ENVIRONMENT OF THE TITANOTHEHES 



79 



places highly colored "badlands." Apparently the 

 greater part if not all of these tuffs were distributed 

 from unlinown eruptive volcanic centers by wind; but 

 at four periods they were deposited in great shallow 

 playa lakes and partly worked over by stream, delta, 

 and flood-plain deposition. From the general absence 

 of coarse materials such as would be transported by 

 streams of high gradient, it is inferred that the Bridger 

 formation accumulated in a relatively level area. 

 (Sinclair, 1906.1, p. 279.) 



Exploration of the Bridger formation. — The Bridger 

 formation has been explored almost continuously by 

 geologists and paleontologists, first by Hayden (1869- 

 1871), next by King (1878), who regarded the Bridger 

 as an ancient lake basin deposit, then by Osborn and 

 Scott (1877-1878), and again by Endlich (1879). 

 In 1902 the American Museum parties, guided by 

 Matthew and Granger, under the direction of Osborn, 



undertook to determine whether the Bridger can be 

 divided into a series of life zones. After four years of 

 careful geologic field work by Granger and Matthew 

 (1902-1905), who had at hand the level record of every 

 specimen, the Bridger was subdivided lithologically 

 and faunistically into five levels, A to E. Bridger A 

 is relatively barren. Of these levels A and B were 

 grouped into the lower Bridger (Palaeosyops paludosus- 

 OroMppus zone), characterized by the absence of 

 Uintatherimn, and C and D, the upper Bridger 

 {Uintatherium- Manteoceras- Mesatirhinus zone), distin- 

 guished by the appearance and great abundance of 

 TJintaiherium. Similar faunistic surveys in the 

 Washakie Basin, east of the Bridger Basin, and 

 in the Uinta Basin, south of the Uinta Mountains, 

 have given very complete correlation of the local 

 subdivisions of the section as follows: 



Correlation of middle and upper (?) Eocene sections of the Uinta, WashaTcie, and Bridger Basins 



Volcanic ash deposits. — The petrographic analysis of 

 the rocks of the Bridger formation serves to support 

 their correlation with the deposits of the Washakie 

 Basin, to the east, and of the Uinta Basin, to the south. 

 The recognition by Sinclair (1906.1, pp. 273-280) of 

 the fact that the entire Bridger series was in large 

 part originally volcanic dust and the later careful petro- 

 graphic analysis by Johannsen (1914.1) led to the 

 conclusion that the Bridger rocks are largely tuffs 

 perhaps modified in part by sufficient transportation to 

 add the numerous grains of quartz they contain, and 

 that these grains may be of sedimentary origin 

 although the material of the tuffs is mostly andesitic. 

 Johannsen's analysis of the Bridger rocks is essentially 

 as follows: 

 Bridger D. Irregular grains of quartzite, feldspar, hornblende, 



etc. : dacite tuff. 

 Bridger C. Fragments of quartz and hornblende; groundmass of 



glass tuff. 



Bridger B. Smith's Fork; fragments of quartz, feldspar, horn- 

 blende: ?dacite tuff. 



Bridger B. Church Buttes; fragments of quartz, feldspar, etc.: 

 altered tuff, probably dacite tuff. 



Bridger A. North of Church Buttes, fragments of quartz, feld- 

 spar, hornblende. No glass tuff seen. 



Thus the Bridger is composed chiefly of dacite tuff, 

 of altered dacite, and of glass tuff containing irregular 

 grains of quartz, feldspar, and hornblende, which 

 are at some places contained in a groundmass made up 

 of entirely coarse angular particles of stringy glass 

 full of bubbles. The Huerfano formation of Colorado, 

 which is in large part older than the Bridger, is com- 

 posed largely of glass tuff. The deposits in the 

 Washakie Basin, east of the Bridger Basin, are com- 

 posed chiefly of dacite and glass tuffs. 



Playa lalce deposits. — Conspicuous features of the 

 Bridger formation are four hard "white layers,'' 

 which were laid down at intervals in the series of beds. 



