2IO HENRY F. OSBORN 



Middle. — The Lower Oligocene, or Titanotherium Zone, most 

 accurately surveyed by Hatcher, is succeeded by the Middle Oligocene 

 or Oreodon Zone, broadly comparable with the Superior Tongrian 

 and Stampian of France, both containing similar types of amphibious 

 rhinoceroses and many other mammals. One of the chief points of 

 interest here is the sharp separation discovered by Matthew between 

 the plains-living mammals buried in the so-called clays, or finer 

 deposits, and the forest-living mammals buried in the coarser intrusive 

 river sandstones. 



Upper. — The close of the Oligocene takes us into the John Day 

 tuff deposits of Oregon, and is generally parallel with the Aquitanian 

 Stage of France, typified by St. Gerand-le-Puy. It is the Dicera- 

 therium Zone, or the climax of the evolution of the pair-horned 

 rhinoceroses in both countries. We pass also into the Upper Mery- 

 cochoerus Zone at the summit of the John Day and at the base of the 

 Arikaree formation extending along Pine Ridge of South Dakota. 

 Here we are again in diihculty in determining just when the American 

 Oligocene should be regarded as closing and the Miocene as beginning. 

 An abundance of diceratheres and entelodonts still betokens Oligocene 

 times, but it is possible that we may be in the Miocene. This is one 

 of the doubtful points requiring further investigation. 



MIOCENE 



The solution of the Lower and Middle Miocene sequence in 

 America through the discoveries of Hatcher, Peterson, and of Matthew 

 marks another great advance of recent years. 



Lower. — There is no question that in the Upper Arikaree, the 

 Upper Harrison of Hatcher, and the Upper Rosebud of Matthew we 

 are fairly in Lower Miocene times corresponding with the Burdigalian 

 of Europe. There is now considerable faunal difference between 

 the New and Old Worlds. The Proboscidea certainly enter Europe 

 at this time, and one of the debated points is when they first appear 

 in North America. 



Middle. — The Vindobonian, or Middle Miocene of Europe, 

 divided into the three successive stages of Sansan, Simorre, and St. 

 Gaudens, is again with considerable confidence compared with the 

 Deep River of Montana, and the Pawnee Buttes of Colorado, through 



