686 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



doubtfully referred to the Dakota are now" regarded as in part referable to the 

 Kootenai and in part probably to the Bear River. 



In an unpublished manuscript F. C. Calkins describes and correlates the Colorado 

 formation in the Philipsburg quadrangle, Montana, as follows: 



Immediately above the Kootenai formation in the Philipsburg district is a very homo- 

 geneous accumulation of fissile black shale whose upper part contains thin beds of low-grade 

 coal near Drummond. On Mount Princeton the shale is about 450 feet thick. The greater 

 part of the shale, is barren of fossils, but according to Dr. Stanton it Hthologically resembles 

 the Colorado shale of areas farther east. Immediately above the coal near Drummond a Unio 

 which may be of Colorado age was found. 



At the top these shales pass by a rapid gradation into beds dominantly arenaceous, attaining 

 a thickness of more than a thousand feet. They consist mainly of greenish flaggy sandstones 

 but comprise considerable shale and several beds of conglomerate. 



In the lower part of the sandstone on Mount Princeton and about 4^ miles east of Drum- 

 mond, very imperfect specimens doubtfully referred by Dr. Stanton to Mactra and Callista were 

 collected which appear to be of Colorado age. About 1^ miles southeast of Drummond a small 

 collection of plants from a horizon a few hundred feet above the base of the sandstones is reported 

 upon as follows by Dr. Knowlton: * 



"There are three things in this little collection — a Marchantia which is probably new, a 

 conifer which is probably a Glyptostrobus, and a fern that is pretty close to if not identical with 

 Aspidium oerstedi Heer. The latter is from Patoot, which = Senonian = Fox Hills. I can not 

 place this material definitely, but should incline to regard it as possibly Upper Cretaceous." 



In 1910 Mr. Pardee collected more satisfactory fossils from a limestone outcrop in Coberly 

 Gulch, 10 nules north-northwest of the northeast corner of the quadrangle. The exact position 

 of the horizon is not known; the hmestone bed, however, is underlain by about 400 feet of 

 sandstone and shale similar to those a;bove the black shale, so that it is virtually certain that 

 the fossils occur at least 400 feet stratigraphically above the main body of black shale. 



The list of forms identified by Dr. Stanton and his comment on them follows: 



6552. Specimen No. 10-P, 4. NE. i NW. i sec. 34 and SW. i sec. 27, T. 10 N., R. 12 W., Granite County, Mont. 

 Modiola sp. 

 Cyrena securis ^\%ite. 

 Corbula sp. 



Glauconia coalvillensis Meek. 

 Admetopsis subfusiformis Meek. 



" The horizon is in the Colorado group. The same association of forms occurs in the Oyster 

 Ridge sandstone member of the Frontier formation of southwestern Wyoming." 



By way of summary it may be said that lithologic and paleontologic evidence put the 

 correlation of the black shale and about 400 feet of the overlying beds with the Colorado beyond 

 reasonable doubt. It is probable, however, that some beds of Montana age occur in this district. 



Calvert "° describes the Cretaceous of the Lewistown coal field, in central 

 Montana, as follows: 



The Lewistown coal field * * * lies between longitude 109° and 110° 15' west, and 

 the 47th parallel divides it into two nearly equal portions. It is thus located in the center 

 of Montana, including the west-central part of Fergus County and a few square miles of north- 

 eastern Meagher County. The greater part of the field lies in the Judith Basin, a name apphed 

 to the upper drainage area of Judith River. * * * The Little Belt and Big Snowy moun- 

 tains are in the southern part of the field and the Judith and South Moccasin groups he well 

 within its northern borders. The area as described includes about 1,500 square miles. * * * 



The rocks of the Lewistown coal field range in age from lower Carboniferous to Quaternary, 

 inclusive. * * * The succession of the rocks and the relation of the principal coal-bearing 

 beds to strata above and below are shown in the following generahzed section: 



"■ Stanton, T. W., comment on manuscript. 6 Personal communication to Dr. Stanton. 



