OF WESTERN NORTH AMERICA. 



69 



the transition beds are sharply demarcated positively (1) by the sud- 

 den appearance among Artiodactyla-Oreodontidse of Fr ornery cochcerus 

 followed in higher levels by Merycochcerus and Merychyus; (2) by the 

 survival of progressive species of Leptauchenia in the same family; (3) 

 among Artiodactyla also, 3 families of earlier horizons apparently 

 have become extinct, namely, Anthracotheriidae (which also disappear 

 in the Aquitanien of France), Leptochoeridse, and Oreodontidae- 

 Agriochoerinae. 



COMPARISON WITH EUROPEAN HORIZONS OF UPPER OLIGOCENE AGE. 



The upper part of the John Day formation, or Pr ornery cocho^rus 

 zone, of the Mountain region of Oregon, as well as the Gering and 

 Monroe Creek formations of Hatcher, the Gering or lower Arikaree of 

 Darton, the Rosebud (lower levels, see fig. 12) of Matthew, all in the 

 Plains region of South Dakota, may be regarded as covering the transi- 

 tion between the Oligocene and Miocene epochs, as these divisions are 

 employed in France. They resemble chiefly the upper Oligocene of 

 France. (1) The upper part of the John Day of the Mountain region 

 is somewhat older than the lower part of the Rosebud of the Plains, 

 although both contain PromerycochcRrus. (2) Recent explorations in 

 the upper portion of the Harrison and equivalent formations ® reveal 

 a fauna which partly resembles that of the upper Oligocene of France 

 (an Oligocene character is given by the survival of Elotfierium) ; at tlie 

 same time, it contains a primitive AmpMcyon, a characteristic'Miocene 

 form. The newer fauna of these beds is slightly subsequent to that of 

 St. Gerand-le-Puy (generally regarded as upper Oligocene or Aquita- 

 nien, although several authorities place it in the lower Miocene). (3) The 

 resemblance to the Aquitanien consists in the presence of Rhinocero- 

 toidea-Diceratheriinae, in the nonappearance of Mastodon among Pro- 

 boscidea, and in the nonappearance of Teleoceras among Rhinocero- 

 toidea. (4) It differs from the Aquitanien proper in the survival of 

 Artiodactyla-Elotheriidae, which disappear in the middle Oligocene of 

 France. (5) It contains Chalicotheriidae apparently near Macrothe- 

 rium, a Miocene stage. (6) From a recent comparison of these fauna, 

 Matthew writes (March, 1907): 



The above comparisons indicate that the Rosebud faunae are later than the upper 

 Oligocene and earlier than the middle Miocene of the European standard. Their 

 position is thereby fixed as lower Miocene, representing an earlier and a later stage. 



It is concluded that the upper part of the John Day, for the present, 

 may be somewhat arbitrarily separated as the American upper Oligo- 

 cene, while the partly contemporaneous and partly sequent Plains 

 formations may be termed lower Miocene. 



a Matthew, W. D., A lower Miocene fauna from South Dakota: Bull. Am. Mus. Nat. Hist., vol. 23, 

 1907, pp. 169-219. 



' ' From these discoveries it appears that the Miocene section from the Oligocene to the top of the 

 Nebraska l)eds, in this general locality, may pterhaps have to l)e regarded as lower Miocene."— Peterson. 

 O. A., The Agate Spring fossil quarry: Ann. Carnegie Mus., vol. 3, No. 4, 190G, p. 491. 



