FAULT M'.Ai; BOULDER. 119 



east be produced until they intersect above the mountain profile, and the 

 whole series of beds be then imagined as turned back from their inclined 

 position through their angle of dip to a horizontal position, the strata are 

 found to be compressed on the fracture plane, the beds above or east of 

 this plane having been moved up relatively on those beneath or west; the 

 hade is distinctly from the downthrown side. Moreover, if the tests which 

 are given in the analysis of faults under the preceding heading be applied, 

 the evidence for the iv\ erse type of the fault is xtill more conclusive, for a 

 trial lint — for instance, one perpendicular to stratification plane- not within 

 either of the acute angles in such a manner as to make with the planes a 

 smaller angh — would penetrate the same series of beds twice, a series 

 reduplicated to an extent equal to the stratigraphic throw of the fault itself; 

 on the other hand, a line falling within the acute angles of the stratification 

 and fault planes, the vertical line of the figure, for instance, would fail to 

 cut the same series of beds at any point whatever. The obscurity in the 

 proper reference of this fault arises chiefly from the position of the fault 

 plane and the planes of stratification with regard to the earth's horizon, but 

 it is influenced in a measure, also, by the amount of erosion to which the 

 rocks involved have been subjected. 



From the analysis of the South Boulder Peaks fault, it is evident that 

 it is a fault of compression induced at the time of the general uplift of the 

 range. 



THE FAULT IN THE FOOTHILLS SOUTH OF SOOTH BOULHEB CREEK. 



'This is similar in character to the South Boulder Peaks fault, but 

 has not been studied in detail owing to its lying beyond the established 

 limits of the survey. Its length is unknown, the •ends being obscured in 

 granite. The maximum throw is estimated at approximately 3,-000 feet. 

 The inclination of the fracture plane, as seen in the walls of the deeply 

 eroded gulch near its southern end, is to the west at an angle of about 80 

 with the horizon, and the occurrence is apparently in strict accord with 

 that of the fault to the north — that is, it is a "reverse" fault. 



THE DEEE CREEK FAULT. 



The greatest displacement along the Deer Creek fault is aboul 1,200 

 feet, midway its length, from which point it gradually diminishes to either 



