Sixteen years after his discovery, he published a geological map of England showing the rocks of different geologic time eras.Methods for relative dating were developed when geology first emerged as a natural science in the 18th century.The Law of Superposition, which states that older layers will be deeper in a site than more recent layers, was the summary outcome of 'relative dating' as observed in geology from the 17th century to the early 20th century.The regular order of occurrence of fossils in rock layers was discovered around 1800 by William Smith.A fundamental principle of geology advanced by the 18th century Scottish physician and geologist James Hutton, is that "the present is the key to the past." In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now." The principle of intrusive relationships concerns crosscutting intrusions.In geology, when an igneous intrusion cuts across a formation of sedimentary rock, it can be determined that the igneous intrusion is younger than the sedimentary rock.however, this process is not enough to allow the layers to change their positions.This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed.
Prior to the discovery of radiometric dating in the early 20th century, which provided a means of absolute dating, archaeologists and geologists used relative dating to determine ages of materials.
As a result, xenoliths are older than the rock which contains them.
The principle of original horizontality states that the deposition of sediments occurs as essentially horizontal beds.
A similar situation with igneous rocks occurs when xenoliths are found.
These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix.