Page:The Journal of geology (IA journalofgeology21894univers).pdf/173

equivalency of stratigraphical position by the structure from some region in which such marks are evident.

The Pocono and the Mauch Chunk are, as formations, but the continuation of the Catskill formation upward and are of local value as formation-names, but of little or no value as elements of a time-scale.

Thus the Catskill may continue to appear in the list of formations of New York, and the Appalachian province, where it generally appears as a Neodevonian formation, but it is useless to define or to discuss its more exact position in the time-scale, because its own time-criteria range through the whole Devonian Era, and its relations to formations whose time-criteria are more exact is indefinite and inconstant.

If we grant the truth of the great principle here set forth, it is evident that the increase of formation names can be no more objectionable than the increase of the names of mountains; and that there is no more reason for applying the same name to two formations which are of the same age, but differ in position and structure, than there is in giving two names to mountain ridges of the same range. The classification of formations into groups or systems will depend upon considerations of geological structure, and only secondarily upon time considerations; and even in the latter case, only because the same general geological events have affected the sedimentation in a similar way for a wide extent geographically.

On the other hand, the time-scale will depend for its classification upon the fossils and not upon structure. Stratigraphical sequence, of course, is important in reading fossils, but only as in a sentence, or on a page, the sequence of the words is important.

An organic species, or a genus, or an order lived only during a particular period of time, and it is for this reason that the fossils have an intrinsic time-value, but a mineral, or a rock may have been formed under like conditions and present like characteristics at any period of geological time. I refer of course to clastic rocks, and those formed through processes of sedimentation.