He did not realize that the Earth mantle was convecting, and this invalidated his estimate.
In 1895, John Perry produced an age-of-Earth estimate of 2 to 3 billion years using a model of a convective mantle and thin crust.
Other naturalists used these hypotheses to construct a history of Earth, though their timelines were inexact as they did not know how long it took to lay down stratigraphic layers.
Kelvin calculated the age of the Earth by using thermal gradients, and he arrived at an estimate of about 100 million years.
By measuring the concentration of the stable end product of the decay, coupled with knowledge of the half life and initial concentration of the decaying element, the age of the rock can be calculated.
Thus the age of the oldest terrestrial rock gives a minimum for the age of Earth, assuming that no rock has been intact for longer than the Earth itself.
However, they assumed that the Sun was only glowing from the heat of its gravitational contraction.
The process of solar nuclear fusion was not yet known to science.