Before the development of radiometric dating techniques
That concept began with eighteenth-century French naturalist Georges Cuvier, picked up steam with Charles Lyell, and it has been in vogue ever since.
Why else do we find marine fossils on the tops of all the major mountain ranges?
Both the physical geologists and paleontologists could point to evidence that much more time was needed to produce what they saw in the stratigraphic and fossil records.
As one answer to his critics, Kelvin produced a completely independent estimate -- this time for the age of the Sun.
His result was in close agreement with his estimate of the age of the earth.
The solar estimate was based on the idea that the energy supply for the solar radioactive flux is gravitational contraction.