FORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 25 



The No. 3 beds occupy an irregular area, somewhat more elongate 

 east and west than north and south, bounded by the Crazy Mountains 

 on the west and by the loop of Lebo exposures on north, east, and 

 south (pi. 1). 



From their position over the Lebo, it may be supposed that the 

 No. 3 beds correspond in their lower part to the Tongue River and 

 perhaps in their upper part to the Sentinel Butte, but such a correla- 

 tion is at present totally unwarranted. They are very distant from 

 the typical or from any unquestionable exposures of the Tongue River 

 or Sentinel Butte, their litholog}^ is no more like either Tongue River 

 or Sentinel Butte than like many other formations and certainly is not 

 close enough, in itself, to warrant correlation \\dthout tracing them 

 laterally continuously or nearly so into true Tongue River and Sentinel 

 Butte, which is impossible. Their age is well established, in part, as 

 brought out below under "Correlation", but that of typical Tongue 

 River and Sentinel Butte is not, and paleontological correlation wdth 

 those members is not now possible. 



Under these conditions it is certain!}^ less helpful than conducive to 

 serious error to adopt the frequent practice of assuming that a corre- 

 lation exists. Still worse is the practice, also exemplified by some 

 work on the Fort Union, of assuming that both Tongue River and 

 Sentinel Butte must occur here and dividing the beds, in which no 

 natural division has been established in the field, according to the 

 proportionate thickness of these members in a widely different area. 



I therefore propose the local name Melville for the lithologic unit, 

 from the town of Melville, which is situated on these beds and is 

 surrounded, within a few miles, by excellent and typical exposures of 

 them. The lower boundary of the formation is well established, as 

 shown on the accompanying map. The upper boundary is still 

 uncertain. The name is proposed to include at least the lower 3,000 

 feet of the No. 3 beds, to about the top of Cay use Butte, and tenta- 

 tively for the whole No. 3 of this field, to the highest sediments on 

 Porcupine Butte. Later discoveries might make it advisable to 

 remove some of these uppermost strata from the formation, and the 

 pertinence of still higher beds around the mountain flanks is wholly 

 dubious. 



Concretions are common throughout the Fort Union here. Limy 

 concretions, weathering rusty yellow, may be of great size, up to 10 

 feet or more in greatest diameter, and locally characterize a definite 

 stratum, but these appear to be truly concretionary and not a true 

 sediment or limestone. Shell limestones do occur locally in the No. 3 

 beds, but in the No. 1 and No. 2, while shells may be fairly abundant 

 in relatively small lenses, they are generally in a shale matrLx and do 

 not form a true limestone. In the No. 3 beds there are a few thin 

 bands of comminuted shells, or shell breccia (notably at the Scarritt 

 Quarry), generally mixed with clay. 



