Words relating to dating
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.
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.
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 principle of cross-cutting relationships pertains to the formation of faults and the age of the sequences through which they cut.Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization (although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal).The law of superposition states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it.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.