OLIGOCENE OF EUROPE, NORTH AFRICA, AND NORTH AMERICA 227 



descended from any loiown American ancestor, therefore probably a new- 

 comer; it exhibits two pairs of short, plated or rounded, bony horns, or 

 bosses, and canine tusks like those of the muntjacs. In these sandstones 

 was discovered also the finely preserved dicerathere Ccenopus tridactylus, 

 a descendant of the Ccenopus occidentalis of the Oreodon Zone, in which 

 paired rugosities on the nasal bones of old male specimens are prophetic 

 of the prominent horns of the later Dicer atherium. In the same sandstones 

 are found tapirs {Protapirus validus) of larger proportions, and small- 

 headed horses (Mesohippus intermedius) of considerably larger size, which 

 more nearly approach the Anchitherium type of the Lower Miocene of 

 Europe; in fact, the six species of horses found on this level represent a 

 great advance upon those of the Oreodon Zone. Three of these species 

 pass into the higher Miohippus stage of the Upper Oligocene, or John Day. 

 Among Carnivora it is noteworthy that Cynodictis survives and that the 

 machaerodonts now include Eusmilus. 



The adaptive radiation into plains and lowland mammals beginning in 

 the Titanotherium and Oreodon Zones is thus still more strongly marked, 

 but the plains tjrpes are relatively little known. Among artiodactyls, the 

 entelodonts, peccaries, anthracotheres, oreodonts, hypertragulids, and 

 camelids are all represented, the latter passing into the somewhat more 

 advanced side branch of camel development known as Pseudolabis. 



Second Phase of the Upper Oligocene, Diceratherium Zone 



John Day Formation of Oregon. — This grand formation carries us for 

 the first time into the mammal life of the extreme northwestern states, 

 which has become knd^vn through the successive explorations and re- 

 searches of Condon, Sternberg, Wortman, Cope, Merriam, and Sinclair.* 

 We owe the discovery of these beds to the veteran geologist of Oregon, 

 Professor Condon. It is a sharply defined volcanic deposition of a total 

 thickness of 1,650 feet, composed of tuffs, divided into lower, middle, 

 and upper levels by two rhyolitic flows, and sharply bounded at the 

 summit by a gigantic basaltic outflow two thousand feet in thickness, 

 known as the Columbia River Lava, which is regarded as of Lower 

 Miocene age. As described by Merriam,- the volcanic materials of the 

 John Day deposits were chiefly wind-blown, or aeolian. There is little 

 evidence of fluviatile conditions. The greater portion of the series is 

 presumably made up of slow accumulations of ash which fell mainly on 

 open plains, upon which shifting, shallow lakes may have existed from time 

 to time. The John Day beds show a remarkable evenness in their strati- 



' Principal titles given in Bihliogniphy. 



^ Merriam, J. C, A Contribution to the Geology of the John Day Basin. Univ. Cal., 

 Bull. Dept. GcoL, Vol. II, 1901, p. 269 fol.; and, Carnivora from the Tertiary Formations of 

 the John Day Region. Univ. Cal., Bull. Dept. GeoL, Vol. V, 1906, pp. 1-64. 



