An extrapolation of the isotope ratios of lead ores also gives an age of 4.6 Ga for the Earth. Lead 204 is not produced by radioactive decay and is therefore constant throughout geological time. After 704 Ma, half the original amount of 235U is converted to 207Pb. After another 704 Ma, half of the remaining 235U decays and so on. So after 4.6 Ga, very little 235U is left.
Lead ores are concentrations of the lead that is normally dispersed in the crust of the Earth.
The lead 206 formed from the decay of uranium 238 goes with the older lead 204 into the lead ores. So the ratio of lead 206 to lead 204 increases with time. An extrapolation of the curve derived from the dated lead ores intersects the value obtained from the meteorite. This is good evidence that the Earth and the meteorites were formed at the same time, about 4.57 billion years ago. The geological time scale Most fossils come from the period of time known as the Phanerozoic .