DIKES IX CKANDALL BASIN. 231 



No dikes of volcanic rocks were found in the gneiss and granite along 

 Clark Fork, although the bare and smoothly glaciated surfaces offered 

 ample opportunity for detecting- them if present. 



The structure of the high ridge north of the upper portion of Crandall 

 Creek is shown in the escarpment on its northern side — that is, along the 

 southern side of Clark Fork and of the creek south of Index Peak. The 

 basaltic breccias appear to be rudely bedded at various angles and are cut 

 by numerous dikes. The top of the western end of the ridge, as already 

 stated, consists of twenty or more basalt sheets, which are nearly horizontal. 

 When viewed from Hurricane Mesa it appears that the low, rugged peak 

 about 4 miles north of the gabbro core consists of rough beds of breccia 

 dipping steeply eastward. In the middle of the ridge which is being 

 described there is a low arch of breccia beds that are rough and irregular. 

 Higher up on the peaks to the west the bedding is more regular and dips at 

 a low angle to the southwest. 



At the eastern end of the ridge the breccias resting on the limestone 

 are traversed by dikes trending S. 10° E. One of special importance is 3 

 feet wide and trends S. 25° E. It rises 8 or 10 feet above the surface of 

 the ground and exhibits horizontal prisms. It will be referred to again on 

 account of its composition and of the presence of large crystals of primary 

 quartz. 



The southeastern spur of Index Peak is cut by dikes trending north- 

 west and southeast, and some more northerly. Numerous dikes cut the 

 northern and northwestern spurs of the same mountain, and a few were 

 observed on the ridges surrounding Republic Creek. 



In the neighborhood of Cook City we approach another center of 

 eruptive action, which manifests itself in the form of intrusive sheets 

 of porphyry that occur within the Cambrian strata along the valley of 

 Soda Butte Creek, and more especially in the mountains north of Cook 

 City, just beyond the limits of the mapped area. Dikes of similar rocks 

 cut the breccia of Mount Miller, one of the peaks of this group of moun- 

 tains. Several intrusive sheets occur in the limestone south of Cook City, 

 and are located on the map. They include a dense porphyry, a fine- 

 grained gabbro-porphyry, and hornblende-andesite. A few miles west of 

 Cook City there are two small intrusions of dacite-porphyry. 



