XXVIII INTRODUCTORY REMARKS. 



fessor Capellini visited together the localities on the Missouri River where 

 these leaves were found in this rock, they were both entirely convinced, as they 

 have since stated in separate papers, that it is clearly Cretaceous, as we had 

 from the first maintained, and as is now admitted by all* (See Bull. 

 Soc. Geol. Francais (2e ser.), XXII, 1864; and Mem. Soc, Helvetique, Sci. 

 Nat., 1866.) 



Professor Mudge and others collected some of these same plants, and a 

 number of other species, from this rock in Eastern Kansas, all of which have 

 been studied and described by Professor Lescpiereux, who, although at one 

 time inclined to -sustain Professor Heer's opinion that they belonged to the 

 Tertiary epoch, now admits that they are Cretaceous species. 



Fort Benton group. — Respecting this and the three succeeding 

 divisions of the section above belonging to the Cretaceous, there have never 

 been any differences of opinion. As usually developed, this rock is composed 

 of dark-gray laminated clays or shale, with, at places, thin seams of lignite. 

 Among its-characteristic fossils may be mentioned Ostrea congesta, Inocera- 

 mus problematicus, Prionocydus Woolgari (— Ammonites Woolgari, Mantell), 

 Scaphites Warrenanus, scales and other fragments of cycloid fishes, etc.f 

 On the Missouri, between the mouths of Big Sioux and James Rivers, it 

 was first observed, and at some localities there it is seen resting upon the 

 Dakota group already described. Here it seems not to attain a thickness 

 of more than ninety to one hundred feet; but at some localities in the Rocky 

 Mountain region it is much thicker. From the incoherent nature of this 

 rock in Eastern Nebraska and the adjacent country, it does not usually form 

 perpendicular outcrops, and consequently is rarely so well exposed as the 

 rock immediately above and that below it. 



This group, or subdivision, has a wide geographical range in portions 

 of some of the States and Territories west of about the longitude of Eastern 

 Nebraska. It is also known to occur, overlaid by heavy deposits of drift and 



* At a still later date, after all the facts iu relation to this formation and the different opinions in 

 regard to its age had been for some time published, we were naturally somewhat surprised to seo Pro- 

 fessor Marcou credited, in an address by the president of a European scientific society on the progress 

 of geological investigations, with having made the remarkable and interesting discovery of a Tertiary 

 flora beneath well-marked Cretaceous strata in Nebraska. This was, however, undoubtedly a mere 

 oversight by tho distinguished author of the address; and it is also duo to Professor Marcou to stato 

 here that he did not himself claim it as an original discovery of his own. 



t Professor Cope has described the following fishes and reptile from this rock, viz : Lamnat cus- 

 pidate, a shakr; Pelecorapis vurius, related ta the flying fishes ; Apsopelix sauriformis, supposed to bo 

 related to the last ; and a Crocodilian that he has called HypoBaurus Vcbbii. 



