GEOLOGY LIGNITIC GROUP. 



23 



braska Tertiary rocks consists of three or four groups, three of which 

 at least (and probably four) evidently belong to separate and distinct 

 epochs. They usually occur in isolated basins, but have, with one ex- 

 ception, all been seen in such connection as to leave no doubt in regard 

 to their order of superposition. 



" Their prevailing lithological characters, estimated maximum thick- 

 nesses, and order of succession will be seen in the section given below. 



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Subdivisions. 



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Localities. 



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Fine loose sand, with some layers of limestone ; 

 contains bones of Canis, Felis, Castor, Equus, 

 Mastodon, Testudo, etc., some of which are 

 scarcely distinguishable from living secies. 



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On Loup Fork of Platte Eiver, 

 extending north to Niobrara 

 Eiver, and south to an un- 

 known distance beyond the 

 Platte. 



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White and light drab clays, with some beds of 

 sandstone find local layers of limestone ; fos- 

 sils, Oreodon, Titanotherium, 'Cheer opotamus, 

 Rhinoceros, Anchitherium, Hyaenonodon, Ma- 

 chairodus, Irionyx, Testudo, Helix, Planorbis, 

 IAmncea, petrified wood, etc., etc. All extinct. 

 No brackish-water or marine remains. 



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Bad Lands of White Eiver, un- 

 der the Loup Kiver beds on 

 Niobrara, and across the 

 country to the Platte. 



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Light gray and ash-colored sandstones, with 

 more or less argillaceous layers. Fossils, 

 fragments of Trionyx, Testudo, with large He- 

 lix, Vivipara, petrified wood, etc. No marine 

 or brackish-water types. 



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Wind Eiver Valley, also west 

 of Wind Kiver Mountains. 



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Beds of clay and sand, with round ferruginous 

 concretions, and numerous beds, seams, and 

 local deposits of lignite ; great numbers of 

 dicotyledonous leaves, stems, etc., of the gen- 

 era Platan/us, Acer, TJlmus, Populus, etc., with 

 very large leaves of true fan palms. Also, 

 Helix, Melania, Vivipara, Corbicula, TJnio, Os- 

 trea, Potainomya, and scales of Lepidotus, with 

 bones of Trionyx, Emys, Compsemys, Crocodi- 

 lus, etc. 



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Occupies the whole country 

 around Fort Union, extend- 

 ing North into the British 

 possessions to unknown dis- 

 tances ; also southward to 

 Fort Clarke ; seen under the 

 White River group, on North 

 Platte Eiver, above Fort Lar- 

 amie ; also on west side of 

 Wind Eiver Mountains. 



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"The Fort Union, or Great Lignite group, occupies extensive areas of 

 country in Nebraska, and has been seen beneath the White Eiver group 

 at several distant localities. It was evidently deposited in large bodies 

 of water, which were at first brackish, and then gradually became fresh. 



« The great number of fossil leaves, and numerous beds of lignite con- 

 tained in it, clearly show that the shores of these ancient estuaries, 

 lakes, etc., in which this formation was deposited, supported dense 

 forests of large trees, and a growth of other vegetation, far exceeding 

 in luxuriance anything now met with in these latitudes. 



" Indeed, the presence of true fan palms, of large size, and 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 remaius of Mollusca found in these beds is that they, being 

 fresh and brackish water types, bear little or no analogy to those of the 



