The principle of Uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time.
These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix.
As a result, xenoliths are older than the rock which contains them.
Due to that discovery, Smith was able to recognize the order that the rocks were formed.
Sixteen years after his discovery, he published a geological map of England showing the rocks of different geologic time eras.
These strata make up much of the famous prominent rock formations in widely spaced protected areas such as Capitol Reef National Park and Canyonlands National Park.