30 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



and form a narrow wedge between this fault and the gneiss. The Gallatin 

 fault, which crosses the west base of Three River Peak, trends in a north- 

 northwest direction, crossing Crowfoot Ridge three-quarters of a mile west 

 of Three River Peak, and crossing the ridge north of it at the saddle one 

 mile north of the g-neiss, thence following down the drainage, to die out 

 where it joins the short fault east of Section Ridge. The trend of the fault 

 is nearly parallel with that of the Gallatin River, as will be seen on the map. 

 The maximum displacement is about 2,000 feet. 



The long, low, flat-topped ridge lying between this fault and Gallatin 

 River consists of nearly horizontal beds of Carboniferous limestone capped 

 by the white Quadrant quartzite or sandstone occurring at the top of the 

 Carboniferous series. The dip of the beds is about 5° NE. From this it 

 is evident that there must be a fault or a fold between this ridge and the 

 higher one east of Gallatin River. A fold exists west of Bighorn Pass, but 

 it was not followed down the valley. On both sides of the low ridge west 

 of Gallatin River are bodies of intrusive igneous rock, related to the dacite- 

 porphyry of the Holmes bysmalith in composition and petrographical 

 character. The rock is lithoidal and holds small mica phenocry sts ; it is 

 fissile near the contact with sedimentary rocks, and massive a few feet dis- 

 tant. It crosses the fault line and is found on its western side intruded in 

 the axis of an anticlinal fold in Cambrian rocks. Its intrusion followed or 

 accompanied the faulting. On the eastern side of the flat ridge it appears 

 as an intrusive sheet, about 50 feet thick, forced between beds of Carbonif- 

 erous limestone. This exceptional occurrence of igneous rock as an intru- 

 sive sheet in massive Carboniferous limestone is of limited extent and is in 

 the immediate neighborhood of a fault, with which it is directly connected. 

 Similar rock has been intruded into the west base of Three River Peak, and 

 it may be assumed that the Holmes bysmalith was connected with the same 

 line of faulting. The intrusion of this mass has been shown to have been 

 subsequent to the upheaval that permitted the intrusion of the Indian Creek 

 laccolith; hence it follows that the more steeply upturned position of the 

 gneiss and Cambrian strata west of this fault was due to a later movement 

 than the general uplifting of the body of the range. This steeper uplift was 

 limited on the east by the fault last mentioned, and by that cutting across 

 the northwest end of Crowfoot Ridge, which faults are probably contempo- 

 raneous and were accompanied by a slight faulting east of Section Ridge. 



