THE BOULDER VALLEY REGION. 133 



the old openings in the bluff easl of the No. 5 mine tin- fracture was 

 actually encountered. If present, its trend is about X. 62 E.; its extent, 

 half a mile; the maximum dislocation, about 100 feet; the dip of the fault 

 plane, undetermined. 



The terminal cross-fault is a simple fracture extending across the 

 brow of the Davidson mesa a few feel wesl of the triangulation station, 

 between the southern and middle branches of the Marshall system. Its 

 trend is a little wesl <>t' north, the downthrow on the west, the displacement 

 nut over 30 feet. It is probably connected with the sharp anticlinal fold 

 appearing in the southern lace of the mesa and forming here a crumple in 

 the western rim of the Davidson syncline. 



The Davidson faults. — These embrace the two fractures mi the northern 

 slopes of tin- Davidson mesa, near its eastern end. They intersect at the 

 crossing ol the Davidson ditch and Colorado Central Railroad, ami run, 



the one north along the deep cut of the ditch, the other S. 30 W. across 

 the old Davidson coal property. 



The southern of these faults belongs to the X. .'!<l E. series of dislo- 

 cations, and owes its development to the same forces that threw the strata 

 into the many gentle folds of like trend; the northern fault belongs to the 

 scries of north-and-south fractures that characterize the entire breadth of 

 the northern third of the Boulder Yallev region, and which owe their 

 development to the forces acting in a due east-and-west direction. 



The extent of the southern fault can not !»• definitely determined. Its 



single exposure occurs at the deep cut of the Davidson ditch at the railroad 

 crossing, but its presence to the southwest, in the vicinity of the old 

 Davidson mines, is proved in the superficial succession of the beds to be 

 found between the mine openings ami the upper part of the Davidson ditch 



to their east. At the mines and a short distance eastward the beds of the 



lower Laramie underlie the surface, having a northwesterly dip of 30 ; a 



little to the east they are succeeded by outcrops of the Fox Hills: as the 

 Davidson ditch is approached, however, this formation is abruptly suc- 

 ceeded, with but slight changes of level, by the iron tones and clays which 

 form a portion of the coal measures and which plainly outcrop in the ditch 

 itself at the point where it turns directly northward after its long course 



