BNVIKONMENT OP THE TITANOTHERES 



85 



TJintaiherium is closely related to the ancestral BatJiyop- 

 sis, which is found in the Wind River Lambdotherium 

 zone and in the long antecedent first Wasatch zone. 

 The pseudotapir Isedoloiyhus is related in tooth struc- 

 ture to Systemodon, which is characteristic of the third 

 Wasatch zone. We are therefore disposed to regard 

 the life of the upper Bridger ( Uintatherium) zone as 

 the result of a local immigration from the adjacent 

 Rocky Mountain or Plains region into the Bridger 

 Basin, and not as the result of a continental immigra- 

 tion such as is made manifest in the lower Eocene. 



Bridger D. — Upon the "Lone Tree white layer" lie 

 the 375 feet of strata that form Bridger D, in which 

 are found five faunistic levels, D 1 to D 5. The fos- 

 siliferous sediments of this closing period of the 

 Bridger consist of 350 feet of "gray and greenish-gray 

 sandy and clayey tuffs, with one or more ash beds," 

 including the upper "white layer," which lies about 75 

 feet below the top of the formation. Among the tita- 

 notheres of this zone are descendants of species of 

 Palaeosyops, Limnohyops, Manteoceras, and Telma- 

 therium, which continue to increase in size and which 

 represent advancing mutations that are exhibited in 

 the comparative measurements shown in the tables on 

 pages 304, 313, 341, 364. It is noteworthy that there 

 is no very marked faunistic change in the species of 

 titanotheres that persisted from Bridger C to Bridger D. 

 For example, Manteoceras manteoceras persists from the 

 lower to the higher levels, and Mesatirhinus peter soni is 

 recorded in both C 2 and D 3. Exceptions to this 

 slow evolution are seen in two species — Palaeosyops 

 copei, which represents in certain characters an ad- 

 vanced stage of evolution allied to a stage found in the 

 lower sediments of the Washakie Basin, and Telma- 

 therium validum, assigned to Bridger D, which shows a 

 distinct advance upon Telmaiherium cultridens, as- 

 signed to Bridger C 5. 



Bridger E. — Bridger E is theoretically correlated 

 with Washakie B and Uinta B (upper Eocene). The 

 topmost beds of the Bridger formation, 500 feet thick, 

 include sediments that are almost barren of fossils, 

 but the few fragments of mammals they have yielded 

 are of undoubted Bridger age. The 500 feet of soft 

 banded tuff containing at intervals thick layers of 

 volcanic ash indicate increasingly active volcanism. 

 The layers of gypsum found at this horizon were 

 probably deposited in playa lakes (Sinclair, 1906.1), 

 like those in the Humboldt Basin of the present time. 

 The dark-red bands in Bridger E may indicate an arid 

 climate. The correlation of Bridger E with Washakie 

 B, to the east, is purely conjectural, for neither con- 

 tains determinable remains of mammals. Matthew 

 (1909.1, p. 306) attributes the paucity of life in this 

 zone to violent volcanic eruptions, observing that the 

 thick and generally unsorted beds of ash indicate great 

 volcanic activity and that the large amount of gyp- 

 sum and the absence of fossils might be due to the 



consequent destruction of vegetal and animal life, 

 which converted the region into a barren plain that 

 was alternately submerged and desiccated. 



The UintatJierium zone in the Washakie Basin 

 (Washakie A) is described on pages 85, 87, in the 

 description of the deposits of that basin. The barren 

 deposits in the Uinta Basin (Uinta A) that correspond 

 to the Uintatherium zone are described on pages 

 91-92, in the description of the Uinta Basin. 



WASHAKIE BASIN, WYO. 

 STRATIGRAPHY OF THE BASIW 



Deposits and faunal zones. — The Washakie Basin 

 lies about 50 miles east of the Bridger Basin, and the 

 two contain similar volcanic sediments. The basin 

 was described by Hayden in 1869-70 (1871.2, p. 73), 

 and more fully by Cope in 1873 (1873.4). Its fau- 

 nistic levels were studied by the Princeton expedition 

 (Osborn and McMaster, 1881.8) and by expeditions 

 of the American Museum of Natural History, under 

 Wortman (1893, 1895) and Granger (1906). Granger 

 (1909.1, pp. 13-32) gave the first complete and accu- 

 rate description of the geology of the Washakie Basin 

 in his "Faujial horizons of the Washakie formation 

 of southern Wyoming" (1909.1, pp. 13-32). King 

 treated the deposits of the Washakie Basin as of 

 Bridger age and of lacustrine origin. Osborn (Osborn 

 and McMaster, 1881.8) favored the theory of separate 

 deposition, and Scott (1899.1) showed that where the 

 fauna of the Washakie Basin departs from that of the 

 Bridger it approaches that of the Uinta. The dis- 

 covery of the true upper Bridger fauna in horizon A 

 of the Washakie Basin was due to the American 

 Museum expeditions of 1893, 1895, under Wortman. 



The Washakie Basin, with its vivid coloring and its 

 alternation of hard and soft layers of tttft' and sand- 

 stone, affords the most picturesque geologic views to 

 be found in the Rocky Mountain Eocene basins. 

 Haystack Mountain ("Mammoth Buttes" of Cope), 

 a long ridge of badlands near the north end of the basin, 

 which in places rises 400 feet above the plain, forms 

 the northern border of an extensive semicircular 

 "central basin" that has the appearance of a gigantic 

 crater. The floor of this basin is rather level and 

 regular, being broken only by a few low tables and 

 buttes, which have long been preserved from erosion 

 by their capping of hard sandstone, though their 

 sides are trenched by innumerable deep, vertical- 

 walled canyons, which present a great variety of 

 architectural forms that are illuminated by brilliant 

 coloring. 



Washalcie A {TJintaiherium zone, middle Eocene). — ' 

 The "lower brown sandstone" of the Washakie Basin, 

 known as Washakie A (fig. 60), contains the fauna of the 

 Uintatherium- Manteoceras- Mesatirhinus zone. It was 

 deposited contemporaneously with the upper Bridger 

 (Bridger D), to the west, and probably with the non- 



