44 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



porphyry have been intruded exactly as on the southern and eastern sides of 

 the bysmalith. The lowest one exposed, however, is beneath the Juratrias 

 beds, immediately above the Carboniferous limestone. It appears around 

 the head of Fan Creek, thinning out northward. Within the Ellis shales, 

 beneath the Dakota conglomerate, there are five thin sheets of intrusive 

 rock on the northern slope of Joseph Peak. Three of these have been 

 recognized north of the saddle between Fan Creek and Gardiner River. 

 They grow thinner and less noticeable to the northwest, and may be traced 

 down the east slope of Joseph Peak, where, on account of the position of 

 the beds, they form isolated patches. These sheets vary in thickness from 

 15 or 20 to 100 feet. 



Above the Dakota conglomerate and sandstone, the shales and sand- 

 stones that alternate with one another through a thickness of nearly 3,000 

 feet, constituting the Colorado and Montana formations, take part in the 

 quaquaversal arching already described — that is, on the northern side. On 

 the south they have been removed by erosion. In the ridge north of Fan 

 Creek they dip to the northwest and north, curving over to a northeasterly 

 dip in the ridge connecting this with Electric Peak, throughout which 

 latter ridge they maintain a generally uniform dip to the northeast, con- 

 tinuing the same attitude beyond the boundary of the Yellowstone Park to 

 the synclinal trough at Horr. In the ridge between the head branches of 

 Gardiner River, these beds curve from an easterly dip near its west end 

 to a northeasterly one farther down the ridge, and in Little Quadrant 

 Mountain they also maintain a general northeasterly dip, as already noted. 



The alternation of shales and sandstone layers seems to have been 

 particularly favorable to the intrusion of sheets of igneous magmas. The 

 fissile shale offered numerous planes of weakness and parting, while the 

 sandstone layers tended to stiffen the strata and cause the splitting to follow 

 more nearly constant horizons, for though there is some cross fracturing of 

 the sedimentary beds, where the igneous rock may be seen crossing the 

 strata to higher horizons, yet the persistency of the intrusive sheets is one 

 of their marked features. This is observed both upon actual exposures 

 over long distances and upon the comparison of geological sections made 

 across the strata by several observers in numerous localities. 



In the ridge north of Fan Creek the Colorado shales form the northern 

 slope and steep spurs and a small portion of the western end. Directly 



