It was only with the discovery of radioactivity that geology made a great step forward in this field.
By studying certain radioactive substances, it was discovered that, in the course of time, they lose their radioactivity at a steady, invariable rate. The element uranium, for example, loses its radioactive properties over a very long period of time and changes into lead. We know that one gramme of uranium takes a thousand years
to produce 1/7,000,000 of a gramme of lead. With this knowledge it is possible to calculate the age of a rock containing uranium by comparing the amount of lead produced in it with the amount of uranium still present.
Tests made on radioactive rocks all over the world have shown that the oldest were formed about 3,800 million years ago. However, from the ages of meteorites, the Earth is thought to have formed about 4,500 million years ago.