22 GEOL()(iY OF THE EUREKA DISTRICT. 



single ridge structure, as seen toward the north, the configuration of 

 the country shows a broad, rough mass of mountains, from 4 to 5 miles 

 in width, of very divei'sified topographic forms and deeply scored by 

 narrow gorges. In the region of Atrypa Peak, Gray's Peak, and Lookout 

 Mountain a classification of the mountain masses becomes a matter of much 

 difficulty, the orographic structure being complex, and the resultant of 

 forces in some respects different from tliose wliicli elevated Prospect Ridge 

 or the Fish Creek Mountains. Southward from Castle Peak the latter 

 mountains become a distinct range, and with a north and south trend stretch 

 off southward several miles l^eyond the limits of this survey. They ai'e 

 situated in the extreme southwest corner of the Eureka District, and are 

 sharply defined by the broad valley of Fish Creek on the one side and An- 

 telope Valley on the other, which partially disconnects them from the 

 Eureka Mountains. They measure about 5 miles in width and rise over 

 2,000 feet above the adjoining Quaternary plain. They present the im- 

 pressive appearance of a solid mountain mass gently inclined to the west, 

 but falling off somewhat abruptly on the east, accompanied by a steep es- 

 carpment just beneath and parallel with the summit of the ridge. The 

 structure is that of an anticlinal fold whose axial ])lane coincides with the 

 escarpment along which there has been a downthrow of 600 feet. The 

 origin of the escarpiTient is due to the faulting. At the base of the cliff the 

 faulted strata are unifoi'mly inclined toward the valley at an angle of 

 about 15°. Along the west side of the anticlinal axis the beds lie at much 

 lower angles, exhibiting first a slight synclinal fold followed by an equally 

 gentle anticlinal, beyond which for nearly 2 miles they fall away with a 

 nearly uniform dij> toward Antelope Valley. 



The Fisli Creek Mountains may be considered as essentially made up 

 of Silurian rocks, in marked contrast with Prospect Ridge, which is, as lias 

 been already shown, formed of Cambrian strata with outlying slopes of 

 Pogonip Hmestone and Eureka quartzite. Here are exposed the two lower 

 members of the Silurian in a manner which can hardly be excelled for sim- 

 plicity of structure elsewhere in the Great Basin. Nearly all the more ele- 

 vated portions of the mountains consist of Upper Pogonip limestone, the 

 axis of the fold occurring not far below the top of the horizon. Tlie Eureka 



