26 BULLETIX 16 9, UNITED STATES XATIOISrAL MUSEUM 



The relatively small quantity of carbonaceous material is note- 

 worthy in ail three subdivisions. There are a few thin impure and 

 local lenses of coal in the No. 3, and a little prospecting has been done 

 on these, but none are of any commercial value. Aside from these 

 thin seams, generally only an inch or two thick, there are a few car- 

 bonized tree trunks, occasional very local lenses of coal a few feet in 

 diameter and less than an inch in thickness, and locally many minute 

 carbon fragments and filaments. In marked contrast with the Fort 

 Union of most other areas, these rocks can be classed as not coal- 

 bearing. 



Several workers, especially Stone and Calvert and Silberling and 

 I, have attempted to follow out the development of this Fort Union 

 series into regions beyond the local field, but with indifferent success. 

 To the eastward it ends against the Cretaceous, and the Fort Union 

 reappears, considerably modified in thickness and character, after a 

 long gap, in the Bull Mountain Field (see Woolsey, Richards, and 

 Lupton, 1917; also Ellis and Meinzer, 1924). The lower part of the 

 Fort Union is there predominantly a shale member, dark and greenish 

 in color, with some sandstone (not closely similar to that of the No. 1 

 in the Crazy Mountain Field) and coal, about 200 to 300 feet in 

 thickness. This is commonly correlated with the Lebo, which is 

 probable on lithologic grounds, but there is no evidence that it repre- 

 sents the whole Lebo or that it is not a lithologic facies of somewhat 

 different span in time. The correlation is here more probable than 

 in any other area where the Lebo is supposed to occur, but it cannot 

 be considered as established bej^ond doubt. The upper part of the 

 Fort Union is here only 1,650 feet thick and contains more pale shale, 

 more and different limestone, more numerous and persistent hard 

 sandstones above the base, and much more coal than does the No. 3 

 of the Crazy Mountain Field. The Bull Mountain Fort Union is an 

 isolated mass, completely surrounded by Cretaceous outcrops. 



The Fort Union encircles the Crazy Mountains, which are formed 

 principally by intrusive masses thrust into it, and has been followed 

 in some detail by Stone and Calvert (1910). (Silberling and I have 

 also made a reconnaissance around the mountains on which, as well 

 as on Stone and Calvert, my remarks are based.) The Upper Creta- 

 ceous formations and the Lebo, steeply folded and much disturbed 

 and altered by later igneous activity, swing around and into the 

 northeastern end of the mountains. The Hell Creek here appears to 

 contain more andesitic debris and comes to resemble more closely the 

 Lennep and the Lebo. Toward the southwest, and southward on the 

 western flank of the Crazies, the Hell Creek, Bear, and Lebo have not 

 been distinguished. It seems probable that they here merge litho- 

 logically with the Livingston, as Stone and Calvert believe, or they 

 may possibly be absent. There remains, however, the possibility 



