COPE ON GEOLOGY AND VERTEBRATE FOSSILS. 575 



the Judith Eiver is continuous and uuiuterrupted by any non -conformity 

 or hiatus. They appear to have been deposited in regular sequence, 

 and without any other disturbance than that oscillation of the bed of 

 the sea which causes change in the character of the sediment. As has 

 already been shown by Hayden in other localities, the transition from the 

 marine to the lacustrine stage has been gradual, and the observation here 

 recorded of a bed of Oysters at the summit of thelacustriue series indicates 

 a return by subsidence to the brackish condition at least. In this view of 

 the history of the deposits, the striking and abrupt change in the animals 

 becomes interesting. The Pyrulas and Neritas are replaced by JJnios 

 and 2Ielanias. The Sharks, Chimseroids, and Encliodus are succeeded by 

 Lepidosteus. The Sauropterygia and Fythonomorpha, with their paddle- 

 like extremities adapted for marine life, utterly disappear, and are fol- 

 lowed by a numerous population of Dinosaiiria, whose pillar-like limbs 

 bespeak support on firm ground. The Crocodiles and Turtles, whose 

 habitat is on the shore, taking to land or water with equal facility, 

 are present in both, although the excess of Turtles is largely in the 

 fresh- water formation. 



That this marked change was due to extinction of the marine forms 

 on the one hand is highly probable, but that it was produced by the 

 new creation of terrestrial types on the other cannot be admitted until 

 the evidence in favor of the entrance of the land animals into the newer 

 portions by migration from the older regions of the land has been ex- 

 plained away. The JDinosauria formed part of the terrestrial animal 

 population of the northern hemisphere from the period of the Trias, 

 and it is to be supposed that regions peopled by them during the time in 

 which the country of the Missouri was beneath the ocean, furnished the 

 immigrants for the new territory so soon as it rose above the waves. 



Such a change in the physical condition of the surface of the earth as 

 is exhibited in the region above described, is regarded by geologists as a 

 time-boundary, such as defines the close of one period and the commence- 

 ment of another. The question to be solved in the present instance is 

 whether this boundary separates entire formations, or only the epochal 

 divisions of one formation. Dr. Hayden, Mr. Lesquereux, and others, have 

 given an affirmative answer, believing that the change from the marine 

 to lacustrine deposits marks the passage from Cretaceous to Tertiary 

 time. Others, including the writer, have maintained that the fresh- water 

 beds represent but a phase of Cretaceous time, and that the separation of 

 this formation and the Eocene Tertiary is between this horizon and 

 that of the Wahsatch epoch. In support of this vi6w, I have hereto- 

 fore* cited the presence of the Mesozoic types of lieptiUa, the Bino- 

 sauria and Sauropterygia, as having been found in the Lignitic beds of 

 this epoch at various localities. The only opposing evidence which is 

 to be found in the Vertebrate fauna is the presence of the genus Lepi- 

 dosteus, which is only known to occur otherwise in Tertiary formations. 



* Trans. Anier. Philos. Soc, 1869, p. 243 ; Bull. U. S. Geol. Suiv. Terrs., ii, lti74 (2). 



