COPE ON GEOLOGY AND VERTEBRATE FOSSILS. 567 



Dr. Haydeu has already pointed out the great extent of this formation 

 traversed by the Missouri. He remarks (1. c, p. 120) : — " Near the mouth 

 of the Niobrara River, No. 4 appears upon the summits of the bluffs, 

 surmounting No. 3. At the foot of the ' Big Bend ', No. 3 passes 

 beneath the water-level of the river, and is succeeded by No. 4, which 

 occupies the country to Grand River, where No. 5 makes its appearance 

 on the summits of the hills. Near the mouth of the Cannon Ball River, 

 the Lignite Tertiary begins to overlap the Cretaceous strata, but does not 

 entirely conceal them along the banks of the river until we reach Square 

 Buttes, about 30 miles below Fort Clarke. From this point to Milk 

 River, in latitude 48°, longitude 106°, only the Miocene beds of the 



great lignite basin are exposed The Tertiary beds continue 



to overlap the Cretaceous, gradually thinning out upon the summits 

 of the hills until we reacli the mouth of the Muscleshell River, 

 where the Cretaceous bed No. 4 occupies the whole country." This 

 account of the distribution of the strata is confirmed by my own obser- 

 vations, but I would add that it does not include clear mention of the 

 extensive exposure of the Pierre formation over the country between 

 the Poplar River, 80 miles above old Fort Union, and the Round Butte, 

 220 miles farther up. 



In approaching the mouth of the Judith from the northwest, the bluffs 

 were observed to have the section represented hy Fig. 1, Plate 30. 



The section rexjresented by Fig. 2, Plate 30, was taken from a bluff 

 formed of strata uplifted at an angle of 40°, dipping northeast. It is on 

 the north side of the Missouri, twelve miles east of the above locality: — 



An exposure very similar to the last was observed on Dog Creek, 

 three or four miles from its mouth, where the thick bed of lignite already 

 mentioned occurs. In the black shale at its summit, I obtained con- 

 siderable numbers of a small and elegant Baculites. 



These sections resemble very closely two given by Dr. Hayden (1. c, 

 p. 127) as occurring on the eastern border of the Judith Basin near 

 Amell's Island, 60 miles distant. This identity of structure of the 

 opposite borders of the basin has an important bearing on the question 

 of the relative age of the Fort Union beds lower down the river. 



As regards the homotaxy of the black shale No. 4, some light is 

 thrown on it by the Vertebrate fossils which I obtained, although these 

 are not numerous at any locality that I visited. [ obtained bones of 

 several species of Pythonomorpha, one of which is a Mosasaurus. An- 

 other is represented by a humerus which resembles that of a Clidastes 

 or Pl'ttecarpns. The greater part of the skeleton of a huge Elasmosaunis 

 was discovered,* whose vertebne so strongly resemble those of E. orien- 

 talis that I am unable to distinguish them. The premaxillary bone of 

 Enchodus is a common fossil. Now Elasmosaurus orientalis is cbaracter- 



* And obtained chiefly through the assistance of George B. Clendenniug, of Carroll, 

 and Capt. Nicholas Bnesen, of Yankton, to whom I must express my sense of the obli- 

 gation I am under. 



