Rogers — Persistent Parting in a Coal Bed. 299 



Art. XXIII. — The Occurrence and Genesis of a Persistent 

 Parting in a Coal Bed of the Lance Formation /* by G. 

 Sherburne Rogers. 



During the examination of the Tullock Creek coal field, 

 Montana, in 1912-13, the writer discovered a coal bed contain- 

 ing everywhere a curious parting which, because of its uncom- 

 mon physical characters, seems to merit a brief notice. The 

 material resembles a slightly carbonaceous sandstone and was 

 so considered in the field ; but, owing to its persistence 

 throughout the area examined, it w T as later studied micro- 

 scopically and was found to be an almost pure aggregate of deli- 

 cate crystals undoubtedly formed in situ. The occurrence of 

 such a deposit in the midst of a mass of heterogeneous and 

 largely continental sediments is surprising. The writer has 

 seen similar partings in other coal beds in this region, and 

 several other geologists of the U. S. Geological Survey have 

 mentioned like instances both in the Cretaceous and Tertiary 

 coal fields of the west and in the Carboniferous fields of the 

 east ; it is believed, therefore, that this phenomenon may be 

 not uncommon and may have some general bearing on the 

 accumulation and alteration of sedimentary rocks. The writer 

 is indebted to Dr. F. W. Clarke and Mr. E. S. Larsen, of the 

 U. S. Geological Survey, for advice and helpful criticism. 



Occurrence. — The Tullock Creek coal field is located on the 

 east side of Bighorn River, Montana, in the angle formed by 

 its junction with the Yellowstone. The coal bed here 

 described outcrops also in the Pine Ridge field on the west 

 side of the Bighorn, and a brief examination of that area was 

 also made by the writer. The data here presented were 

 gathered in both fields, which together form an area 36 miles 

 in an east-west direction and 21 miles in a north-south. 



The coal bed in which the parting occurs belongs to the 

 Lance formation, which is Cretaceous or Tertiary in age. 

 The Lance in this district comprises about 1,200 feet of 

 sandstone and shale, the lower 900 feet of which contains no 

 coal and is probably in part of brackish water origin. The 

 upper 300 feet is coal-bearing, is somewhat different in color 

 from the lower portion, and is probably of continental origin. 

 The strata in most of the area lie almost flat and have suffered 

 practically no metamorphism. The Lance formation is of 

 widespread extent, occurring as far east as central North and 

 South Dakota, and as far south as central Wyoming ; and it 

 is everywhere characterized by the lenticular and irregular 

 habit of the strata, and especially the coal beds. 



* Published by permission of the Director, U. S. Geological Survey. 



