Absolute dating on rocks
As the magma cools, grains of different minerals begin to crystalize.
In the late 19th century Lord Kelvin devised an experiment to try to calculate the age of the earth.that there was no new heat being produced inside the earth. And pushed the Earth's age back into the billions of years.The discovery of radioactivity by physicists in the very late 19th century, allowed geologists to discover the absolute age of a particular rock. The only assumption made with absolute dating by radioactive decay...From this they were able to deduce that the lower rocks were generally older than the higher rocks.They also observed that certain fossils were found in certain layers, and were not found in other layers.