HELL CREEK BEDS, CERATOPS BEDS AND EQUIVALENTS 21'J 



tain that a time interval is represented during which certain of the 

 Upper Cretaceous sediments and, if my view be correct, also certain 

 lower Tertiary beds, including the Arapahoe and Denver or their 

 equivalents, were removed. It is therefore demonstrated beyond 

 reasonable doubt that the beds under consideration are the same 

 throughout the wide area over which they have now been traced. 

 As they are above an unconformity they can, on the basis of 

 stratigraphy, no longer be considered as a part of the 'conformable 

 Cretaceous series.' 



Relations Between Lower and Upper Members of the Fort 



Union. 



With the exception of the limited area covered by the Kingsbury 

 conglomerate east of the Bighorn Mountains, which as already shown 

 was a comparatively local affair consequent upon the Bighorn up- 

 lift, the lower member of the Fort Union is conformably overlain 

 by the upper member throughout the entire region. 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. The strong lithologic difference, as 

 already described, makes it possible to separate them usually with 

 little difficulty, but the sedimentation appears to have been contin- 

 uous. The possibility of unconformity by overlap, always a diffi- 

 cult condition to demonstrate, has not been overlooked, but if present 

 it should have been detected on the borders of the areas along the 

 mountains, which does not seem to have been done. Therefore, it 

 appears that the lower and upper members cannot be separated on 

 structural grounds. 



Stratigraphic Relations of the Lower Member of the Fort 

 Union to the Laramie. 



The beds that are here regarded as constituting the lower member 

 of the Fort Union have usually been — and by many are still — called 

 'Laramie,' but when it is recalled that, by original definition, the 

 Laramie is the uppermost member of the conformable Cretaceous 

 series above the Fox Hills, it is seen at once that they cannot be so 

 considered since they are separated from the Cretaceous by an uncon- 



