Page:The Journal of geology (IA journalofgeology21894univers).pdf/162
Potsdam as names of geological divisions of the scale of which there are a definite number. These arranged in a definite order, according to English sequence of systems: Cambrian, Silurian, Devonian, and Carboniferous, etc., constitute the standard geological scale.
These formations have also been called Epochs and Periods, and because the succession of time is continuous, the strict application of the method has required the filling of the whole interval, from the Cambrian to the Carboniferous, by these formations, except when unconformity gives marked evidence of break in the record. The old system requires that in all the sections, the lines separating the formations from one another shall coincide with the stratigraphical divisions, and progress has been checked by the authoritative rebuke, from those who are supposed to know, of any timid suggestion that the geology of a newly discovered section does not conform to the standard. We are already familiar with the proposition that there are such systems, groups and formations, on the one hand, and Ages, Periods and Epochs, on the other, but our whole nomenclature and classification is applied and used as if the divisions indicated by the two categories were strictly synonymous; in fact, the nomenclature of the International Congress went no farther than to propose that the names of the divisions of the one category, viz.: group, system, series, stage, should be applied to the same concrete geological facts as the corresponding names of the other category, era, period, epoch, age, and that these names of the categories should be universally used; as if, a century ago, zoölogists had proposed that class, order, family, genus, be used in a uniform manner by all naturalists. It is the essential idea contained in this differentiation of nomenclature, in two directions, which I would here emphasize and elaborate.
When geologists consider the two scales, the time-scale and the formation-scale, it is found that the divisions are not synonymous, but that there are two distinct sets of facts confused in our present nomenclature and classification. There is a geological time-scale, and, however we subdivide it, or however we