140 GEOLOGY OF THE DENVEK BASIN. 



surface, and though ihe fracture was followed for a distance of nearly 1,200 

 feet in the old Boulder Valley mine to the south of it, no reliable data 

 concerning its features can now be obtained. The dip of the Fox Hills at 

 the fracture is vertical; of the basal sandstones of the Laramie, 200 feet to 



the north, 45° northwest; of sandstone C, LU0 feet still farther north 10 ' 

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

 South of the fracture the strata are much less disturbed, the dip being 

 southeast, away from the break, 5 to 10°. The block north of the fault 

 is liadK fractured and has apparently suffered the greater amount of 

 displacement. 



Other possible faults to the north of the Erie fault. AllOllt T ( >< > feet llOllll of till' Erie 



fault there probably exists another, a parallel, fault, with downthrow to 

 the north, making of the intervening portion an uplifted block of lower 

 Laramie and Fox Hills between two masses of coal measures. There 

 is no evidence of this second fault, however, beyond the rapid succession 

 of the basal sandstones by the coal measures, and a report of its observ- 

 ance in now abandoned mines beneath its suspected locality. 



Still farther north, at the northern edge of the northern block of work- 

 able coal, near the summit of the grade three-fourths of a mile east of 

 Erie, there is said to exist a third fault, met with in mining though not 

 appearing at the surface. 



Besides the actual and possible faults of this region, the coal has been 

 thrown into a number of very gentle parallel folds, the axes of which have 

 approximately the same trend as the faults. 



The relation of this series of faults and folds, especially of the princi- 

 pal or Erie fault, to the general system of the Boulder Valley is apparently 

 that of cross-fractures, their trends all forming a wide angle with the 

 predominant faults and folds. At the same time it is quite possible that 

 their development iua\ be due to a reappearance at this point of the forces 

 which produced the similarly disposed fractures and folds in the southern 

 and western parts of the held, notably the Baker and Harper faults and 

 the southern and middle fractures of the Marshall subsystem. 



The jackson-star fault. — 'This break appears only in the two mines from 

 which it has received its designation, the surface of the ground between 

 them affording no evidence of its presence. 



