356 GEOLOGY OF THE DENVER BASIN. 



relatively higli inclination of the beds west of the fracture and tlie sharp- 

 ness of their curve to horizontal; and the rapid succession in outcrop from 

 Fox Hills to coal measures, the latter coming in at dips between 45' and 

 15°, usually nearer the latter. 



A break in the continuity of the rim occurs opposite the town of Erie. 

 Here, by displacement along the Erie fault — a cross fracture having a X. 

 G4 E. trend with downthrow to the south — the upper beds of the Fox 

 Hills and the coal measures have been brought in opposition, the former 

 appearing on the north of the fracture, the latter on the south. The dip of 

 the Fox Hills at the fracture is vertical; of the basal sandstones of the 

 Laramie 200 feet to the north, 45°; of sandstone C, 100 feet still farther 

 north, 10° to 15 . beyond which an approximately horizontal position is 

 assumed. South of the fracture the beds are much less disturbed, the dip 

 in the ( >ld Boulder Valley mine being reported at 5 to 10° SE. 



The southwestern extension of the fault is uncertain: it may be but 

 little over a mile long', or may continue well toward the western rim of the 

 syncline. 



The foregoing structure has resulted in a line of outcrops as follows: 

 From the point at which the coal measures first reach the surface along the 

 western side of the Coal Creek fault a southern trend prevails to within 200 

 or 30<> feet of the Erie fracture; at this point the line runs sharply west- 

 southwest, paralleling the latter fault; its continuation in a southwest direc- 

 tion depends on the extent of the Erie fracture, hut the measures, with 

 their included coal seams, finally sink beneath the surface, and, upon the 

 disappearance of the fracture, become continuous with their fellows on 

 the opposite side. It is possible that this fault is the structural division 

 between the Mitchell and Jackson subbasins, and that to the elevation of 

 the strata on the north is due a large superficial area of sand. A gentle 

 anticline in the measures has been already suggested as the alternative of 

 this structure. 



North of the Erie fault, immediately south of the deep railroad cut 

 in the basal sandstones at the crest of the valley bluffs, there is possibly 

 another fault, parallel to the Erie, though much shorter linearly, by which 

 there may be a second indentation in the measures similar to the foregoing. 



