24 GEOLOGICAL SURVEY OF THE TERRITORIES. 



Tertiary of the States bordering on the Atlantic, nor are any of them, so 

 far as known, specifically identical with foreign forms. 



"When we bear in mind, however, the fact, that wherever this forma- 

 tion has been seen in contact with the latest Cretaceous beds, the two 

 have been found to be conformable, however great the upheavals and 

 distortions may be, while at the junction there seems to be a complete 

 mingling of sediments, one is strongly impressed with the probability 

 that no important member of either system is wanting between them. 

 This view is also rendered more probable by the fact that the formation 

 under consideration is known to hold a position beneath the White River 

 group, which is characterized by the remains of an entirely different 

 fauna, clearly of Miocene age. 



"Again, the occurrence in this lower group of remains of the genus 

 Lepidotus, which is, we believe, in Europe unknown above the Eocene, 

 while the other vertebrate remains found associated with it have been 

 compared by the distinguished comparative anatomist, Professor Leidy, 

 with types even older than the Tertiary, are facts strengthening 

 the impression that this Fort Union Lignite group probably repre- 

 sents the Eocene of Europe. 



" It should not be forgotten, however, that an extensive and beautiful 

 series of fossil plants from this formation, although not yet thoroughly 

 investigated, have been thought by Dr. Newberry to be most analogous 

 to Miocene types. 



"Yet even if this formation should prove to be of Eocene age, this 

 would only be in accordance with what is now known in regard to the 

 earlier introduction of particular types of plants in the Cretaceous sys- 

 tem of this country than in that of the Old World. 



" As the Wind Eiver deposits have not yet been seen in contact with 

 any well-marked beds of the other Tertiary formations of this region, 

 and few fossils have yet been found in them, their position in the 

 series remains doubtful. It is, therefore, only provisionally that we 

 have placed this formation between the Fort Union and White River 

 groups in the foregoing section. It may possibly belong to the horizons 

 of one of these rocks, or even represent them both in part, or, what is 

 more probable, it may occupy an intermediate chronological position. 



" The only fossils yet found in this formation are fragments of Trionyx 

 and Testudo, together with the shells of two species of Helix and a cast 

 of a Vivipara. One of these Helices is more like H. Leidyi from the 

 White River group than any of the other species yet known from any 

 of these rocks, while the other is a very large depressed species of 

 southern type, quite unlike any of those hitherto found in any of the 

 other Nebraska rocks. The Vivipara seems to be indistinguishable 

 from our V. trocliiformis from the Fort Benton group, though, as it is 

 a mere cast, it cannot, be identified with certainty with that 

 shell. No marine or brackish-water fossils have been found in these 

 beds. The White River group is the formation that has furnished the 

 extensive and interesting collections of vertebrate remains which have 

 been so ably investigated by Professor Leidy. It occupies a consider- 

 able area in the region of White River, and is seen beneath the succeed- 

 ing formation on the.Niobrara and Platte Rivers. Its position above 

 the Fort Union or Great Lignite group has also been clearly and satis- 

 factorily determined. 



" This formation is mainly composed of a series of whitish, indurated 

 clays, which have been worn and cut, by the streams, rains, and other 

 atmospheric agencies, into numerous deep valleys and ravines, so as to 

 leave various peaks, isolated columns, towers, etc., presenting, as seen 



