Up to James Hutton's time, everyone thought the Earth was just a few thousand years old. James Hutton looked at lots of patterns in the rocks where he lived in Scotland. He observed them closely, drew pictures of them, and thought about them.
He noticed how the rock wasn't just in lots of layers, but sometimes the layers were broken by other rock coming through from volcanoes. This made him wonder if deep down in the Earth it is really hot.
He saw this on Arthur's Seat, Edinburgh - basalt from a volcano, wrapped round other layers of rock: