Page:Popular Science Monthly Volume 77.djvu/79

Rh Tropidoleptus carinatus, Cypricardella bellistriata, Rhipidomella vanuxemi, Spirifer marcyi and Delthyris mesacostalis (= D. consobrinus) and others; and the Owego and Swartwood zones appear in the midst of a characteristic Chemung fauna both above and below them. In the Owego recurrent zone both Phacops rana and Dalmanites calliteles occur.

The Van Etten recurrent zone lies entirely below the range of Spirifer disjunctus and associated species of the Chemung formation. On following the sections eastward from the Waverly quadrangle the species of the Chemung fauna become scarce, and east of the Chenango River very few species of the typical Chemung fauna have been detected—although they are still abundant in the Chemung rocks to the southeast and southward across Pennsylvania, Maryland and Virginia.

§ 5. These facts have been interpreted as evidence not only of a general shifting of faunas coincident with a rising of the land along the eastern edge of the present continent, but of oscillation of conditions and alternate occupation of the area by two sets of faunas coming from opposite directions and temporarily living in abundance in the area of central New York.

§ 6. The lithologic changes in the sediments containing the different faunas are not sufficient to account for the change in fauna. In quite a number of sections there is no appreciable difference in lithologic constitution between the strata which for a hundred feet thickness have been filled with characteristic Chemung species and the immediately following thin zone (of a foot or two) with scarcely a trace of the Chemung species, but holding, in great number, species which if found by themselves would be undisputed evidence of the Hamilton formation.

§ 7. It becomes necessary therefore to suppose that the controlling cause determining the presence of one or other fauna is not the character of the bottom on which the sediments which preserved the fauna were laid. We are thus led to conclude that the qualities of the ocean water have determined the shifting or migration of the faunas. The conditions to which the faunas were adjusted were evidently those of depth, salinity or temperature of the waters in which the species lived; and their change of habitation was occasioned by change in the direction, path or extent of flow of oceanic currents.

This leads us to consider the principles of migration as affecting marine organisms.

§ 8. Migration of Species and Shifting of Faunas.—Migration as commonly applied in natural history means the movement of large numbers of the same species from one place to another in a general definite direction at more or less regular periodic times. So birds migrate northward with the advance of warm weather; some fish migrate