Page:The Journal of geology (IA journalofgeology21894univers).pdf/71
Richmond coal, which are not in New Jersey. The unit is peculiarly definite in that its lower and upper limits are marked by conspicuous unconformities, while its strata are everywhere conformable with one another. Its composition, though not uniform, is so little varied that attempts to unravel its stratigraphy and structure have been successful in but few districts.
2. The name should include a local geographic term.—In the nomenclature of historic geology there are two parallel sets of terms, the one representing larger or smaller bodies of strata, the other representing larger or smaller divisions of geologic time. As the divisions of geologic time are based upon the classification of strata, their names have been mostly derived from stratigraphy, and there are many circumstances under which it is a matter of indifference whether a given term be construed in its stratigraphic or in its chronologic sense. Partly in this way there has arisen a widely prevalent habit of confusing strata and time. This confusion has an unfortunate influence on the treatment of problems of correlation, as it leads to language implying that the stratigraphic units of distant lands, for example, Europe and America, are the same. As I understand the case each portion of the general geologic time scale was based upon the stratigraphy of some district, usually in Europe. Correlation at a distance, for example, in America, does not determine the existence in America of the European formations, but only the existence of local formations deposited (in whole or part) in the same portions of geologic time. Or, in other words, correlation arranges the formations of a country in accordance with a standard time scale.
When the time relations of a formation or other stratigraphic unit are unknown or are imperfectly known, a name derived from the time scale can be employed only provisionally. As knowledge of fauna and flora increases, opinions change as to time relations, and experience shows that at any stage in the accumulation of paleontologic data conflicting opinions may be held by different students. Time names are thus unstable; but a geographic name, depending as it does on simple relations readily