Stratified extrusive volcanic rocks and metamorphic rock bodies of distinct sedimentary or volcanic origin are generally recognized as lithostratigraphic units. Intrusive igneous rock bodies and nonstratiform bodies of metamorphic rocks of undetermined origin should also be recognized as such. All are defined, identified, and mapped on the basis of their distinguishing lithology, which is the diagnostic criterion for establishing lithostratigraphic units. The name of formal igneous and metamorphic lithostratigraphic units, both layered and non-layered, should be composed of an appropriate local geographic term combined with either (1) a unit-term indicating its rank—group, formation, member—or (2) a simple field lithologic term—granite, gneiss, schist—indicating its predominant rock type, or both. Rank terms are more clearly suited for layered extrusive volcanic rocks and for metamorphic rocks whose original sedimentary and/or extrusive volcanic derivation is readily recognizable. A simple field lithologic term may be more appropriate in the case of nonlayered intrusive igneous bodies or for metamorphic rocks of homogeneous lithology and undetermined origin. The term “complex” may be used for igneous and/or metamorphic rock bodies of heterogeneous and irregular lithology, whether or not having suffered strong and complicated deformation and/or metamorphism.