INTRODUCTORY REMARKS. XL VII 



back, is al the line separating the Niobrara from the Fori Pierre group: and 

 thai the Upper Missouri beds above this horizon, as stated above, represent 

 the Upper or .White Chalk, and those below il the Lower or Gray Chalk, 

 and perhaps also in |nir! the Upper Grreensand, of the English geologists. 

 Hence this line of division assumes importance from a broader view of the 

 subject than from any reasoning based on its known persistence from Fort 

 Benton in the far north, to Texas and New Mexico in the distant south. 



FRESH- AND BRACKISH- WATEK LIGNITE DEPOSITS OF THE UPPER MISSOURI. 



In regard to the Cretaceous age of all the rocks of the Upper Missouri sec- 

 lion given on pages xxiv and xxv. it will be seen that there is now no difference 

 of opinion. Just here, however, that is, in passing from the Fox Hills group 

 or tipper member of that section into the lignite-bearing rock above, we enter 

 upon disputed ground. Although not very much prominence has generally 

 been given to the fact, especially by those who have attempted the correlation of 

 these deposits with particular rocks elsewhere in the far-west, it is never- 

 theless true, that there appear to be two distinct groups of lignite-bearing 

 strata in the Upper Missouri country, that are often spoken of and viewed 

 as if they formed one homogeneous rock. These are (he Judith River and 

 Fort Union groups. 



Although we have long regarded the Judith River beds as forming a 

 distinct group older than the Fort Union deposits, the difficulty, without 

 more detailed stratigraphical examinations throughout considerable areas, of 

 deciding to which of these horizons particular species only known from 

 isolated localities properly belong, prevented the arrangement of the figures 

 of the fossils from these groups in all cases on separate plates. For the 

 same reason, the species have all, from both of these groups, been described 

 together in the body of this work under one general head, though the horizon 

 to which each belongs has, when certainly known, been definitely stated in 

 connection with its description; and where doubts existed on this point, the 

 horizon to which species are believed to. belong, has been less confidently 

 stated. In these remarks, however, the two groups will be separately con- 

 sidered as follows: 



Judith River group. — The first information in regard to this group 

 was derived from the observations and collections of Dr. Hayden during his 

 two years' sojourn in this distant country. It was first examined by him at 

 the typical locality near the mouth of the Judith River on the Upper Mis- 



