THE PETROLOGY OF SEDIMENTARY ROCKS oO 
east to the Bull Mountains, about 50 miles west of the area under 
discussion, where it was mapped by C. T. Lupton.'. Between this 
area and the Little Sheep Mountain field the beds are raised in a 
gentle anticline which exposes the Pierre shale, and the Fort Union 
and Lance are eroded, so that there is no way of actually tracing 
the Lebo member from the one field to the other. 
Stratigraphic relations and structural features of the Lebo shale 
member.—The distinctive appearance of this member facilitated the 
recognition of the fact that it is wedge or fan-shaped. In the 
eastern part of T. 12 N., R. 51 E., directly opposite Terry, it is 
lacking. Coal bed U, which is taken as marking its base, is here 
Fic. 1.—View of badlands in T. 12 N., R. 50 E., P.M., Montana, looking north; 
and showing the yellow beds of the Fort Union formation in the background overlying 
the gray Lebo shale member. 
directly overlain by the yellow beds of the Fort Union and under- 
lain by the light yellowish gray beds of the Lance. In the north- 
east quarter of sec. 9, T. 12 N., R. 51 E.,-there are about 12 feet of 
dark-gray shale above this coal bed. A half-mile to the west the 
strata above it are yellow and gray, alternating in about equal 
quantity; a mile farther west there are 200 feet or more of dark-gray 
material above it with only occasional beds of dirty yellow (see Fig. 
1). Insec. 29 of T. 11 N., R. 50 E., seven miles southwest of the 
1C. T. Lupton, “The Eastern Part of the Bull Mountain Field, Montana,” Bull. 
U.S. Geol. Survey, No. 431, 1909, pp. 163-89. 
