2l8 KNOWLTON 



formity. Mr. Barnum Brown ^^ has clearly recognized the validity 

 of this position, concerning which he writes as follows: 



Strictly following King's definition of Laramie, neither of these 

 deposits [''Hell Creek beds," "Ceratops beds." etc.] can be con- 

 sidered as such, for neither one represents a continuous sedimen- 

 tation from the marine Fox Hills. They should therefore be 

 grouped with the Livingston, Denver, and Arapahoe beds and may 

 be considered Post-Laramie. 



This interpretation by Brown makes the unconformity at the 

 base of the lower member of the Fort Union the same as that 

 demonstrated by Mr. Whitman Cross as occurring at the top of 

 the Laramie in the Denver Basin of Colorado. In the latter 

 area, while an unknown thickness (estimated by Cross at 12,000 to 

 15,000 feet) has been removed, a considerable thickness of Laramie 

 beds still remain, whereas in the areas covered by the lower Fort 

 Union, the Laramie, as well as the Arapahoe and Denver, or their 

 equivalents, if ever present, has been entirely removed so far as 

 known, and the beds rest on other members of the Upper Cretaceous 

 series. A comparison of the conditions of sedimentation in the two 

 sets of beds, as indicated by their invertebrate fauna, confirms their 

 distinctness. Thus, the Laramie is described as a series of brackish- 

 and fresh-water beds, indicating transition from estuarine or marine 

 conditions, while the Fort Union was laid down in fresh water, and 

 affords little or no convincing evidence of even temporary or occa- 

 sional incursions of the sea.^^ 



Paleontological Characters of the Lower Fort Union. 



I. plants. 



We may now proceed to the consideration of the various lines of 

 paleontological data, beginning with that of the plants. On review- 

 ing the foregoing account it appears that plants have been obtained 



" Bull. Am. Mus. Nat. Hist., vol. 23< 1907. P- 845. 



•' The possible exceptions are a single species of Corbicida reported by Whit- 

 field from the upper portion of the "Hell Creek beds, " this genus being found 

 in both brackish and fresh waters, and Ostrca glabra reported from the sec- 

 tion near Yule, North Dakota. 



