. The age of mammals in Europe, Asia and North America. Mammals, Fossil; Paleontology. OLIGOCENE OF EUROPE, NORTH AFRICA, AND NORTH AMERICA 231 of specific forms. Similarly the suillines, or dicotylids, are found in great numbers (Perchoerus) and in a variety of specific forms. Of the oreodonts the Agriochoerus phylum reaches a climax preceding its extinction. The typical cursorial oreodonts pass into the Eporeodon stage. The hyper- tragulids are represented only by Hypertragulus. Third and Final Phase, Promerycochoerus Zone Upper John Day of Oregon. — Especially noteworthy in the Upper John D
. The age of mammals in Europe, Asia and North America. Mammals, Fossil; Paleontology. OLIGOCENE OF EUROPE, NORTH AFRICA, AND NORTH AMERICA 231 of specific forms. Similarly the suillines, or dicotylids, are found in great numbers (Perchoerus) and in a variety of specific forms. Of the oreodonts the Agriochoerus phylum reaches a climax preceding its extinction. The typical cursorial oreodonts pass into the Eporeodon stage. The hyper- tragulids are represented only by Hypertragulus. Third and Final Phase, Promerycochoerus Zone Upper John Day of Oregon. — Especially noteworthy in the Upper John Day is the sudden appearance of Promerycochoerus, an unheralded oreodont, which is highly characteristic of the close of the Oligocene and opening of the INIiocene of North America. This Upper John Day level also contains members of the camel series (Paratylopus), animals of larger size, and similar to those in the Middle Oligocene. It is noteworthy that the only camels from the John Day o])tained by .the University of California expeditions come from this upper level. Great Plains of Dakota, the 'Harrison^ and' Rosebud.'—While the John Day Forma- tion was gathering in Oregon, either a con- tinuous but non-fossil-bearing deposition or a brief erosion interval followed on the Great Plains of Dakota, and the great formations variously known as 'Arikaree,' 'Gering,' 'Monroe Creek,' 'Harrison,' and 'Rosebud' began to be deposited conformably or unconformably on the summit of the White River group, or Leptauchenia Zone (Fig. 96). This great formation was recog- nized as Horizon D by Hayden as early as the year 1869. It is indicated by the horizontal lines in the map on p. 211, and is extensively exposed along the White, Niobrara, and North Platte rivers. It opens with partly flood plain or fine deposit conditions, interspersed with partly fluviatile or channel conditions, which are indicated liy broad beds of conglomerates and river sandstones (Gering and Monroe Creek formations)
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectpaleontology, bookyea
