122 GEOLOGY OF THE EUEEKA DISTEICT. 



several lesser ones branch off, nearly all of them lying approximately 

 parallel with the same northeast trend. 



On the summit of the Fish Creek Mountains, midway between Belle- 

 vue and White Cloud Peaks, occurs a vertical dike of granite-porphyry only 

 a few feet in width. It is made up of feldspar, hornblende and mica 

 imbedded in a groundmass of quartz and feldspar, possessing typical 

 microgranitic structure. Apparently this dike itself exerted little, if any, 

 influence on the adjoining country, and the only geological interest at- 

 tached to the occurrence consists in its being closely allied to the larger 

 bodies of coarse granite-porphyry, from which it is most likely an offshoot. 

 It is quite possible that the quaquaversal dip of the strata from White Cloud 

 Peak, of which mention has already been made, may be due to an under- 

 lying mass of intruded crystalline rock, of which the dike is the only 

 evidence upon the surface. 



Coinciding in direction with the secondary off-shoots from the main 

 dike occur narrow dikes of granite-porphyry penetrating the Lone Moun- 

 tain limestone of Castle Mountain. They are exceptionally fine grained, 

 with a characteristic microgranitic grouudmass. In their mode of occur- 

 rence they resemble the dike near Bellevue Peak, and doubtless have the 



same common ongm. 



As the geological and petrographical features of the granite-porphyry 

 are discussed with some detail in chapter vu, devoted to the discussion of 

 the pre-Tertiary crystalline rocks, it is needless to enter more at length 

 into the subject here. By reference to the map (atlas sheet xi) the 

 position of the main body of granite-porphyry and its relations to the 

 primary and secondary offshoots from the parent mass may be readily seen. 



Ridge west of Wood Cone. In many respects the best locality to study the 

 Pogonip of the Eureka District is the long, narrow, monotonous ridge which 

 stretches westward from Wood Cone. Here the beds abut against the 

 southern end of the main granite-porphyry body, standing invariably at 

 high angles, in most places nearly vertical, but sometimes inclined westerly 

 and again easterly. Just west of the limestone saddle, which separates the 

 two bodies of porphyry, there is a fault in the limestone which brings up 

 the lower beds. There is apparently a synclinal fold, to the west of which 



