Inclusions (also known as Components) (Charles Lyell, 1797-1875). The components of a rock unit are older than the actual age of the rock unit itself. This principal is self evident in sedimentary rocks. By definition, sedimentary rocks are composed of the remains of preexisting rocks. Therefore, the particles that are in a conglomerate were actually formed as part of another rock at an earlier time. (This fact prevents geologists from radiometrically dating the age of most sedimentary rock units since the material containing the radioactive isotopes within the sedimentary rock unit originally formed in a preexisting rock.)

This law is beneficial when trying to determine the relative ages of two different sedimentary rock units that are in contact with each other. The younger unit will contain components of the older. This occurrence is most often seen when the lower and older sedimentary unit has been mechanically eroded on its’ surface. When the next sedimentary bed is deposited, fragments from the older bed are incorporated in lower portion of the younger bed.

This law is also beneficial when trying to determine the relative ages of sedimentary beds that are in contact with igneous or metamorphic rock units. If the sedimentary beds are younger than the igneous or metamorphic units, the sedimentary beds will contain fragmental components of the eroded igneous or metamorphic material. If the sedimentary beds are older than igneous units, the igneous material may contain inclusions of the sedimentary rocks called xenoliths.