INTRODUCTORY REMARKS. !,V 



there, as they maintain, at many places in this region; that is, there is 

 evidence of the erosion into little irregularities of the upper surface of the 

 bed just below this horizon previous to t he deposition of that just above; 

 while (hey argue that no other similar break occurs in this region at any 

 other horizon at which the line could be drawn. 



Those gentlemen, however, at the same time frankly admit that there is 

 no corresponding change in the general character of the organic remains at 

 this horizon. It' they are correct, however, in drawing the line here, this 

 would accord almost exactly with the original reference by the author, of the 

 fossils from the Hallville mine to the Lower Eocene, and those from near 

 Point of Rocks to the Cretaceous, without any knowledge of the strati- 

 graphical relations of the beds at these localities, or of the existence of any 

 apparent physical break between the two horizons Still, after seeing the 

 remarkable uniformity of the fossils above and below this horizon, the genera 

 being nearly all the same, and the species often very closely allied, it is 

 difficult to remove from the mind the impression that the whole should go 

 together as one group. 



Fort Union GROur. — Nearly all of the more extensive and important 

 lignite beds of the Upper Missouri country seem to occur in this group, 

 which also occupies a much more extensive surface-area there than the Judith 

 River group. Whether or not there is any kind of physical break between 

 them where they occur together, the writer has no satisfactory information; 

 but it is very probable that there is none whatever. Owing to this fact, it is 

 also doubtful whether we have been able to refer a few of the fossils described 

 in this report exactly to their proper horizons. This is probably also the ease 

 with some of the vertebrates that have been described by others from the 

 Upper Missouri as coming from the Fort Union group, or in a more general 

 way referred to the Upper Missouri Lignite formation. At any rate, the 

 invertebrate fossils from the Judith River group at the typical locality, differ 

 from those here regarded as the characteristic Fort Union group types to 

 such an extent as to warrant the separation of the beds from which they 

 came as a distinct group. The Fort Union species occupy plate 44 and 

 part of plate 43. 



Wherever seen resting on the weft-marked. Cretaceous, the Fort Union 

 group is always found to be conformable to the latter, whether upheaved or 

 lying undisturbed. 



Lewis and Clarke, it is believed, were the first explorers who brought 



