1872, (publisliecl on August 12,) is apparently regarded by Mr. Meek 

 also as representing a distinct epoch.l He says: "The invertebrate 

 fossils yet known from this formation are in their specific relations, with 

 possibly two or three exceptions, new to science and different from those 

 yet found either at Bear Eiver, Coalville, or indeed elsewhere in any 

 established horizon, so that we can scarcely more than conjecture from 

 their specific affinities to known formvS as to the probable age of the 

 rocks in which we find them." On this account, and because of the 

 great stratigraphical differences exhibited by the Bear Eiver and Evans- 

 ston coal strata, I have followed Hayden in regarding the Bear River 

 group on the west side of the Bridger Basin as representing a distinct 

 series of rocks, with present knowledge. On this account I omit, as 

 heretofore, allusion to determinations of age of the latter formation as 

 irrelevant in discussing the age of the Bitter Creek epoch.* 



My own observations on the relations of these rocks, made during 

 the summer of 1872, have been in measure anticipated by the detailed 

 reports of Messrs. Meek and Bannister, t which, with the older observa- 

 tions of Dr. Hayden and Mr. Emmons, (of King's survey,) leave little 

 to be added. However, as none of these gentlemen paid especial atten- 

 tion to the vertebrate paleontology, the bearing of this department in 

 relation to the stratigraphy remains to be explained. 



As Dr. Hayden remarks, the Union Pacific Railroad, at Black Butte 

 Station, passes through a mouoclinal valley, the rocks on both sides 

 having a gentle dip to the southeast. This dip continues to the east- 

 ward to near Creston, where the beds pass under the newer tertiary 

 strata. Following the railroad westward from Black Butte, the same 

 dip continues to near Salt Wells, w^here we cross an anticlinal axis, the 

 dip of the strata being gentle to the northwest. There are minor vari- 

 ations in the dip, but the general result is as stated. They disappear, 

 five miles east of Rock Spring Station, beneath the latter beds of the 

 Green River Tertiary, which at this point presents a line of strike, ex- 

 tending northeast and southwest across the raib-oad in the form of a 

 range of bluffs of considerable elevation. They are composed of lighter 

 colored and softer material than the Bitter Creek strata. The latter 

 consists of alternating beds of hard and soft sandstone, with argillaceous 

 and carbonaceous strata. The upper part of the series contains eleven 

 coal-strata; at Rock Spring I was informed that the upper was ten feet 

 in thickness, and the next four feet. Returning eastw^ard, the heavier 

 bedded sandstone is low in the series at Point of Rocks, in consequence 

 of the southeast dip ; and the upper beds are softer and abound in fossil 

 shells. At Black Butte Station, the heavy sandstone bed disappears 

 from view toward the east, and the eleven coal-strata appear above it. 

 About twenty feet above the sandstone, between two of the thinner 

 beds of coal, the bones of the Agatliaumas sylvestris were found embedded 

 in leaves and sticks of dicotyledonous plants, cemented together by 

 sand and clay. Where the heavy sandstone bed disappears below the 

 level of the track of the railroad, in the course of its eastern dip, a 

 thin bed of coal just above it soon follows ; then a bed of shells con- 

 taining oysters, more and less nu.merous at different points, may be 

 traced for some distance before it also disappears. Near the latter point 



tHayden's Annual Report, 1872, pages 459, 461, pnblislied April, 1873. 



* This course has been misunderstood by Mr. Meek and others as implying a design 

 to ignore those determinations. Both Mr. Emmons and Mr. Meek are clear in the ex- 

 pression of their conclusions as to the age of the Bear Eiver epoch. 



t See Hayden's Annual Report, 1872, pp. 457, 525. 



