformitarism, and organic evolution.
Radioactivity was used to increase the total age of the earth and to confirm the already decided geological column.