16 



SHORTER C0NTRII3UT10XS TO GENERAL GEOLOGY, 1019. 



are in outward physical appearance. They 

 arc arkosic and consist of angular, subangu- 

 lar, and rounded grains of orthoclase, plagio- 

 clase, quartz, and black chert, with small 

 amounts of muscovite and biotite, inclosed in 

 a matrix or cement of calcite, which is more 

 or less stained with iron oxide. Grains of 

 limestone are numerous in one specimen of 

 fresh-water origin and are rather well rounded; 

 but they are not observed however in any of 

 the other specimens examined. A rather 

 surprising feature is the slight alteration of a 

 considerable proportion of the feldspar. Many 

 of the grains, especially of the plagioclase 

 variety, are perfectly fresh and clear and 

 show no sign of kaolinization. In the sec- 

 tions examined the calcite constitutes 50 per 

 cent or more of the bulk of the rock; of the 

 granular material feldspar is in general the most 

 abundant, followed by black chert and quartz, 

 which vary in amount, in some specimens 

 the one and in some the other predominating. 

 Another interesting feature is the small propor- 

 tion of well-rounded grains. The grains of 

 chert and quartz are subangular to rounded, 

 whereas those of feldspar are predominantly 

 angular and give to the thin sections the 

 appearance of being made up largely of angu- 

 lar to subangular fragments. This marked 

 angularity does not accord well with the 

 highly assorted condition of the material, 

 which is remarkably uniform in size and free 

 from silt or fine particles. This highly as- 

 sorted condition suggests considerable agita- 

 tion, either by waves or by currents, whereas 

 the angularity of the grains suggests but a 

 moderate amount of abrasion. It is probably 

 to be attributed to the cleavage of the feldspar 

 and the smallness of the grains rather than to 

 the amount of abrasion of the particles. The 

 texture is fine and varies somewhat in the 

 different specimens, the grains ranging from 

 about 0.075 to 0.2 millimeter in average 

 diameter. 



Relations. — The Judith River is conforma- 

 ble with both the underlymg and ovei'lying 

 formations. In the western part of the area 

 the fi'esh-water facies overlies about 600 feet 

 of the Claggett, including a sandy transition 

 zone about 35 feet thick, and is in turn overlain 

 by 1,000 to 1,100 feet of Bearpaw shale. The 



marine facies overlies 700 feet (Vananda well 

 record) of the Claggett and is overlain by 

 about 900 feet of the Bearpaw. In other 

 words, the marine and fresh-water facies are 

 at the same stratigraphic horizon, and the 

 overlying shale is a unit from the locality of 

 the one to that of the other. The marine 

 phase represents the seaward extension of the 

 continental deposits and marks the maximum 

 retreat of the sea during this stage of minor 

 oscillation. 



BEARPAW SHALE. 



The Bearpaw shale includes the marine 

 shale lying between the Judith River and 

 Lance formations. It consists chiefly of dark- 

 gray shale that is similar in lithologic character 

 to the shale of the Claggett except that so far 

 as observed it contains no concretions show- 

 mg cone-in-cone structure. In the upper part 

 of the formation there is a sandy zone 25 feet 

 or more thick which consists of sandy shale 

 and some thin sandstones forming a transition 

 to the Lance. This is probably the representa- 

 tive of the sandy shale from which, on Hell 

 Creek, 35 or 40 miles farther north, Barnum 

 Brown obtained a Fox Hills fauna and which 

 he therefore designated the Fox Hills forma- 

 tion.' A small collection of marine fossils, 

 all representing a single species of Lingula, 

 was obtained by the writer in a sandstone that 

 caps a small outlier in the SW. \ sec. 1, T. 14 

 N., R. 32 E. This sandstone is probably near 

 the top of the transition zone just described. 

 The species of Lingula ob tamed is not suffi- 

 ciently diagnostic for purposes of close time 

 correlation. It therefore does not establish 

 the existence of the Fox Hills at this place^ 

 but it indicates that the sandy transition zone 

 at the top of the Bearpaw is marine. 



The thickness of the Bearpaw ranges from 

 1,000 or 1,100 feet in the western part of the 

 area to about 900 feet in the eastern part. As 

 determined by the fossils which occm- in it, the 

 formation is of Montana age and is equivalent 

 to a part of the Pierre and possibly includes 

 also the representative of the Fox Hills. Its 

 correlation in this area is rendered certain not 

 only by its included fossils but b}" its continuity 

 with the undoubted Bearpaw of other regions. 



> Brown, Barnum, The Hell Creek beds of the Upper Cretaceous of 

 Montana: Am. Mus. Nat. Hist. Bull., vol. 23, p. 829, 1907. 



