24 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



The shores of the Gallatin Lake, and the small hills adjoining, are 

 formed of the laccolith rock, andesite-porphyry. The western boundary 

 of this rock runs northward along' a gully, west of the drainage from the 

 lake, and through a small shallow pond to the saddle of Bighorn Pass. To 

 the south the porphyry extends to the base of Three River Peak in the rear 

 of the lake, and foi'ms the saddle in the pass between Indian Creek and 

 the valley of the Gallatin River; from here northward the exposure extends 

 along the western base of the ridge to Bighorn Pass. 



The position of the strata seen in the precipitous northern face of 

 this peak is shown in PI. VII. At the western base of this mountain they 

 dip sharply over the edge of the laccolith, changing from nearly horizontal 

 to 50° or 70° W., and gradually decreasing again westward. Into the axis 

 of this abrupt bend a vertical offshoot from the andesite-porphyry has been 

 intruded, showing that the limestones were ruptured at this place. 



The limestone strata of Three River Peak are traversed by dikes of 

 lithoidal igneous rock at various angles. One broad dike,' 100 feet thick in 

 places, cuts diagonally across the northern face, appearing on the eastern 

 slope about halfway up to the summit. Another, about 10 feet thick, 

 without phenocrysts, lies horizontally between the strata and might easily 

 be mistaken for a compact sandstone. A narrower dike cuts nearly 

 verticallv through the western side of the mountain. 



At the west base of the peak the gneiss is faulted against the lime- 

 stones by the southern extension of the Gallatin fault, The position of the 

 sedimentary beds Avhich abut against the gneiss, as well as their fracturing, 

 shows clear evidence of the presence of the fault. An intrusive body of 

 igneous rock related to the Mount Holmes rock occurs at this locality. 



On the saddle between Three River Peak and Echo Peak, near the 

 contact of the Holmes bysmalith with the andesite-porphyry and lime- 

 stone, the latter rocks are seen to have been turned up, so as to dip 40° 

 N., away from the bysmalith, and to be greatly shattered and dislocated, 

 producing slickensides and a pulverizing of the rock along the fracture 

 planes. 



BIGHORN PASS. 



Bighorn Pass is a low divide between the head waters of the Gallatin 

 River and the drainage of Panther Creek, and affords an easy passage from 

 the valleys west of the mountains across the range to the central region of 



