COAL. 341 



terrace is northeast. The second terrace disappears a half mile west of 

 Burnt Knoll, the coal outcrop shortly passing into the northern or main branch 

 of the Marshall fault system, which a little beyond occupies the hod of Dry 

 ('reek. Southeast of the fracture, for the greater portion of its length, 

 beds of a much lower horizon, even the upper portion of the Fox Hills, 

 occupy the surface of the country. Only near the western end of the fault 

 has the coal on the south escaped erosion. 



The coal of the interfault block, between the middle and southern 

 branches of the fault system, rapidly rises from west to east, occupying the 

 northern bluffs of tin- Davidson mesa as tar as the steep-dipping portion of 

 the syncline. Here its outcrop turns northward across the low country, 

 paralleling the main Marshall fault at the distance of about a quarter of a 

 mile. On this portion of the trend is opened the Allen-Bond mine. 



The south branch of the Marshall fault system is also of especial 

 importance in its influence upon the coal outcrop. In effect this fault has 

 determined a line of bluffs to the south of a topographic depression at 

 Marshall, lowering the beds on the north and leaving the outcrop of the 

 coal in those to the south. The western end of the break probably lies in 

 the main Marshall fault, at the point of the bluffs where the vertical dip 

 becomes shallow, the outcrop of the coal here splitting, one portion passing 

 into the lower 1 duffs along the periphery of the field, the other along the 

 higher bluffs to the south of the Marshall depression. In the latter bluffs 

 tlie coal outcrop extends eastward in a nearly horizontal line, midway their 

 height, for nearly a mile, where, at the steeply dipping portion of the 

 Davidson syncline on the eastern edge of the district, it turns down, 

 crosses the gulch, and on the opposite side again rises, only to immediately 

 sink in the depression which occurs on the northern side of the fault. The 

 eastern end of this fault is in the told just mentioned. 



The possible bluff fault, a few hundred feet south of the foregoing, if 

 present, is repetitive, the downthrow being on the north, the displacement 

 slight. 



The cross-fault at the west end of the Marshall mesa is also one of 

 slight throw, appearing in cross-section in the north and south bluffs of the 

 mesa. 



