Page:The Journal of geology (IA journalofgeology21894univers).pdf/170

The table on page 155 presents the facts regarding these formations, and what I conceive to be the chief difficulties in attempting to use a single nomenclature and classification. The question at once arises how will a dual nomenclature help us over these difficulties.

On the left I have placed a part of the time-scale adjusting the formation-scales for the several sections, approximately as I think the fossils indicate their time relations to have been.

Formations may be described for a particular geological section with precision as to kind of rock, stratigraphical sequence and thickness, and, as we compare one section with another, we are obliged to become more indefinite in our description and speak in generalities, or averages; and therefore the wider we extend the use of a formation name, the less accurate and the more indefinite does it become.

On the contrary, in the distinguishing of time-relations or position in the time-scale, the individual facts, the local handful of fossils are the more indefinite, indicating only, it may be, Paleozoic or Mesozoic age. The extension of the comparison to the study of fossils above and below, and to the comparison of fossils in adjoining sections, and to their geographical distribution for a wide extent, increases the precision in locating position in the geological time-scale.

It happens, therefore, that, while formation definitions are best constructed in the field in the presence of the actual rock section, the time definition becomes more accurate the more thorough the investigation of the fossils is made. Hence in constructing a time-scale, it begins properly with the grander divisions, and it is built up and perfected by subdivision, whereas the formation-scales begin with the lesser strata and are elaborated and completed by adding together successive strata.

The grander divisions of the time-scale are already in use. These are universally known as the Paleozoic, the Mesozoic, and the Cenozoic. The primary sub-divisions of these geological times are also very widely understood and used. They may be called eras, and are named Cambrian, Ordovician, Silurian, Devo-