Stratigraphy, from the Latin stratum and the Greek graphia, has traditionally been considered the descriptive science of rock strata. In the last few decades, the critical value to stratigraphy of the information provided by nonlayered rock bodies—sedimentary as well as intrusive igneous rocks and massive metamorphic rocks of undetermined origin—has become evident. Non-layered rock bodies not only are the source of geochronometric (numerical) ages determined by isotopic methods, but they also provide crucial age information through the establishment of their cross-cutting and boundary relationships with layered and/or nonlayered rocks with which they are associated. The definition of stratigraphy should, therefore, be broadened to include the description of all rock bodies forming the Earth's crust and their organization into distinctive, useful, mappable units based on their inherent properties or attributes. Stratigraphic procedures include the description, classification, naming and correlation of these units for the purpose of establishing their relationship in space and their succession in time. As such, stratigraphy is concerned not only with the original succession and age relations of rock bodies, but also with their distribution, lith-ologic composition, fossil content, and geophysical and geochemical properties—indeed, with all observed properties and attributes of rock bodies and their interpretation in terms of environment or mode of origin and of geologic history. All classes of rocks—igneous and metamorphic as well as sedimentary, unconsolidated as well as consolidated—fall within the general scope of stratigraphy and stratigraphic classification.