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VI. — THE FORT UNION OR LIGNITE GROUP. 



With this o[K)ch wo enter debatable grouiul, and begin to consider strata 

 deposited in brackisli or Cresh waters, which were more or less inclosed by 

 the elevation of parts of the Rocky Mountains and other western regions, and 

 which are, therefore, more interrupted in their outlines than tlu; marine for- 

 mations which underlie them. Dr. Hayden has recognized and located a 

 number of formations of this character, to some of which he has applied the 

 name of " transition-beds.'' That the period of their deposit was one of 

 transition from marine to lacustrine conditions is evident; and that a succes- 

 sion of conformities in position of beds may be traced from the low-cst to the 

 highest of them, and with the Tertiary strata above them at distinct locali- 

 ties, beginning at the south and extending to the north, is also proven by 

 Hayden and others. It appears impossible, therefore, to draw the line satis- 

 factorily without th(! aid of paleontology; but licre, whih; cnidence of inter- 

 ruption is clear, from I he relations of the plants and vertel)riife animals, it is 

 not identical in the two cases, but discrepant. I therefore append a synopsis 

 of the views expressed by authors, with a presentation of tiic evidence which 

 is accessible in my department. 



Hayden has named the following as distinct epochs of transitional char- 

 acter, all of v,hich he originally referred to the Tertiary period. I give them 

 in the order of age which he has assigned to them : ' (1.) Placer Mountain ; 

 locality. New Mexico. (2.) Caiion City coals, Southern Central Colorado. 

 (3.) Fort Union or Lignite group, Dakota, Montana, and Wyoming. (4.) 

 The Bitter Creek series, end)racing the Bitter Creek coals, Wyoming. (5.) 

 Bear River group, Western Wyoming. To these may l)e added the Judith 

 River beds of Montana, which Dr. Hayden has placed, with reservation, 

 below the Fort Union series, leaving their final location to I'utu re discoveries. 



No vertebrate remains having come undcu" the author's notice from the 

 Placer Mountain and Caiion City formations, no further notice can l)e here 

 taken of them beyond the statement that they are, as Meek iiulicates, of Cre- 

 taceous age, not llir removed from the horizon of the coals of WcIxt Kivcr, 

 Utah. The presence of iimnionitcs and liaculites above and below Ihcm h.id 

 alr(;a(ly indicated such a conclusion to Leconte," as it has in the case of the 



' Gi;i)loj;ical Survey ut Colorado, 1869, p. 90. 



- I'cjiorf nil tlic Oi'olosy of tbc Smoky Hill Paeitic Kailroad route, 18(i8, p. 6(i. 



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