REGION NORTH OF GALLATIN RIVER. 43 



southwest. The corresponding intrusive sheets beneath the Dakota con- 

 glomerate in the southern slope of Gray Peak thin out toward the east, and 

 near the end of the spur one of them breaks upward as a dike-like body 

 across the Dakota conglomerate. 



The porphyry forming the axis of this small arch extends south, 

 constituting the ridge of Fawn Pass. It extends east down the valley of 

 Fawn Creek as an intrusive sheet near the base of the Juratrias shales, and 

 extends south of Fawn Pass as an intrusive sheet at the same horizon, and 

 has been traced as a ledge along the ridge south and westward to the cliff 

 first described north of Granatin River. It becomes thinner as it is followed 

 farther from the bysmalith, and it is evident that the intrusive sheets in 

 this vicinity proceeded from the Gray Peak intrusive mass. 



The sedimentary beds forming the mountain side south of Gray Peak 

 dip into the mountain toward the north and northeast at a low angle and 

 encounter the igneous rock of the bysmalith which forms the highest 

 portion of the mountain mass from Gray Peak to Joseph Peak, and extends 

 down the east slope to a level of 9,000 feet and down the west side to below 

 this altitude. The igneous rock extends along the north face of the ridge 

 west of Gray Peak. From it also proceed sheets of porphyry intruded 

 between the Juratrias strata which are exposed along the south face of 

 Little Quadrant Mountain and may be traced around the northern slopes. 



On the northern side of the mass the sedimentary beds dip toward the 

 southeast, into the igneous core Again, as at the southern side of this 

 body, the highest horizon is that of the Dakota conglomerate which is 

 found at the summit of Joseph Peak in contact with the intruded mass. 

 As may be seen from the map and cross sections (Pis. IX and X), there is 

 a quaquaversal arching of the strata, the center of which is located in the 

 head of Fan Creek, northwest of Joseph Peak. From this point the beds 

 dip south, southeast, east, northeast, north, and northwest. In the three 

 valleys heading against the ridge surrounding this arch the beds dip to 

 the east, northeast, and north at angles not far from 10° — in some cases 



reaching 25°. 



At the west end of the ridge north of this part of Fan Creek, the beds 

 arch over to the west and southwest with a dip of 20°, and pitch against 

 the same fault plane noted north of Gallatin River which let down the 

 volcanic breccia. Between the sedimentary strata, sheets of andesite- 



