INTRODUCTORY REMARKS. 



LIX 



Professor Dawson, the geologist of the British American Boundary Commis- 

 sion, at different localities north of the boundary-line. Some of the brackish- 

 water deposits, however, mentioned by him, as already stated, almost certainly 

 belong to the older Judith River group. 



No continuous detailed section showing all of the beds of the Fort Union 

 group has ever been made out, but Dr. Hayden has roughly estimated its 

 entire thickness at about two thousand feet. The following section, taken by 

 him, of the few beds exposed at and near Fort Union, will give an idea of 

 the general lithological characters of the group as seen at this typical locality, 

 in descending order. 



Section of Fort Union group beds at Fort Union.* 



Of course, the beds of such a formation vary greatly at different locali- 

 ties and different horizons. At some places, too, the beds of lignite are much 

 thicker and of better quality, having a smaller proportion of earthy matter 

 in their composition ; this being one of the more marked distinctions between 

 the Judith River and Fort Union groups. In general, the fossils of this 

 o-ronp (the invertebrates) are strictly fresh-water types, with a few land-shells. 

 Sometimes, however, as at Fort Clarke, we find a species of Corbula among 



* Havden's memoir Geol. and Nat. Hist, of the Upper Missouri, in Amer. Philos. Soc, XII, 96. 



