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VI. — THE FORT UNION OR LIGNITE GROUP. 



With this epoch we enter debatable ground, and begin to consider strata 

 deposited in brackish or fresh 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 the 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 lowest 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 the aid of paleontology ; but here, while evidence of inter- 

 ruption is clear, from the relations of the plants and vertebrate 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 the evidence which 

 is accessible in my department. 



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

 acter, all of which 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.) Canon City coals, Southern Central Colorado. 

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

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

 Bear River group, Western Wyoming. To these may be added the Judith 

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

 below the Fort Union series, leaving their final location to future discoveries. 



No vertebrate remains having come under the author's notice from the 

 Placer Mountain and Carion City formations, no further notice can be here 

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

 taceous age, not far removed from the horizon of the coals of Weber River, 

 Utah. The presence of ammonites and baculites above and below them had 

 already indicated such a conclusion to Leconte, 2 as it has in the case of the 



1 Geological Survey of Colorado, 1869, p. 90. 



2 Report on Ibe Geology of tbe Smoky Hill Pacific Railroad route, 1668^ p. 0(3. 



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