34 



of a ? worm in a hard layer. On top of these are clay and slate rocks of a 

 muddy-yellow color, with their various ledges rising to perhaps two hundred 

 feet. Continuing now to the southeastward, along the old stage-road, we cross 

 South Bitter Creek at the old Laclede station. Some miles south and cast 

 of this point, a band of buff sandstones form a bluff of fifty or more feet in 

 elevation. Below it lie more white or ashen beds, which contain i-emains of 

 mammals and turtles, rather decayed. A short distance beyond these, and 

 forty miles from Black Butte station, we reach tiie base of the enormous pile 

 of sediment which I have called the Mammoth Buttes. These form a horse- 

 shoe-shaped mass, the concavity presenting south and eastwardly, the summit 

 narrow, serrate, and most elevated to the east, and descending and widening 

 toward the south. I estimated the height of the eastern end to be at least one 

 thousand feet above the plain surrounding it. Numerous mammalian remains^ 

 demonstrated that this mass is a part of the Bridger Eocene; although, as Mr. 

 Emmons, of King's survey, informs me, no continuous connection with the 

 principal area west of Green River can be traced. The total thickness of 

 the Green River and Bridger formations on this section cannot be far from 

 two thousand five hundred feet, at a very rough estimate. 



The point of transition from the Cretaceous to the Tertiary deposits, as 

 indicated by the vertebrate remains, is then in the interval betvi'een the last 

 plant-bed at the summit of the buff mud-rocks and the mammal-bone deposit 

 in the lowest of the ash-gray beds. Below this line, the formation must be 

 accounted as Cretaceous, on account of the presence of the dinosaurian Aga- 

 thau?nas sylvestris, and those above it, as I have already pointed out, Eocene," 

 on account of the types of Mammalia contained in them. 



The authorities on the Bitter Creek formation have presented views 

 more or less at variance with those entertained by the writer, or of such a 

 dubious character as to fall very far short of the requirements of evidence. 

 Dr. Hayden has regarded them as Tertiary and as transitional from Creta- 

 ceous to Tertiary. Mr. King, in his very fall article on the Greon River Basin, 

 definitely refers the lower part of the series to the Cretaceous, in the follov/- 

 ing language:^ "We have then here the uppermost members of the Cretaceous 

 series laid down in the period of the oceanic sway, and quite freely charged 



'See tlio Monster of MaiDuioth Buttes, Penn Monthly Magazine for .ingnst, 1873. 

 ^Ou Bathmodon, an extinct genus of Ungulates, February 10, 1872, Hayduu's Annual Keport (1872), 

 p. 431 ; Annual Eeport for 1872, p. 645. 



3 Exploration of tlic Fortieth Parallel, p. 458. 



