156 STANTON & KNOWLTON — LARAMIE AND RELATED FORMATIONS. 



very reasonable query whether the beds containing them at these places 

 also are not younger than the true Laramie. The facts we have presented 

 relative to the stratigraphy and paleontology of the Black Buttes dino- 

 saur horizon seem to us convincing that it is in the Laramie and near 

 the base of that formation. It is less than 200 feet above the marine 

 Cretaceous, and there is no evidence of a break * nor any abrupt litho- 

 logic change. The character of the flora and of the invertebrate fauna 

 also, so far as the species have a distribution in recognized horizons else- 

 where, favors its reference to the Laramie. If the Dinosaur bed of Black 

 Buttes is not Laramie, then the Laramie is either absent or is represented 

 only by about 100 feet of sandstone. The overlying beds up to and in- 

 cluding strata with a Fort Union flora seem to form a continuous series 

 that is indivisible either structurally or lithologically, and we can see no 

 reason for placing the top of the Laramie lower than the base of the 

 lowest bed with a Fort Union flora. 



Closely similar conditions are seen in Converse county, the principal 

 difference being a greater development of the beds. 'the sandstones at 

 the base overlying the Fox Hills are a few hundred feet thick, and the 

 variable, more argillaceous, higher beds, with a fresh-water fauna in 

 large part identical with that at Black Buttes and a flora that also in- 

 dicates the same horizon, have a much greater thickness. Here again 

 there seems to be no break in a series that has Fort Union plants in its 

 upper member. The abundant occurrence of such a species as Campe- 

 loma multilineata throughout all but the lowest portion of the series 

 argues strongly for continuous sedimentation. 



The difficulty of recognizing unconformities in beds so little disturbed 

 has not been overlooked, and the possibility that there may be such un- 

 discovered breaks in these two areas is freely admitted, though it does 

 not seem to us probable. From the facts now available it seems most 

 probable that in Converse county and in the Bitter Creek valley the time 

 representatives of the Denver and Arapahoe beds are undifferentiated 

 portions of a continuous series and cannot be separated from the Lara- 

 mie.f The Fort Union beds are apparently distinguishable by means 

 of their flora, and these mark the upper limit of the Laramie in the areas 

 in question. 



*The supposed unconformity between Powell's Point of Rocks and Bitter Creek groups has no 

 bearing on this question, since it is below marine beds belonging to the Fox Hills. 



fThis is essentially the view suggested by Dr C. A. White in Bull. U. S. Geol. Survey no. 82, 

 p. 150. 



