In the 1890s, the British physicist Lord Kelvin refined Buffon's calculation, and derived an age of 20-40 Million years. This turns out to be an underestimate because natural radioactivity in rocks helps keep the Earth warmer that it would be in the absence of any other sources of heat. Previous idea was decay from initial creation. Lyell is the most famous of Hutton's students. Used strata of different rocks to separate the ages of previous geological history Noted change in fossils in strata as ways to fix relative ages.
Old rock are like young rocks, but life shows great variation with time.
More lead of both isotopes, but in different isotopic proportions because of the different radioactive decay rates of the parent Uranium isotopes. Gyr One measures the detailed isotope ratios in crystalline inclusions, exploits chemical differences to distinguish original elemental content from radioactive decay products, etc. to form the basis of dating the time since the rock solidified. It is far too complicated to go into in detail here, but it is now a well-defined, if exacting technique that gives good results. Once you make a rock, its elements are locked in other elements are locked out. Radioactive dating therefore gives you the time that has elapsed since the rock solidified .