218 DOUGLASS — CRETACEOUS AND LOWER TERTIARY. [Aprils, 



Viburnu??! asperiun ? Newb. • 



Populus cuneaia Newb. 



Populus sp. 



Plat anus nobilis Newb. 



Platanus basilobata Ward. 



Viburnum sp. 



Paliurus sp. 



Grewiopsis viburnifoUa Ward. 



Populus .? n. sp. 



Mr. Knowlton says : '' The species are all Fort Union beyond a 

 doubt." 



Of a {^\N shells which I enclosed, he writes: ''The shells I 

 showed to Mr. Stanton, and he says that the two large ones are 

 Unio Couesi White ; and the other pretty near to Unio Endlichi 

 White." 



The Mammals were found in the shale. The collection of fossil 

 leaves was made in the sandstone a little higher up, though there 

 are concretions and layers of sandstone that contain leaves in the 

 same beds as the Mammalian remains. A portion of the collection 

 was obtained on Sweetgrass Creek north of east of Big Timber, in 

 the locality mentioned above. 



General Observations. 



The problem of greatest interest connected with the study of 

 this section is that relating to the transition from Mesozoic to 

 Cenozoic times. Of course, if deposition had been continuous, or 

 nearly so, and there were no great faunal or floral migrations, there 

 could be no distinct boundary between the two. There is a great 

 difference between the Cretaceous as a whole and the Tertiary as a 

 whole, but where are we to draw the line? If there was a time of 

 widespread or general upheaval throughout the western portion of 

 the continent, or of the Rocky Mountain region, this might form 

 a convenient division. Upheavals and great volcanic activity cer- 

 tainly occurred in restricted localities, but we cannot at present 

 prove that such were general or that they did not occur in different 

 places and at different times. If we could point to any time when 

 the Di7iosaui's ceased to be and the higher orders of Manwials took 

 their places, then the matter would be easy ; but heretofore most of 

 the Cretaceous Dinosaurs, in fact nearly all of them, have been 



