Throughout the history of life, different organisms have appeared, flourished and become extinct.
Many of these organisms have left their remains as fossils in sedimentary rocks.
Geologists have studied the order in which fossils appeared and disappeared through time and rocks. Fossils can help to match rocks of the same age, even when you find those rocks a long way apart.
This matching process is called correlation, which has been an important process in constructing geological timescales.
There was probably radiogenic lead in the mineralizing fluids plus possibly lead leached out of the host rocks.
In addition there is another stable isotope, lead 204, that is entirely primordial and does not form via radioactive decay at all.
Thus any of the radioactive isotopes and its lead daughter product can be used for dating, or a combination may be used.
Some fossils, called index fossils, are particularly useful in correlating rocks.
For a fossil to be a good index fossil, it needs to have lived during one specific time period, be easy to identify and have been abundant and found in many places. If you find ammonites in a rock in the South Island and also in a rock in the North Island, you can say that both rocks are Mesozoic.