THE LANCE FORMATION 353 



the sea was present while the lower non-marine member of the Lance was 

 being laid down, so that there were continuous marine conditions from 

 Fox Hills time on until the whole of the marine member was deposited. 

 The fossils do not include an}'^ that especially suggest an Eocene fauna, 

 and the geographic location makes it extremely improbable that a marine 

 Eocene fauna could have reached the area, which when located on Pro- 

 fessor Schuchei"t's^^ paleogeogTaphic map of the Eocene is seen to be al- 

 most exactly "at the farthest point from known marine Eocene that can 

 be found in the United States, the nearest Eoeene localities being the 

 Mississippi Valley near the mouth of the Ohio on the one side, and Ore- 

 gon west of the Cascade Mountains on the other. If there was general 

 uplift and long continued erosion throughout the province preceding the 

 Lance, as one current view requires, there would be no place on the conti- 

 nent where the marine fauna of the Lance could have been retained, and 

 it must have come in from some ocean area at least a thousand miles 

 away. In that case it is not probable that it could have been so closely 

 related to the provincial Fox Hills fauna or that it could have escaped 

 bringing Eocene elements with it. That a marine Cretaceous fauna could 

 have persisted in this region after the Eocene was well advanced in the 

 Gulf coastal plain of the United States is, to my mind, too improbable to 

 deserve serious consideration.^* 



Conclusion 



In my opinion, therefore, the conclusion is justified that the Cretaceous 

 period did not end in the Interior Province until the sea had completely 

 retreated from the province, and that the Lance formation should be 

 assigned to the Cretaceous. 



The final retreat of the Cretaceous Sea from the Interior Province was 

 doubtless associated more or less closely with local orogenic movements 

 which caused active erQsion to begin or to increase in various areas ; but 

 in other areas within the province the products of this erosion were laid 

 down as terrestrial deposits, which taken together practically bridge the 

 gap between Cretaceous and Tertiary. The boundary between the two 

 systems in such areas is not marked by an important break caused by 

 general diastrophism, because the breaks and discordance and erosion 



13 Charles Schucliert : Paleogeography of North America. Bull. Geol. See. Am., vol. 

 20, 1908, pi, 96. 



1* That this hypothesis does not seem unreasonable to many of my colleagues is evi- 

 dent from the fact that since this paper was read before the Geological Society the 

 United States Geological Survey has decided to classify the Lance formation, including 

 Its marine member, as Tertiary (?), although the Cretaceous aflSnities of its marine 

 fauna were acknowledged by those who made the decision. 



