Early scientific estimates of Earth's age used observations of exposed marine fossils requiring a drop in sea level (Benoit de Maillet), cooling of a hotter Earth (Lord Kelvin), and the salinity of the ocean (John Joly)

Question

What age did each of these estimates come up with? What assumptions were made for each of these estimates that we now know to be incorrect?
What will be an ideal response?

Answer 1

Benoit de Maillet came up with an age of 2.4 million years that would account for the locations of these fossils if they were left behind by the fall of a once all-encompassing deep sea. Geologists now know that sea level has risen and fallen countless times during Earth history, and that both falling sea level and uplift of land can explain the presences of marine fossils found above present sea level.
Lord Kelvin determined a most likely age of 20–40 million years for Earth. It turns out that the most important assumptions in Kelvin's calculations were suspect. If, as geologists now think, Earth's mantle convects, then heat moves upward in the convecting mantle to maintain warm geothermal gradients near the surface for billions of years. Therefore, the geothermal gradient required to maintain high temperatures in deep mines can be maintained much longer than the 40 million years calculated by Kelvin. Kelvin's conduction assumption, therefore, yields an imprecise minimum age for Earth.
Joly calculated an age of 80–100 million years. Modern geologists also see many problems with Joly's calculation. Oceanographers later explored the ocean depths and now know that oceans hold a much larger volume of seawater than estimated by Joly. Subsurface explorations for oil and gas and field excursions in remote lands reveal that rock-salt deposits also are much larger than known by Joly. Applying Joly's logic to these newer data would result in a substantially older age for Earth.