IV LETTER TO TOE SECUBTAKY. 



If ohjoction is inaile to the use of the term "Lignitic" Group, I would 

 say that, in this work, it is restricted to a series of coal-bearing strata lying 

 above the Fox Hills Group, or Upper Crelaceous, and these are embraced 

 in ihc divisions Laramie and Fort Union Gronjis. It is well known tliat 

 there are in various parts of the West, especially along the fortieth ])arall('l 

 and southwestward, very thick beds of coal in the various divisions of tlx; 

 Cretaceous, extending down even into the ITpper Jurassic. Had this not 

 been the case, the more general term Lignitic would have been retained l)y 

 this Survey, in preference to any other. 



As far back as 1859 it was my belief, tbunded on what appeared to be 

 sufficient evidence, (hat the sequence between the well-characterized Creta- 

 ceous strata and those of the LigniticGroup, as defined at that time, was 

 continuous, and that the chasm which was supposed to exist between the 

 Cretaceous and the Tertiary epoch would be found to be bridged over. This 

 belief was not based on strictly palceonfological evidence, for no well-marked 

 Cretaceous fossils were then known to pass up into the Lignitic or brackish 

 beds. But the physical conditions under which the sediments of the upper 

 strata of the Fox Hills Group were deposited indicated a gradual change, 

 from deep, quiet marine seas to shallow waters, which became at length 

 brackish and finally entirely fresh waters, during which time the purely 

 marine inverteljrate fauna perished, a brackish and purely fresh-water fauna 

 taking its place. This condition of the Lignitic Group covered a vast area 

 in 1hc Xorthwest, extending far southward, alonij the eastern slope of the 

 Rocky Mountains, to Denver, Colorado. As we proceed southward and 

 westward from the Missouri River, the brackish beds increase in thickness 

 until along the fortieth j)arallel they become three thousand feet or more, 

 indicating, so far as can l)c determined, no break in the sequence from the 

 Fox Hills Group to the purely fresh-water strata of the Wahsatch Grouj). 



Dr. C. A. White, Pateontologist to the Survey under my charge, has 

 made a critical examination of these formations during the past season, and 

 he says tliat his investigations have fully confirmed the views expressed by 

 mc some years ago, and in(ricated i)y the pahcontological sludies of Mr. ]\Ieck, 

 that the Fort Union beds of the Upi)ei; Missouri River are the equivalent of 

 the Lignitic formation as it exists along the base of the Rocky Mountains in 

 Colorado. He also testifies to the equivalency of the latter with the Bitter 

 Creek series west oflhe Rockv i\I()untains. The.-<c views of Dr. Whit(> are con- 



