A FRACTURE VALLEY SYSTEM 99 



lines of the zigzag Third Canyon. These lines persist to the 

 southeast within the boundary of the Yellowstone National 

 Park, the fault itself being covered by younger volcanic lavas. 



As already remarked, the great fault appears to end at Cin- 

 nabar Creek, but the Yellowstone River follows a line parallel to 

 it through the gorge at Yankee Jim, and parallel lines of lesser 

 drainage are plainly shown in the vicinity. Whether the fault 

 actually ceases at C^linnabar Creek cannot be determined, because 

 of the covering of volcanic rocks formmg the surface of the 

 mountainous country westward. Beyond these lavas, however, 

 in a direct line a similarly profound fault is exposed in the 

 Madison Range in the Three Forks Quadrangle. This was accom- 

 panied by a similar throw of the sedimentary strata south of 

 the crystalline schists, and is in a direct line with the fault just 

 described. The total length of the combined fault lines is over 

 sixty miles. 



Parallel to this great fault are several well defined ones of less 

 extent, the largest lying north of Mill Creek Basin. Here 

 there are two sub-parallel faults which unite in the head of the 

 North Fork of Mill Creek. The dominant one of these has been 

 traced from the Boulder Canyon westward to the Yellowstone 

 Valley, a distance of twenty miles. The shorter fault has been 

 traced for thirteen miles. They are not observed west of the 

 Yellowstone Valley in the Gallatin Mountains because of the 

 covering of volcanic lavas, but they appear again with the same 

 characters in the Madison Range just west of the lavas, evidently 

 passing beneath them. The western faults are exposed for 

 twenty miles, and there can be no doubt of their persistence 

 beneath the lavas. In this case the total length of the principal 

 fault would be sixty-four miles. 



In the bare gneisses two miles north of the fault line at the 

 head of the North Fork of Mill Creek joint planes or small 

 faults are clearly visible, parallel to the main fault and having a 

 hade to the south, indicating normal faulting. 



In other parts of the region there are faults parallel to those 

 just described. A small one within the area of crystalline schists 

 occurs in the southeast corner of the quadrangle, crossing 



