The principle of faunal succession is based on the appearance of fossils in sedimentary rocks.As organisms exist at the same time period throughout the world, their presence or (sometimes) absence may be used to provide a relative age of the formations in which they are found.
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.
There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills and dikes.
Cross-cutting relations can be used to determine the relative ages of rock strata and other geological structures.
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 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.
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.
He also found that certain animals were in only certain layers and that they were in the same layers all across England.