CANNONBALL MARINE MEMBER OF LANCE FORMATION 545 



record. Obviously, if a definite correlation can be made of this 

 unconformity in the Coastal Plain region with an unconformity in 

 the Rocky Mountain region a long step has been taken toward 

 settling the problem for all of the western region. 



This leads directly to a second line of evidence — the fossil 

 flora. The recent work of F. H. Knowlton^ and E. W. Berry has 

 shown a close relationship between the floras of the Midway and 

 Wilcox formations of the Gulf region, which is indisputably of age, 

 with the floras of the Raton and the Denver formations Eocene 

 of New Mexico and Colorado. This evidence, together with the 

 fact that the Raton and the Denver and Arapahoe formations rest 

 unconformably on underlying formations which contain totally 

 different fossil floras, has led Knowlton to the conclusion that the 

 Raton and Denver formations are of Tertiary age. This conclu- 

 sion is now accepted by the U.S. Geological Survey, but some 

 geologists question the correlation, as it depends mainly on the 

 evidence of the fossil flora. 



When we turn to the evidence gathered from a study of the 

 marine invertebrates we arrive at a diametrically opposite con- 

 clusion. As has been pointed out in preceding pages, the marine 

 fauna of the Lance formation is distinctly a Cretaceous fauna 

 very closely related to the fauna of the Fox Hills sandstone. It has 

 been pointed out too that the Lance and Fort Union formations 

 should be grouped together both on account of the close relation- 

 ship of their fossil floras and because in places no lithologic separa- 

 tion of the two can be made. Thus the presence of a marine 

 Cretaceous fauna in the Lance indicates that the Lance and prob- 

 ably also the Fort Union should be placed in the Cretaceous 

 system. This would make the line between the Cretaceous and 

 Tertiary correspond with the major unconformity above the 

 Fort Union. It must be admitted that in the Rocky Mountain 

 and the Great Plains regions no marine fauna of undoubted Ter- 

 tiary age is known with which the Lance fauna may be compared, 

 and there is a possibility that in this area the Fox Hills marine 



' F. H. Knowlton, "Results of a Paleobotanical Study of the Coal-bearing Rocks 

 of the Raton Mesa Region of Colorado and New Mexico," Am. Jour. Sci., 4th ser., 

 XXV (1913), 526-30. 



