130 GEOLOGY OF THE DENVER BASIN. 



also reported in an earlier mine opening, several hundred feet northeast of 

 No. 3. Its surface location, however, is easily established within loo or 

 200 feet from local exposures of the rooks on either side — on the west, the 

 Laramie sandstones, coal, and fossil oysters peculiar to this horizon; on the 

 east, in marked contrast, the clays, characteristic concretions, and fossil 

 Mollusca of the Fox Hills or the basal sandstones of the Laramie itself. 

 Toward the northeastern end of the fault Dry Creek occupies the line of 

 fracture, and by its erosion has nearly severed a projecting mass of the 

 Laramie from the main body of the formation to the east. 



The trend of the main fault is slightly wavy but approximately X. 30° 

 E. The downthrow is uniformly on the northwest, the stratigraphic throw 

 reaching a maximum of about 350 feet along the middle portion of the 

 fracture. The fault plane, where observed in the Marshall mine, dips 4;")° 

 S. 28° E., making the fracture a reverse fault. Whether this i> local 

 or holds for the entire extent of the break is a matter of conjecture; 

 the reverse type is, however, frequently met with in the Boulder Valley 

 region. 



The middle fracture, although of minor economic importance, is never- 

 theless of much structural interest, both from its relation to the associated 

 fractures and from its own somewhat peculiar genesis. From its point of 

 departure from the main fault, which, as nearly as can be determined, is in 

 the vicinity of the line of the Denver, Marshall and Boulder Railroad, it 

 extends northeastward a distance of about 2 miles along the northern face 

 of the Davidson mesa, where it abruptly terminates in the sharp fold 

 already mentioned as constituting the western rim of the synclinal trough 

 which crosses the mesa diagonally at this point. Its reentrant angle with 

 the fold is very acute, approximately 30°. 



The presence of the middle fault is attested by the relative displace- 

 ment of the beds on either side and hv the included, irregularly disposed 

 fragments of rock along its line. In the northern face of the mesa, at the 

 foot of which the fault runs, from the triangulation station on the western 

 point to the synclinal fold at the eastern end of the break, occur the 

 approximately horizontal coal measures of the Laramie. The very base 

 of the bluff shows the summit of the basal sandstones, succeeded in a few 



