18 BULLETIN 16 9, UNITED STATES NATIONAL MUSEUM 



As already noted, there are no exactly identifiable mammals from 

 Loc. 65, but the fragments found here are of Tertiary rather than 

 Cretaceous aspect, and the mammals from Loc. 78 are certainly 

 Paleocene and probably Middle Paleocene. It seems probable that 

 the whole Fort Union No. 1 here is of Middle Paleocene age and all but 

 certain that it does not include the earliest Paleocene. 



There are two possible interpretations of this section. First, the 

 usual view, in analogous cases, that the Bear is Cretaceous, equivalent 

 to part of the Lance, with the necessary corollary here that part of 

 the Paleocene, a Puerco equivalent, is probably lacking. This in- 

 terpretation is possible, but it seems to me decidedly the less likely of 

 the two. The invertebrates indicate Paleocene and probably not 

 earliest Paleocene. Even if it be decided that they should be con- 

 sidered basal Fort Union rather than uppermost Bear, they are inti- 

 mately associated with the latter and in beds that grade into it in- 

 sensibly. This gradation itself opposes this interpretation, for it 

 makes the presence of an unconformity here, or a gap representing 

 Puerco time, seem unlikely. If a break exists it is more likely to be 

 within or below the Bear. The absence of dinosaurs, with the pres- 

 ence of such reptiles as did survive into the Tertiary, in most of the 

 considerable thickness of the Bear is also evidence against this view, 

 negative but of some weight. 



The second possible interpretation is that most of or all the Bear 

 belongs in the Paleocene, probably representing the Lower Paleocene, 

 and that the Cretaceous-Tertiary boundary is in its basal part or below 

 it. All the items of evidence mentioned above favor this view. Op- 

 posing it are the possible presence of dinosaur bones in the basal Bear 

 and the absence of mammals. The dinosaur fragments are, however, 

 such as could very readily be redeposited from erosion of the Hell 

 Creek, and they are confined to the lower 80 feet of a series about 500 

 feet thick. It is entirely possible that there is a disconformity, or at 

 least a valid epoch boundary, in this series at the base of a sandstone 

 above ther level of these bones, and stUl more likely that the dinosaur 

 bones are not really original fossils in these beds. The absence of 

 mammals has no weight. Mammals are rare throughout the field as 

 a whole, are generally excessively rare in the Lower Paleocene, in this 

 field almost never occur in sandstone, which is all that is well exposed 

 here, and even if present would be small forms easily overlooked. 

 The absence of dinosaurs, however, may well be significant, because 

 their bones are large, occur in most terrestrial Cretaceous formations, 

 and are very likely to occur where, as here, there are remains of other 

 types of reptiles. 



Another summary section through the whole Hell Creek and Bear 

 was taken in sec. 15, T. 6 N., R. 15 E. This is continuous with the 

 lower Fort Union section of Stanton and Silberhng given elsewhere. 



