26o HENRY F. OS BORN 



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 difficulty 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 

 the researches of Scott and Matthew. Here we enter the fifth faunal 

 phase, marked by fresh migrations and the first undoubted appear- 

 ance of the proboscideans and short-limbed rhinoceroses in America, 

 both arrivals from the Old World. Physiographic changes are 

 indicated in evidence of increasing summer droughts, numerical 

 increase of animals adapted to plains-living and the semi-arid con- 

 ditions, in the disappearance of most of the browsing types. The 

 correlation is, however, by no means close at present, because the life 

 of Europe and the great American plains is of different local habitat. 



