The categories within stratigraphic classification are all closely related. All deal with the rocks of the Earth's crust, with the picture of the stratified Earth as it presently exists, and with the history of the Earth as interpreted from its rocks. Each category, however, is concerned with a different property or attribute of the rocks and a different aspect of Earth history. The relative importance of the different categories varies with circumstances. Each is important for particular purposes.

Lithostratigraphic units are the basic units of geologic surface or subsurface mapping, and lithostratigraphic classification is usually the first approach in stratigraphic work in any new area. Wherever there are rocks, it is possible to develop a lithostratigraphic classification even when no other stratigraphic procedures are feasible.

Lithostratigraphic units are based primarily on the lithologic properties of rocks—sedimentary, igneous, and metamorphic. The fossil content of lithostrati-graphic units may in certain cases be an important distinguishing element in their recognition, not because of the age significance of the fossils but because of their diagnostic lithologic (physical) properties. Coquinas, algal reefs, radio-larites, oyster beds, and coal beds are good examples.

Inasmuch as each lithostratigraphic unit was formed during a specific interval of geologic time, it has not only lithologic significance but also chrono-stratigraphic significance. The concept of time, however, properly plays little part in establishing or identifying lithostratigraphic units and their boundaries. Lithologic character is generally influenced more strongly by conditions of formation than by time of origin; similar rock types are repeated