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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.
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.
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.
This is because it is not possible for a younger layer to slip beneath a layer previously deposited.
The only disturbance that the layers experience is bioturbation, in which animals and/or plants move things in the layers.
A similar situation with igneous rocks occurs when xenoliths are found.