While digging the Somerset Coal Canal in southwest England, he found that fossils were always in the same order in the rock layers.
As he continued his job as a surveyor, he found the same patterns across England.
There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills and dikes.Geologists still use the following principles today as a means to provide information about geologic history and the timing of geologic events.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.Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault.The principle of inclusions and components states that, with sedimentary rocks, if inclusions (or clasts) are found in a formation, then the inclusions must be older than the formation that contains them.
Though relative dating can only determine the sequential order in which a series of events occurred, not when they occur, it remains a useful technique especially in radiometric dating.