PROSPECT RIDGE. 19 



gives out and the fault enters the Nevada limestone with a course a little 

 east of north, and follows along under the abrupt east wall of Sugar Loaf. 

 A short distance beyond Sugar Loaf the fault coincides with the contact of 

 the Nevada limestone with the White Pine shale, maintaining this course 

 until both the limestone and shale pass beneath the basalt tableland toward 

 the north. That the fault continues beyond this point beneath the basalt 

 is clearly established by geological structure, the Devonian strata of County 

 Peak passing under the tableland on the west side and the Weber con- 

 glomerate and Upper Coal-measures dipping toward it and passing beneath 

 it on the east. There can be no doubt that the Rescue fault sharply defines 

 a great physical break separating the County Peak from the Diamond Peak 

 block. After entering the region occupied by the basalt field, there is 110 

 means of determining the precise course of the fault, everything being 

 obscured by recent lavas. Upon leaving the basalt area the fault probably 

 follows along the east base of Richmond Mountain, but is hidden beneath 

 the andesitic rocks that, flowing eastward, rested against the base of the 

 gently inclined slopes of the Upper Coal-measure limestones of the Dia- 

 mond Range. Beneath the lavas the trend of the fault, while in a great 

 degree conjectural, can not vary far from the course of the contact between 

 the Nevada limestone and the White Pine shale as exposed to the south and 

 the line of the Carboniferous rocks to the north and east. In the region of 

 the volcanic rocks the displacement along the fault can not be measured, 

 although it must be very great, as is shown by the Devonian beds on the 

 one side and the upper members of the great development of the Carbon- 

 iferous sediments on the other. South of the basalt the fault runs wholly 

 within the limits of the upper portion of the Nevada limestone, or else at 

 the base of the White Pine shales. Nowhere along its entire course, from 

 Packer Basin to Fish Creek Valley, does the downthrow apparently exceed 

 3,000 feet of vertical displacement. 



GEOGRAPHIC BLOCKS. 



Prospect Ridge.-This ridge stands out as the most prominent orographic 

 feature of the Eureka Mountains. It is situated in the very center of the 

 mountains and presents a bold, serrated outline, extending with an approx- 



