Page:Popular Science Monthly Volume 76.djvu/593

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imperfections or gaps in the stratigraphic column. Measured according to the sections of twenty-five years ago, the number of these gaps is growing greater and greater, yet with the discovery and intercalation of new formations, the aggregate of which at the present day has almost doubled the thickness of Paleozoic rocks in the last decade, manifestly the great breaks are being reduced. It was not so many years ago that the Potsdam sandstone was supposed to be the oldest fossiliferous sedimentary rock, yet now we know that many thousands of feet of much more highly fossiliferous strata intervene between this formation and the Azoic, and that other thousands occur above the Potsdam and below the Ordovician as then recognized.

With the intercalation of new formations and the consequent diminution in the size of the stratigraphic gaps, it is then probably only a matter of time before the complete faunal succession can be established. The break in stratigraphy at one point will be bridged over in another area, and it is possible that in only a few regions, such as on the borders of the continent, will permanent gaps exist. Faunas are and will be traced from one area to another until in time we shall know their complete geologic history. With these data in hand, the study of their correlation will not only be greatly simplified, but also will not be hampered by time breaks in the record. While imperfect, or possibly irretrievably lost at the dawn, the faunas of succeeding times are ample for all purposes.




IN discussing this subject from the view-point of a vertebrate paleontologist, I am disposed to lay stress on what I believe ought to be, rather than what has been, the degree of interpendence of these two branches of geology. Vertebrate paleontology has been studied very largely from the morphological and genealogical side, a study of structure, adaptation and the evolution of phyla. Stratigraphic geology has been invoked only when it became necessary to know the order of superposition of the various horizons, to determine the true evolutionary succession of a phylum or development of an adaptation.

I have purposely presented this extreme view, not because I believe that such studies may not be classed legitimately as paleontological, but because I wish to emphasize, by contrast, the view-point which we should ever keep before us as paleontologists—the use of our materials as Leitfossilien. The two correlative conceptions of the faunal unit and the zone, a more or less restricted association of animals and the