Identified pre-existing rock fragments incorporated into an igneous or coarse sedimentary host predate their incorporation into that host. The comparison concerns the fragments' prior existence and the host incorporation or formation event. This relationship supplies relative order, without numerical ages or duration. Limit the rule to these pre-existing rock fragments; it does not establish that every mineral grain or later feature inside a rock predates its host. Analyst deduction from the local observational scope: observing separate fragments in one host alone does not establish their order relative to one another.

Primary sources:

• https://geology.utah.gov/map-pub/survey-notes/glad-you-asked/glad-you-asked-how-do-geologists-know-how-old-a-rock-is/