374 F. H. KNOWLTON 



unconformity are so well exhibited are all adjacent to the anti- 

 clinal uplift which Calvert has shown extends southeast from the 

 vicinity of Glendive, Montana, to the western line of the Dakotas. 

 Here the uplift tilted the beds and accelerated the erosion, while 

 in the flat country to the westward in Converse County and adja- 

 cent areas, the erosion of the Fox Hills was relatively uniform, and 

 when the Lance formation was later laid down over this surface 

 the unconformable relations are difficult of detection. But as 

 Cross long ago stated: 1 "The visible conformity between the Cera- 

 tops beds and Fox Hills in Converse County cannot be accepted, 

 contrary to other evidence, as proving the former to have been 

 deposited in the epoch next succeeding the Fox Hills." 



UPPER LIMIT OF THE LANCE FORMATION 



In my original paper on the Lance formation (" Cera tops beds") 

 I stated that everywhere throughout the vast region studied it 

 was found conformably overlain by the acknowledged "yellow" 

 Fort Union, adding that "of the many workers who have observed 

 the field relations at hundreds of points, not one, so far as known 

 to the writer, has recorded the presence of unconformity between 

 them." Field work during the past two seasons has confirmed 

 this statement in every particular, and there is yet to be observed 

 a single locality at which unconformable relations have been even 

 suspected. Hence it seems to have been demonstrated that 

 sedimentation from one to the other was continuous and unin- 

 terrupted. 



At the time the original paper was published it was thought 

 that the Lance formation and the acknowledged Fort Union (the 

 lower and upper members of the Fort Union as they were there 

 called) might usually be separated on lithologic grounds, the lower 

 being generally dark and somber-colored and the upper usually 

 yellow. Subsequent investigation, however, has failed to confirm 

 this, for while in individual sections, or even within limited areas, 

 a provisional lithologic separation may often be made, when 

 regional studies were undertaken it was found that the lithologic 

 difference was so variable within short distances as to be wholly 

 1 U.S. Geol. Survey, Mon. 27 (1896), 236. 



