434 PROCEEDINGS OF THE ACADEMY OF 



the remains of the genus Crocodilus, as well as the affinities of the mollusca 

 found in these beds to southern forms, all point rather to the existence here 

 of a tropical than a temperate climate during their deposition. 



In regard to the relations of this formation to known horizons in the 

 Tertiary of the Old World, we scarcely feel prepared to express a very decided 

 opinion. The difficulty in the way of drawing inferences bearing on this point, 

 from the remains of mollusca found in these beds, is, that they, being fresh 

 and brackish- water types, bear little or no analogy to those of the Tertiaries 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 formation 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 dis- 

 tinguished Comparative Anatomist, Prof. Leidy, with types even older than 

 the Tertiary, are facts strengthening the impression that this Fort Union Lig- 

 nite Group probably represents 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 System of this country, than in 

 that of the Old World. 



As the Wind River Deposits have not yet been seen in contact with any 

 well marked beds of the other Tertiary formations of this region, and few fos- 

 sils 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 horizon of one of these rocks, or even represent them 

 both in part, or, what is more probable, it may occupy an intermediate chrono- 

 logical 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 Vivi- 

 para. 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 

 tho e hitherto found in any of the other Nebraska rocks. The Vivipara seems 

 to be undistinguishable from our V. trochijbrmis, from the Fort Benton Group ; 

 though, as it is a mere cast, itjcannot be identified with positive certainty with 

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



Tue White River Group is the formation that has furnished the extensive 

 and interesting collections of vertebrate remains, which have been so ably in- 

 vestigated by Prof. Leidy. It occupies a considerable area in the region of 

 White River, and is seen beneath the succeeding formation on the Niobrara 

 and Platte Rivers. Its position above the Fort Union, or Great Lignite Group, 

 has also been clearly and satisfactorily 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 



[Dec. 



