14 GEOLOGY OF THE EUREKA DISTRICT. 



Longitudinal Faults. The most profound faults, those which mark the 

 greatest amount of displacement and have exerted the most influence in 

 producing the present structural features of the region, cross the mountains 

 at varying intervals with an approximately north and south trend from Fish 

 Creek Basin to Diamond Valley. These faults constitute the principal 

 factors in outlining the individual orographic blocks, and probably from the 

 beginning of mountain building up to the present time, and certainly 

 through the Tertiary period, have played a most important part in their 

 development. The amount of displacement along those faults that extend 

 the entire length of the mountains is very great, measuring at some points 

 in their course as high as 13,000 feet. 



The four principal lines of displacement are the Spring Valley and 

 Sierra fault, on the west side of Prospect Ridge; the Hoosac fault, separat- 

 ing Prospect Ridge from Spring Hill and Carbon Ridge; the Pinto fault, 

 lying between the Spring Hill and Carbon Ridge on the one side and the 

 County Peak and Silverado Mountain block on the other, and the Rescue 

 fault, on the east side of the latter block. These main faults will be de- 

 scribed here. Numerous other longitudinal faults, while they express 

 powerful orographic movements, are more restricted in their influence and 

 confined within the limits of one or the other mountain blocks into which 

 the country is broken up. They will be mentioned with more or less detail 

 when describing the particular region in which they occur. 



Spring Valley and sierra Fault The Spring Valley fault adheres closely to 

 the west base of Prospect Ridge and sharply defines the ridge both in 

 physical and geological structure from the Mahogany Hills on the opposite 

 side of the narrow valley which has given its name to the fault, and through 

 which the line of the displacement runs Along the base of Prospect Ridge 

 the oldest Cambrian strata yet recognized in the Great Basin come up 

 against the fault and are separated by it from the Silurian and Devonian beds 

 which form the mountains to the west. On the west side of the fault 

 and opposite Prospect Peak, the culminating point on the ridge, the Eureka 

 quartzite of Spanish Mountain is exposed against the fault line. The strati- 

 graphical position of the Eureka quartzite along the Hoosac fault on the 

 east base of Prospect Ridge, where it overlies the great development of 



